key: cord-0057863-48tomzjp
authors: Welford, Mark R; Yarbrough, Robert A
title: Thresholds
date: 2020-10-07
journal: Human-Environment Interactions
DOI: 10.1007/978-3-030-56032-4_4
sha: 8c70d2c25613ebeaae258b2f2bdde2922c69a788
doc_id: 57863
cord_uid: 48tomzjp

Have human activities such as deforestation and urban sprawl, natural resource consumption, and pollution exceeded the capability of the planet to absorb these actions? It does appear that we have exceeded many critical environmental thresholds such as the ocean’s capacity to absorb CO2. As a result, record-breaking increases in local, regional, and global rainfall and temperatures are occurring, and increases in the number, intensity, and frequency of tornadoes and hurricanes are being documented. El Niños are more common and now frequently stretch across the entire Pacific Ocean. Coastal fogs are decreasing in frequency. The polar front is weakening and affecting the frequency and duration of polar vortexes. This begs the question—have we exceeded the earth’s carrying capacity?

After reading this chapter, you will be able to: 5 Evaluate whether human activity and pollution have exceeded the capability of the planet to absorb these actions. 5 Identify areas in which we have exceeded critical environmental thresholds. 5 Assess the threshold cost and consequences for continuing oil use. 5 Evaluate effects on global warming, agriculture and food consumption, travel and innovation, multinationals and nations when we surpass the world's oil cost threshold.

In this chapter, we explore the notion that changes in human activities, resource availability, and environment processes and patterns occur after crossing a threshold or divide. Typically, human-environmental process thresholds involve exceeding some kind of environmental carrying capacity that appears to depend spatially on population, affluence and technological (IPAT-Impact = population * affluence * technology). These thresholds can be one-directional as in the formation of arroyos, or bidirectional in bark beetle population eruptions, or change entire ecosystems as in the near extinction of India's vulture populations. Establishing the cause-and-effect for most human-landscape-atmosphere process relationships is difficult. Typically, environmental-process thresholds are hard to identify prior to threshold exceedance. Moreover, to add to the uncertainty, thresholds that separate different process regimes can be gradual or abrupt. In addition, most environmental systems are highly buffered across space and time-in other words, multiple different process thresholds must be exceeded within a system before environmental change is observable, and when change is observable, it frequently appears chaotic and difficult to ascribe to single causes-and-effects. Nevertheless, global environmental degradation might have reached or in many cases exceeded their critical thresholds, such that the extent and severity of the impacts may prove irreversible. All thresholds are scale-dependent and spatial in nature and appear nonlinear. In other words, they are complex in nature, hard to identify before a threshold is exceeded, and vary across the landscape. Thresholds can be sensitive or insensitive to significant changes in inputs. The acceleration, or graphic hockey-stick, in the global temperatures witnessed over the last decades, that is, the five warmest years on record have occurred since 2014, suggests that global climate change has recently exceeded a critical threshold.

Floods seem to be threshold-dependent. The Cedar and Upper Mississippi River Valleys have recently recorded three 500-year floods in July 1993, July 1999, and June 2008! These events followed large late spring snowfalls, wet springs, and early summers that appear to be associated with a weakening of the polar front and the increased penetration of tropical air masses into the upper Midwest.

Abrupt or gradual changes to the natural environment all involve some form of threshold exceedance. Jonathan Phillips suggests they are typically characterized by the "four Rs." How quickly the system responds to a change in input and how quickly it adjusts to a sustained change in the system or inputs, the resistance of a system to drivers of change, while the degree of change is mitigated by the fragility or resilience of the environment to change, and recursion of a system to either undergo positive or negative feedback.

This can be encapsulated within the transient form ratio (TF ratio) developed to evaluate change within landscapes. 5 TF > 1: the mean recurrence time of events capable of producing change is shorter than the time taken for the system (or component of the system) to recover or equilibrate, there is likely to be a poor correspondence between process agents and resulting forms (be they landforms, ecosystems); that is, forms will be predominantly transient. In this case, the system can be considered fragile. 5 TF < 1: the system has the potential to adjust to new conditions before the next major disturbance so that characteristic forms will tend to prevail after the initial recovery period, leading to more reliable process-response relationships. In this case, the system can be considered resilient. 5 Brunsden and Thornes (1979) Abrupt, episodic shocks to the landscape are typically followed by a return to the preexisting form or pattern, or TF < 1, whereas perturbations that are ramped and continue to increase in magnitude can result in permanent, long-lasting changes to the landscape and exhibit TF> 1. The introduction of fire to the East African savannah by humans 2-3 million years ago is an excellent example of a ramped input that opened up the forest, creating broad, open grasslands. A more nuanced perspective suggested by Phillips and Van Dyke is that landscapes can exhibit high resistance and resilience to change (e.g., TF < 1), rapid relaxation times, and stable recursive feedbacks that mitigate the effects of disturbance. In other words, irrespective of the magnitude of the disturbance, be it human or natural, for example, hurricane-induced flooding, some landscapes quickly adjust to the disturbance as feedbacks minimize the effect of the disturbance, such as a river quickly eroding, transporting, and moving sediments downstream after a flood.

A good example is the Big Thompson Flood of July 31, 1976, where 12 to 14 inches of rain fell in four hours in its upper drainage near Estes Park. By 9 pm, the creek had risen from 18 inches to 20 feet, killing 143 people. Today, other than a stone memorial and the debris fan, little evidence of the catastrophe remains in the canyon. River flow has remodeled its channel to its pre-flood conditions such that the forms created by the flood are obliterated. ◄ In contrast, low resistance and resilience (e.g., TF > 1), slow relaxation, and unstable feedbacks can, according to Phillips and Van Dyke, create large impacts that remain on the landscape for thousands of years. We might be witnessing just such a phenomenon with the persistent shrinkage of the North Polar sea ice.

Recursive and non-recursive landscape feedback loops, mentioned in the preceding section, were first identified by Schumm in the 1960s. Arroyo and terrace formation in the American West illustrate these feedback loops. Arroyos are gullylike landforms that carry water flow only intermittently and were first observed across the American West in the late 1890s. Typically, arroyos exhibit steep or vertical walls in cohesive, fine sediments and have flat and generally sandy floors. They tend to desiccate the surrounding soils as water tables drop to the lower river floor. They coincided with the fencing of the American West that constrained the formerly open range cattle. Cattle over-grazed and locally compacted the surrounding landscape, reducing rainfall infiltration capacities that led to higher volumes of overland flow and higher in-stream flows that increased flow shear stress in the channel perimeter, leading to channel scour and channel entrenchment.

Among terraces, a single perturbation to the river system (where TF > 1) triggered by climate change, grazing, compaction of the drainage surface upslope from the terrace, base-level change in the form of local or regional uplift or local lake drainage or sea-level drop might precipitate channel erosion, sediment transport and removal, and down-cutting of the streambed and abandonment of the floodplain as a nick-point migrates upstream. Simply put, terraces are abandoned floodplains. However, sediment transport can overwhelm stream capacity leading downstream to deposition and aggradation and the formation of lower alluvial stream terraces within the original incised floodplain. Wolman and Schumm observed just such a phenomenon in Douglas Creek in 1978. So, within the channel, riverbeds are eroding at one point at and immediately below a nick-point but aggrading further downstream.

However, streams and rivers are not disconnected from their surrounding watershed slopes. To clarify, a watershed is all the landscape-its soil, its subsurface rocks, its slopes, hills, floodplains, and terraces that contribute water to a stream or river. Rain falling on a watershed moves over slopes, but mostly through the soil and rock, to where it intersects stream or river courses and then flows into the stream channel. If during heavy rainfall events, the instantaneous infiltration capacity threshold of the watershed surface materials-rock or sediment or soilis exceeded, then water flows downslope across surface slopes to streams and rivers. Sheets of overland flow or surface wash are not commonly observed; rather, surface wash becomes concentrated in the form of rills as water is diverted around obstacles. These obstacles can vary from the size of individual sand grains to plant and tree stems and roots. Once rills form and flow, they can entrain larger particles than surface flow. Rills are ephemeral and short-lived and new rill networks frequently develop after each rain storm. Rills are typically less than 0.5 m in depth, do not extend to the head of slope, and run parallel to each other on fresh surfaces, developing a branching network as divides between rills are broken down by lateral migration or rain splash erosion.

Rills develop on unstable, typically weak vegetated or exposed soil slope surfaces that have been burnt or overgrazed by animals and whose surface is compacted or experienced either significant increases or decreases in rainfall due to local or regional changes in climate, or suffered human-induced deforestation, or suffered the spread of cultivation or urban sprawl.

If unstable slope surface conditions persist, rills can on the lower slopes become gullies. Gullies exceed 0.5 m in width and depth and may be in-excess of 10 m across. They are, in fact, enlarged rills and become permanent landscape features. Some gullies extend to permanent stream channels. They are steep-sided, V-shaped, and flat-floored channels. They represent major sites of sediment storage on slopes at the end of major rainfall events.

Gullies are the principal conduit of slope sediment to streams, but their size is controlled by the length of their contributing slopes or catchment area. Gullies result in accelerated erosion from slopes and increase the speed and volume of water reaching streams (. Fig. 4 .1).

Gullies increase flood runoff but decrease the time to peak runoff, leading to more flash floods. However, in the longer term, gullies decrease the capacity of channels to carry runoff as gullies contribute too much sediment to streams and rivers, thereby choking channels.

Adrian Harvey established over a 20-period that within gully, interactions are seasonal, whereas those related to the coupling of slope/streams, where debris accumulation in the form of colluvial cones and fans and their removal by floods and basal scour by streams, produce a cyclicity over 2-5 years. He also established that over decades, progressive changes in gully morphology and revegetation of once unstable contributing valley slopes may lead to negative feedback mechanisms that stabilize the gully, resulting in a finite age of gullies and an upper size limit for modern gully development (. Fig. 4 .2).

Hydraulic gold mining with its associated use of mercury to trap the placer gold among the South Fork of the American River in California is an excellent example of the aforementioned process and has left an environmental disaster. Over 7500 tons of mercury was used between 1849 and 1884 to trap gold in hydraulic mining sluices. Allan James estimates the hydraulic mining also caused in excess of 13 billion tons of sediment to be eroded within the watershed. Banning hydraulic mining in 1884 did not immediately mitigate the effects of mining. The 13 billion tons of sediment continue to episodically move through the watershed. In fact, in the lower Bear Basin within the mining districts, half of the mining sediment remains. These deposits cover 50 km 2 to depths of 2 to 3 meters. Initially, the riverbed of the South Fork aggraded, with sediment washing over banks and submerging adjacent valley sides and forest due to increased frequency of flood events. As a result, spatially disjunct slugs of riverbed sediment have been mobilized where degradation of the riverbed has occurred, as erosional nick-points have moved upriver. Courser gravel-sized sediment has been deposited as alluvial fans upon the Central Valley, east of Sacramento, creating dry xeric soils highly suitable for citrus farms. Further downriver, more mobile finer silt and . Fig. 4 .2 Active and inactive gullies where reforestation in the channel slopes is stabilizing the lower portion of the basin, while several upper slopes are still actively eroding (unknown source) clay-sized particles contaminated with mercury from hydraulic mining are filling San Francisco Bay. Bioaccumulation of mercury has been observed among the bay's Macoma balthica clams. ◄

The recent Ok Tedi environmental disaster in Papua New Guinea mirrors the processes of aggradation, degradation, and contamination observed in the American River. Between 1984 and 2013, over 260 million tons of mine waste were discharged into the Ok Tedi and Fly Rivers. Just below the mine, the river aggraded 10 meters, while a thick toxic gray sludge continues to migrate downstream. Over 1300 km 2 of flood plain has been inundated due to the aggradation of the streambed contaminating taro, bananas, and sago palm fields. While a further 1600 km 2 of forest has died. Approximately 50,000 people have suffered economic or medical hardships. Although a US$28.6 million out-of-court settlement was reached, few have benefited from the settlement, while the mining company was exempted from further fines. ◄ Recent work does suggest that geomorphic systems are responding to recent rapid climate change. Andrew Goudie suggests rates of landscape erosion will increase between 25-50% by 2100, particularly in cold, tropical, and arid drainage systems, while deltas will have to respond to sea level rise and increased sedimentation due to the transfer of these landscape erosion products. James Knox argues that we are already observing large increases in flood magnitude due to modest changes in climate. An intense snow ablation event witnessed in France in 2003 suggests that if global warming is sustained, sediment transport from alpine regions will increase significantly and likely compromise the flood retention and water storage capacities of many of the region's dams.

Knorr and coauthors suggested long-term global warming will lead to significant soil carbon escaping into the atmosphere; according to Sazonova and coauthors, this prediction is coming true as the arctic tundra is venting tens of thousands of teragrams of methane into the atmosphere.

Although change can be minor, cumulative, or abrupt within the natural environment, any change can be detrimental to societal health. Jared Diamond in his book Collapse suggested that environmental damage, climate change, hostile neighbors, and the loss or lack of friendly trade partners could prove instrumental to a society collapsing. In contrast, a society's responses to its environmental problems have, it appears, always proved significant to a society's ability to either stave off collapse or collapse.

Lastly, in many ways, Easter Island foretells the future of the earth, we have no intergalactic trading partners, and we are knowingly and willingly engaged in local, regional, and global environmental destruction because we are economically, politically, and culturally too inflexible to change our consumption habits. k IPAT: local, regional, and global Unintended consequences of IPAT are widespread, but one of the most surprising is the connection between the near extinction of several Indian vulture species and the explosion in rabies cases across India.

In the last few decades, India's economy has boomed, and today India is one of the fastest growing major economies on earth. As the number of elite and affluent households increased, so did the efforts to relieve suffering among India's sacred cows. Beginning in the mid-1980s, the painkiller and anti-inflammatory drug, diclofenac, was increasingly given to cattle, even sacred street cattle. However, among vultures, diclofenac causes visceral gout, a deadly disease that ultimately killed 98% of all lowland and foothill vultures in India. At the same time that vultures vanished from India's cattle carcass dumps, the number of street dogs increased by some 7 million.

In perhaps the most famous example of societal collapse and accompanying localized faunal and floral extinctions, all seabirds, indigenous birds, and palms were expiated from Easter Island by 1450 CE, yet Easter Island was first occupied in 900 CE by the Rapa Nui civilization. In just 550 years, the Rapa Nui had stripped all viable resources from the island, including much of the island's soil as soil erosion intensified when the palm forests were cut down. The pace of decline mirrored the destruction of the island's soils. According to Mieth and Bork, prior to 1280 CE, sustainable agriculture and agroforestry limited soil erosion; however, after 1300 CE, soil erosion accelerated, burying many settlements and ceremonial sites in rill and gully sediments. Today, many of these gullies are still active, illustrating the fragility of the island's slopes and soils.

In the defense of the Rapa Nui, the island's isolation and subtropical/temperate climate were not conducive to large-scale human occupation. Easter Island also lacks fringing coral reefs or a lagoon, which meant that fish and shellfish contributed little to the diet of the Rapa Nui. Instead, protein came from seabirds, landbirds, porpoises, and introduced chickens-no pigs or goats made the long sea voyage. This was coupled with the lack of trading partners, as Easter Island is the most isolated island in the Pacific: lacking trade partners meant the Islanders were on their own. Instead, the Rapa Nui concentrated on transforming the island through deforestation to gardens for cultivating potatoes, yams, taro, bananas, and sugarcane. For building monumental platforms and statues, palm logs were used to transport the increasingly large moai from the highlands to the coast. As a result of both enterprises, Easter Island was completely deforested by 1450 CE. The deforestation also reduced the ability of the Islanders to utilize the limited marine resources. When the first Europeans ventured onto Easter Island, the few remaining canoes were small and leaked, and they were incapable of sustained ocean travel. The pace of deforestation also far outstripped the island's natural ability to reforest because of three factors. First, Easter Island is cool and dry, nearly temperate in nature. Second, Easter Island lies far from the fertile Pacific volcanic dust cloud. As a result, Easter Island's soil is poor and prone to erosion. And third, seed-recruitment failed across the island as the abundant presence of rats ensured that most palm seeds (recovered from the island during this time of environmental stress at the peak of moai production) show evidence of rat tooth marks. By 1722 when Easter Island was first visited by Europeans, the Rapa Nui, the palm forest, and all the indigenous land birds were extinct, and most of the 30 or more breeding seabirds were no longer breeding on the main island.

Cattle carcass dumps are sites where cattle carcasses are dumped, as only 4% of all cows which are considered sacred are eaten by Indians. Before the widespread use of diclofenac, up to 15,000 vultures could be observed at carcass dumps in just New Delhi. However, once the vultures began to decline, street dogs gained more access to cattle carcasses at cattle dumps. This shifted the balance of rabies infections across India. Vultures with their highly acidic stomachs are dead-ends to nearly all the pathogens they consume, including rabies. So as vultures decreased and dogs increased, the incidence of rabies increased. However, where leopards are present, the numbers of rabies deaths have not risen so steeply, as leopards have targeted street dogs but this has meant more attacks on humans. In fact, the first attack on a human by a leopard near Mumbai in an adjacent national park was in 1986. Deforestation and human encroachment into parks was thought to explain leopard attacks. But leopard scat analysis suggests leopards were targeting street dogs and human attacks were incidental. Throughout the 1990s, India suffered more and more leopard attacks on humans, just as India saw more and more street dogs and more and more rabies cases. Since the early 1990s, India has seen an extra 50,000 cases of rabies, and today, in India, there are 36% of all worldwide rabies cases and in excess of 20,000 human deaths per year, at a cost of 750 million rupees per year. The Indian government is funding vulture breeding and reintroducing efforts across India to eliminate rabies. Sky-burials by Mumbai's 3000-year-old Zoroastrian Parsi community at their Tower of Silence have had to be curtailed as the vultures have disappeared. Although raven and kite numbers have increased, these birds are not efficient at consuming human flesh, and as a result the bodies rot and their smell is increasingly disturbing local residents. ◄

The single greatest threat to biodiversity in the USA is suburban sprawl and the expansion of the wildland-urban interface or boundary. Since European settlement of NA, 27 different types of natural communities have declined by 98%. The major single cause of biodiversity loss according to Ehrlich is habitat loss and degradation. Destruction of previously intact ecosystems results in a loss of habitat for multitudes of species and breaks down an ecosystem's ability to function. In fact, around the world, urban populations are growing faster than rural populations. In the USA, Forman and Deblinger found that bird densities are significantly reduced within 1000 meters of roads due to the fragmentation of forest and the increase in brood parasitism by cowbirds, as cowbirds invade the fragmented landscape, while road densities lead to significant loss of wetland-dependent amphibians, lizards, and bird species, although Findlay and Bourdages found that these losses lag a decade behind any road development. Urban sprawl, defined as lowdensity development of natural areas outside of cities and towns and its associated wildland-urban interface or boundary that directly elevates fire risk, according to Syphard and coauthors, is responsible for threatening 188 of California's 286 endangered species. Population density and the distance to the wildland-urban interface appear to determine the spatial pattern of fire-risk, with the highest fre-quency of fire and burnt areas occurring at intermediate levels of human activity, but once population densities exceed critical thresholds, fire frequency declines as less fuel is present on the suburban landscape. Yet effective planning in rapidly urbanizing landscape of Eastern Pennsylvania has diminished by half the proportion of habitats lost.

Sprawl also exacerbates air and water pollution that reduces both biodiversity and critical ecosystem services, where ecosystem services are defined as natural capital assetsthat supply life-support services of tremendous value to the landscape and humans. These services include, but are not limited to, services that purify air and water, mitigate floods and droughts, and detoxify and decompose wastes. For instance, new construction accelerates soil erosion as forest, shrubs, and grasses are cleared for development. Eroded soil is transported downslope into streams through surface flow, and rill and gully erosion (see . Figs. 4.1, 4.2, and 4 .3), resulting in increased stream siltation which can reduce the life of dams. Deforestation immediately adjacent to streams can lead to rising stream temperatures and stream eutrophication, stream bank erosion, stream entrenchment and arroyo development, and increasing flash floods as surface infiltration capacities decline. As natural ecosystems shrink, there is less natural capacity to filter pollutants and detoxify waters and less capacity to cycle nutrients and compost organic wastes. As sprawl increases, species and ecosystem services decrease. Small, fragmented habitats hold less than large, single habitat. In Chicago, "unassociated vegetation," which represents fragmented, degraded patches of nonnative woody and grassy vegetation, increased from 7% to 22% of land use/cover as sprawl expanded north, west, and south of the urban core. Some animals benefit from sprawl such as dietary generalists, previously cliff-dwelling birds, and granivores, in other words, those animals that eat seeds as a main part of their diet. Highly disturbed environments such as urban and suburban gardens and brown sites are dominated by weeds that produce large numbers of easily distributed seeds.

Sprawl is not just a local issue; nonhuman creatures and non-domesticated plants are running out of space even on continents. And what space is left is highly fragmented. At the crux of the issue of space is the species-area relationship. Simply put, the larger the area, the greater the number of individuals and species an area can hold. So smaller islands or smaller continental fragments or smaller patches of habitat have few species, because these small islands or patches of habitat have less space and less variation in environments, while larger islands or larger patches of habitat have more species, more space, and typically a larger variety of habitats which can support a richer variety of ecosystems. This can occur among true islands surrounded by water, or mountains surrounded by lowlands, or patches of habitat surrounded by urban sprawl or arable land. In the case of mountains, in the American West, several mountain ranges lie surrounded by inhospitable deserts. These mountain-islands or sky islands do exhibit species-area threshold relationships (Johnson 1975) , although they display higher species diversity and comparatively low endemism compared to true islands, especially when compared to oceanic islands (McLaughlin 1994) . Yellowstone NP is surrounded by a low-density urban/suburban/arable sprawl that restricts migration of mammals into the park. For all intent and proposes, Yellowstone is a sky-island! It exhibits a lower mammalian and amphibian diversity than other more connected habitat patches in the Rocky Mountains. However, the reintroduction of the wolf in 1995 is changing this dynamic.

Barro Colorado Island (BCI) in Panama was created by the damming of the Chagres River in 1913 to form Gatun Lake that provides water to both sides of the Panama Canal. Since its isolation, the understory of BCI has opened up as tree seedling recruitment has slowed. Two large-seeded, mature-phase rain forest trees Dipteryx panamensis and Gustavia superba are not regenerating. Their seeds have ten times greater likelihood of surviving on mainland than on BCI. About 50-60 bird species have disappeared from BCI. Among undergrowth forest bird species, rarity is not a good predictor of extinction on BCI; instead, it appears that birds that utilize patchy or seasonally variable resources, be they insectivores, frugivores, or nectarivores, are more prone to extinctions (Karr 1982) . In other words, there are insufficient patchily or seasonally variable resources in the small area of BCI to support these bird populations. Many groundnesting birds also disappeared as coatimundis and opossums robbed nests. In fact, 96% of ground-nesting attempts fail on BCI, but only 4% on mainland. It appears that as cats (jaguar) left BCI (only one has ever been observed on BCI in 1987), so coatimundi and opossum numbers exploded, killing off ground-nesting species. The following table illustrates the impact of jaguars disappearing from BCI (Cocha Cashu is on the mainland).

The tropic cascade triggered by the disappearance of jaguars from the BCI forest has led to the local extinction of many bird species, while seed predators such as the agoutis, peccaries, paca, and rats have expanded and increased seed-predation rates within BCI. Asquith and coauthors suggest that the forest of BCI is in fact dying. In a sense, BCI is a microcosm of the earth, both terrestrial and marine apex predators across the globe are in rapid decline, and the resultant tropic cascades will be significant, detrimental, and unpredictable! Rather sadly, the Smithsonian Tropical Research Institute established on BCI is a witness to this ecosystem death, not something the founders expected. ◄ Sadly, the highly endangered helmeted hornbill (Rhinoplax vigil) illustrates the problems of rampant deforestation, hunting, and minimum viable populations. A minimum viable population is a genetically isolated population of at least 50 pairs that have at least a 95% chance of surviving extinction over the next 100 years despite demographic, environmental, genetic stochasticity, and natural catastrophes. Phillipps suggests helmeted hornbills eat large figs exclusively; as a result, each pair needs approximately 7.7 km 2 of territory to breed successfully and 50 pairs require 385 km 2 of intact lowland rainforest. However, in Borneo, an area of only 743,380 km 2 , only two intact areas of contiguous lowland forest in excess of the minimum threshold area, remains: one at Danum Valley Conservation Area of 438 km 2 , and one at Tabin Wildlife Reserve of 1225 km 2 . However, Tabin Wildlife Reserve has an intact, unlogged core area that is significantly smaller than 1225 km 2 . These populations are protected, but other surviving helmeted hornbill populations outside of these two parks also suffer significant hunting pressures. Today, helmeted hornbill's casque, its red helmet-like solid protrusion on its upper mandible, is highly sought after by Chinese ivory carvers. In Laos, helmeted hornbill heads go for $3700. ◄ Even under pre-agricultural, pre-urban conditions where humans existed at quite low densities, we have wielded a big deadly club! Although the explanations for megafauna extinctions in the post-glacial environment are many and varied, one consistent theme is threshold related-the notion of overkill or human hunterfacilitated extinction. In the North America, by 11,700 years BP, some 70% of the megafauna and 45% of the predatory bird genera including a giant vulture Teratornic incredibilis had gone extinct. In a very interesting experiment, John Alroy simulated the impact of growing human populations in North America on megafauna. His simulations assumed megafauna and humans had slow population growth rates, that hunting was random, and hunting effects were low; in other words, few were killed each year. Nevertheless, varied simulations had a 78% success rate in predicting a rapid megafauna extinction in and around 11,700 years BP. Although humans were not a plague on NA by this point, the low reproductive rate of megafauna coupled with slowly rising human populations was sufficient to push megafauna (those animals over 100 kilograms) over a critical population threshold and on to extinction shortly after the end of the Late Wisconsin Ice Age. It probably did not help that this was a time of significant climate and environment change precipitated by the end of the Pleistocene Ice Age and what appears to be a 31-km-wide meteorite impact during the Younger Dryas around 12,900 BP under what is now Hiawatha Glacier in NW Greenland. According to Paul Voosen and Kurt Kjaer, shocked quartz, iridium (both evidence of impacts), and platinum have been found downstream from the Hiawatha glacier and in Greenland ice cores from this time, but from a 1000 km south of the impact site. Firestone and coauthors suggest this event contributed to the NA megafauna extinction and disappearance of the Clovis native American culture.

Even abundant, widespread species can and do go extinct. Ectopostes migratorius, the passenger pigeon was quite possibly the most abundant bird or noninsect animal to go extinct. In 1813, James J. Audubon observed over three days a mega-flock of an estimated 1.11 billion birds. But its extinction is complicated. Certainly, the pigeon was hunted for food. One boxcar near Chicago was discovered containing 38,000 dead birds, and over 5 months in Petoskey, Michigan, in 1878, 7.5 million birds were killed, while the American chestnut tree (Castanea dentata), one its principal food sources, was decimated by the introduction of a chestnut blight. Recent genetic work and modeling by Chih-Ming Hung and coauthors suggest that the passenger pigeon was an "outbreak species" with a small genetically effective population-the product of repeated boom-and-bust cycles in its population due to its diet specialization on acorns, chestnuts, and beechnuts that vary between 12-fold and 136-fold in seasonal production. With the loss of the American chestnuts and subsequent overconsumption of acorns and beechnuts and overhunting, the population of the passenger pigeon crashed in the late 1890s. The huge flocks of passenger pigeon were selected for in order to locate sufficient food because its food underwent such massive spatial variability in seasonal production within the Eastern USA. However, David Quammen believes that once flock sizes decreased below a threshold size, possibly 500,000 individuals, the flocks were unable to locate food and the pigeon spiraled into extinction.

The emergence of new diseases such as Ebola, HIV-1, Marburg virus, Lasa fever, or Hanta virus, or the reemergence of long-known diseases such as tuberculosis, is the product of various environmental factors working together to create conditions that support transmission of a pathogen to humans, the maintenance of the pathogen with a spatial restricted human population, and then the pathogens' transmission to a broader spatial scale and a larger human population. Stephen Morse suggests that among the principle factors that contribute to the emergence of infectious pathogens are human-induced changes in ecological systems; changes in human demographics and behavior; increasing human mobility, industrialization, microbial adaptation, and change; and the failure of public health services (. Table 4 .1). What is frightening is that these factors are not abating; rather, they are getting more severe as human populations continue to grow, consume national resources, and pollute and alter natural ecosystems, making them simpler and more prone to catastrophic fluctuations.

. In order to precipitate a deadly global epidemic, three disease facets must occur: high transmissibility with a relatively high basic reproduction number or R 0 (the R 0 is the number of secondary infections caused by an initial infection), high pathogenicity and lethality, and long-term environmental persistence or frequent reinfection from nonhuman host reservoirs to humans. The 2009 swine flu pandemic exhibited high transmissibility but lacked high pathogenicity and lethality. The 2014 West African outbreak of Ebola displayed relatively low transmissibility, yet very high pathogenicity and lethality. Both previous and recent epidemics suggest significant environmental persistence-in the 2014 case, within fruit bats. In contrast, the first and second plague pandemics, seemingly caused by pneumonic plague, exhibited high transmissibility and high pathogenicity and lethality. However, the third plague pandemic, which is bubonic plague, is typified by low high transmissibility, relatively low pathogenicity and lethality in the last 50 years, and yet persistent long-term environmental survival in various mammalian reservoirs.

Human-environmental relationships have proceeded through four great periods, and each has seen the emergence of new diseases (see Chapter 3, Local to Global impacts; McMichael 2004), and each has borne witness to the evolving nature of human-environmental thresholds and connectivity between fellow humans through trade and transportation that control disease transmission. The spatial extent and lethality of the three plague pandemics-the first pandemic of 541-715 CE, the second pandemic of 1330s-1879 CE, and the third and current pandemic that began in 1894 CE-illustrate these thresholds and changing connectivity. For instance, the first pandemic was restricted to Europe, the second to Europe and Asia, and the third occurs throughout the world.

The first great period of disease emergence occurred 5000-10,000 years ago when humans first became sedentary, living in permanent settlements, cultivating the land and domesticating animals. During this period, measles and pertussis first appeared among humans, suggesting a domesticated-animal origin; however, Pearce-Duvet argues that human modification of the environment might explain other crossovers such as tuberculosis. The second great period occurred 1000-3000 years ago when we began to trade across continents, for instance, the Silk Route and the Greek traders who followed Alexander the Great into India. The Silk Route repeatedly acted as conduit for the medieval Black Death between 1346 and 1815 as plague mutations erupted in Central Asia and were transmitted westward, infecting Europeans along the route a decade or so after their emergence. A third period began in 1492 as humans became intercontinental travelers, and in this case European settlers brought Native Americans measles, flu, and smallpox. For decades, it was thought that Native Americans gave Europeans syphilis, but four skeletons from fourteenth century in Hull, England, and evidence from victims of Vesuvius found in Pompeii suggest syphilis was already well established in Europe before Columbus set sail. Today, we live in the fourth great period of disease emergence, as our global transportation networks move goods and people and diseases, for example, SARS, rapidly around the globe. We live in an increasingly smaller world where millions of people move each day round the globe with comparative ease and frequency and could act, at any time, as perfect vectors for highly lethal, transmissible, newly emergent microorganism.

On January 12, 2010, a magnitude 7 earthquake struck Haiti, devastating the country and its people, killing an estimated 316,000 people and leaving nearly one million homeless. On October 21, 2010, cholera was identified in-country and quickly grew into an epidemic in a country that, in 2008, offered only 63% of its citizens improved drinking water and only 17% of its citizens access to improved sanitation. By December 2011, an estimated 522,335 cases of cholera and 7001 deaths were attributable to cholera. Similarly, on October 31, 2010, an outbreak of the same strain of Vibrio cholerae began in the Dominican Republic, where 86% of the population has access to improved drinking water and 83% to improved sanitation. In contrast, the Dominican Republic suffered 363 fatalities and 21,432 cases were reported. In 2010, 15 cases of V. cholerae serogroup 01, serotype Ogawa, biotype El Tor associated with the Hispaniola outbreak, were reported in the United States, and in 2011, an additional 40 cases were reported. In 2010 and 2011, 17 cases were identified in Florida, 13 in New York, 4 in Massachusetts, 3 each in Ohio and Texas, 2 each in Georgia, Kentucky, and Virginia, and one case in the following states of Arkansas, California, Illinois, Kansas, Missouri, New Jersey, New Mexico, and Pennsylvania. Most of these US cholera cases were tied to US citizens with Hispaniola roots returning to the USA after traveling to Hispaniola to provide technical, logistical, and food aid and emotional support in the aftermath of the earthquake. ◄ The global spread of HIV-1, AIDS, illustrates how spatial thresholds were exceeded within the fourth great transition. We suspect in precolonial Africa, HIV-1 infection rates were less than 1% across Central Africa where simian immunodeficiency virus (SIV) is found within the chimpanzee and gorilla populations in Cameroon, Democratic Republic of the Congo, Republic of the Congo, and Gabon. In fact, the most widespread HIV-1 clad is derived from a SIV cpz found within chimpanzees, specifically those chimps found in SE Cameroon where the chimp SIV cpz is identical to the oldest variation or clade of HIV-1 known to science.

During precolonial times, single sex partners were the norm; migration for work was very limited; transportation within, among, and between regions and countries extremely limited; and deforestation rates were very low. However, colonialism was a game-changer! Deforestation accelerated as colonial programs and multinationals encouraged poor to colonize peripheral forests and cut forest for commercial timber. Unfortunately, many monkeys and apes can still thrive in degraded forest and around human settlements. An unintended consequence we think is that the consumption of bushmeat increased and the transmission of monkey and ape viruses to humans increased. Colonial countries also increased the density of transportation links within countries and forced colonized peoples into a wage-labor market through taxation. Taxation forced colonized people to sell their time and labor for monetary reward. As a result, large numbers of African males in the early 1900s migrated to work-rich environments such as mines, ports, capitals, and other colonial towns. This also precipitated an increased in urban prostitution, facilitating an explosion in sexually transmitted diseases.

David Quammen postulates the following scenario: in the early 1900s a single human hunter is bitten or cut while killing a snared chimpanzee infected with SIV cpz . This probably occurred countless times to countless hunters, but their infection did not cross to other individuals. With the advent of colonialism, mandatory taxation, and increased river trade and traffic, infected individuals had a greater likelihood of transmitting their acquired HIV-1. So, although cut and infected with SIV cpz , the hunter, unaware of the infection, travels downstream into the DRC and infects one or more women in one or more of the river-trading towns in the upper Sangha River basin. These infected women unknowingly pass this SIV cpz /HIV-1 onto subsequent male partners, who migrate further downstream. This movement was probably haphazard and intermittent and contingent on the long incubation period and infectiveness of HIV-1, and so, according to Worobey and coauthors, it was not before 1959/60 that individuals were infected in Kinshasa.

Thereafter, several additional factors assisted in the transmission of HIV-1 to a broader community. Needles and vaccines were first introduced into Africa in the post-WWII environment; however, resupply of needles and syringes was poor, sterilization procedures poor or nonexistent, and, as a result, needles and syringes were reused many hundreds, if not thousands, of times. Furthermore, in the postcolonial environment, political, ethnic, and economic rivals, either disenfranchised by the departing colonial administration or elevated above all others or false-fed propaganda by Cold War rivals, led to the advent of many civil conflicts and lowintensity warfare in the countryside. Poorly led and poorly paid soldiers frequently used beating, murder, and rape to intimidate rural people. Many rural peoples were dispossessed of land or land ruined by warfare, with the result that many of these people either migrated to towns or became involved in smuggling or prostitution. As a result, conditions were ripe for the accelerated transition of sexually transmitted diseases as multiple sexual partners became common place, famine and malnutrition stressed and weakened immune systems, large-scale migrations of people from rural to urban areas increased, and these migrations and low-intensity warfare and population growth devastated or overwhelmed healthcare systems across Central Africa.

By 1968, HIV-1 had crossed into the USA, quite possibly through Haiti and the blood-product trade for hemophiliacs, with the first known victim being a male prostitute in St. Louis. However, today in the USA, according to Adimora and Schoenbach, HIV-1 remains spatially concentrated in socially marginalized groups exhibiting low social capital, high incarceration rates, and high-income inequality.

In contrast to influenza and the cold virus, HIV-1 is difficult to transmit-it is transmitted blood-to-blood rather than through viral aerosols issuing from the lungs through coughing. We now know that there was not a patient zero identified by the world's vitriolic media as Gaetan Dugas, a Canadian flight attendant; rather, HIV-1 transmission velocities were quite slow. In contrast, the 1918 influenza epidemic engulfed the world in two years and killed more victims than WWI-this at a time of a relatively slow oceanic traffic in the form of ocean streamers. Today, we are no more than 48 hours from any place on earth. This is clearly illustrated by the recent global eruptions of SARS, swine flu, and now COVID-19. Whereas SARS was ultimately contained, although three super-spreader centers facilitated transmission beyond Hong Kong, COVID-19 escaped Wuhan, China, and as we write in March 2020, it is still expanding as a global pandemic with ~1 million infections and ~48,000 deaths. What crucially facilitated the emergence of the ongoing COVID-19 pandemic is that Wuhan, China, is fully embedded within the global transportation network while it was not in 2003 during the SARS epidemic. Wuhan has an international airport with direct flights to Europe and much of Asia. Wuhan is also part of China's growing high-speed rail network. In other words, China crossed a transportation threshold sometime between 2003 and today making the likelihood of newly emergent diseases escaping the confines of China all the more likely.

As illustrated by 1918 flu, swine flu, seasonal flu, SARS, and now COVID-19, we long ago as a global community crossed a social distance threshold, whereby pneumonic diseases that spread by aerosol sprays of viruses and bacteria through coughs and sneezes and the subsequent short-term persistence of these pathogens on interior surfaces of building (and our propensity to touch faces) can be transmitted around the world with ease. Our cities are sadly perfect breeding grounds for new emergent pneumonic pathogens! Urbanites live in high densities and endure nearly constant social interactions. Our proclivity to work in crowded office spaces, teach face-to-face, commute on crowded trains and buses, eat and drink out, and work-out in frequently crowded gyms places urbanites at greater risk than those individuals living in rural areas. The critical issue with social distancing according to Rabinowitz and Bartman is that the more viral particles a person is exposed to, the greater the likelihood that the pathogen will overcome an individual's immune system defenses and infect an individual. Simply put, the more social contacts a person is subjected to during the course of a day, week, month, or year, the greater the likelihood that the individual will become infected if the pathogen resides within the confines of that individual's social space. This explains why even nurses and doctors who take precautions while treating infected individuals still have a high likelihood of becoming infected themselves. This also explains why workingclass individuals living in high-density housing also suffer higher disease risks-we are seeing both these phenomena in New York City. New and well-dated evidence of sulfate deposits in Greenland and Antarctic ice cores indicates a substantial and extensive atmospheric acidic dust veil at AD 533-534 ± 2 years. This was likely produced by a large explosive, near equatorial volcanic eruption, causing widespread dimming and contributing to the abrupt cooling across much of the Northern Hemisphere. Two additional eruptions in 540 and 547 CE transformed the Late Antique World by initiating the Late Antique Little Ice Age that ran from 536 to 660 CE. Tree-ring data suggest that this was the most severe and protracted short-term cold episode across the Northern Hemisphere in the last 2000 years. Cold and subsequent droughts led to Mediterranean, Mesopotamian, and Chinese famines, and, according to P. D. Pang and associates, deaths up to 70-80% of the Chinese population (Pang et al. 1989) . Rather, opportunistically, the Justinianic Plague-Yeresina pestis-erupted among Eurasia's impoverished, malnourished, and health-compromised peoples. Perhaps as many as half the population of the Byzantine Empire and Europe was killed between 542 and 565 CE. Support for this contention comes from the Nazi-instigated Dutch famine of 1944-1945. This famine compromised Dutch health for decades, especially among those children born or conceived immediately before or during this famine. McMichael and coauthors found that climate anomalies directly impact health through heat waves, dust, or extreme weather, or indirectly by reducing agricultural yields, reducing water supply, triggering eruptions of climatic-sensitive infectious diseases, displacing human groups, and triggering depression and despair among marginalized, disadvantaged, and failing farm communities.

Given the impoverished, malnourished condition and recently climate-displaced nature of many peoples around the world, are we approaching a critical disease/famine threshold? Certainly, the recent cholera epidemic in Haiti triggered by the chaos of the post-earthquake environment suggests we are ill-equipped to deal with any and all widespread societal problems that climate change is initiating. The recent and continuing collapse in insect populations around the world is equally troubling-insect pollination of the world's agricultural plant crops is at considerable risk. Although it is unlikely we have a reached a critical insect density pollination threshold, we are certainly moving toward a critical threshold triggered by climate change, the widespread use of neonicotinoids insecticides, and seemingly uncontrolled sprawl.

Enacting civilian air flight bans across the USA has been proposed as a means to limit the spread of pandemic diseases such as influenza. The decline in air traffic post 9/11 appeared to explain 27% of the lag in peak of the influenza epidemic that winter/spring (Brownstein et al. 2006) . A more extensive meta-analysis found no difference between the spread of the post 9/11 influenza epidemic than influenza epidemics from 1972-2002 (Viboud et al. 2006) .

Pandemic simulations suggest that isolating just 2% of the largest global cities dramatically reduce the transmission of infectious diseases. However, 27.5% of the principal airline network nodes (e.g., Atlanta International, Heathrow, Dallas-Fort Worth, Frankfurt) had to be shut down to obtain a similar response (Hufnagel et al. 2004 ). The recent 2003-2004 SARS epidemic illustrated that a few infected individuals can infect a high number of susceptibles; these super-spreading events (SSEs) can also rapidly transmit infectious diseases across very heterogeneous spaces. In Hong Kong, three SSEs were identified; one in a hospital, one in a housing estate, and one at a hotel. Other SARS-related SSEs occurred in Beijing, Singapore, and Canada. There is also a strong, positive relationship between relative population concentrations of residents with self-identified ties to Hispaniola and US cholera cases during 2010-2011. This suggests where strong social, cultural, and economic connections exist between places, highly infectious diseases might easily jump across spatial boundaries and move up spatial scales.

As is becoming increasingly transparent, the global community is in the process of failing to prevent a Coronavirus-19 or COVID-19 pandemic. Yet the first cluster of unusual pneumonia cases in Wuhan was identified by an Artificial Intelligence platform BlueDot a little after midnight on Dec 30, 2019 (Niiler 2020)-this some six days before the CDC sent out an alert, and some nine-days before the World Health Organization alerted the world to a novel, newly emergent coronavirus. Yet businesspeople, tourists, and travelers of all sorts continued to fly in and out of China, South Korea, and Italy, three hotspots in the COVID-19 epidemic until mid-March 2020. Most US educational institutions have recalled students on study abroad trips from these destinations. Italy attempted to seal off Lombardy, and initially this worked, but evidence suggests this lock-down was leaky at best! But on March 8, quarantined Lombardy, states around Venice, and the Rimini area effectively sealed in 16 million, shutting down all schools and universities. The long incubation period of COVID-19 has certainly impeded efforts to quarantine areas. As of March 6, 2020, it is thought that COVID-19 was circulating within Washington State, USA, for a few weeks before being detected at a nursing home for the elderly.

What swine flu of 2009 (H1N1), SARS, Middle East Respiratory Syndrome (MERS), and COVID-19 illustrate is that, today, there are few barriers to a pandemic if a human-to-human pathogen is highly transmissible, in other words, (1) exhibits a high level of infectivity, (2) has an incubation period of 5-7 days or more, (3) causes asymptomatic infectious carriers, in other words, people who appear healthy, remain mobile, but infect others, and (4) appears to exhibit a low level of lethality. So, although the world's media is fixated on the coronavirus (as of March 6, 2020) and scientists, politicians, and laypeople are genuinely and rightfully worried about coronavirus, unless very strict quarantines had been imposed immediately upon its discovery, our full globalized society (where every place on earth is 48 hours away from every other place) has crossed a pandemic threshold. In contrast, where pathogens exhibit high lethality and short incubation periods (victims are easily identified and too ill to travel any distance), public health systems can establish effective quarantines. But even under these circumstances, cultural, political, and economic conditions can sometimes optimize local or regional epidemics (rather than a global pandemic) such as the Ebola epidemic that killed over 11,000 in and around Sierra Leone in 2014.

Even by early March 2020, the COVID-19 pandemic had slowed economic activity in China, reduced pollution above China, and triggered a global economic crisis. According to Kasha Patel and NASA, February's NO2 levels above China were 10-30% lower than just a month before.

According to many climate researchers, we are approaching or at the last exit ramp where we can make significant changes to our lifestyles and economic systems that could avert runaway global warming. Yet within the last century, atmospheric temperatures have risen 2.5 °C and the oceans pH has dropped 0.1 units as they have absorbed CO2; as a result, in excess of 35% of the world's coral reefs have bleached and died due to heat-stress and acidification. Estimates suggest 90% of coral reefs will die by 2050. This will have a crippling effect on tropical fish fecundity as many young tropical fish rely on living coral reefs for protection.

Ignoring global climate change, first hypothesized in the late nineteenth century but first measured scientifically in the late 1930s and widely acknowledged after 1960 with the publication of Keeling Curve, appears now to have been extremely short-sighted; Mayer Hillman goes further and suggests we are doomed: » The outcome is death, and it's the end of most life on the planet because we're so dependent on the burning of fossil fuels. There are no means of reversing the process which is melting the polar ice caps. And very few appear to be prepared to say so. Barkham, 26 April 2018. The controversial identification of the "hockey-stick" acceleration in CO 2 and associated temperatures for the late twentieth century by Mann and colleagues in 1998 illustrates how societies, even our modern global society, can conflate, repudiate, attack, and dismiss documented evidence that they are in deep trouble: among the chief climate change deniers are a very vocal array of US citizens, the Republican Party, and Fox News. Yet we have talked to rural, isolated peoples in Ecuador, India, and PNG who have witnessed climate change during their own lifetime and acknowledge its global dimension (see . Fig. 4.1 ). Since 1998, broad scientific consensus has identified unprecedented increases in CO 2 and associated global warming. For instance, the UK Met Office notes that 16 of the 17 warmest years on record have occurred since 2000.

k Tipping Points, Irreversible Changes, and the IPCC The IPCC's Third and Fourth Assessment Reports introduced the notion of tipping points within the earth-atmosphere system whereby abrupt and/or irreversible change will occur if anthropogenic warming exceeds 5 °C above pre-industrial levels. This estimate has proved to be too conservative, and encouraged industries, laypeople, and politicians across the world to be rather tentative in their approach to reducing GHG emissions. Recent IPCC reports Global Warming of 1.5 °C and IPCC Special Report on the Ocean and Cryosphere in a Changing Climate dispel this notion and suggest that abrupt and irreversible change will occur if anthropogenic warming exceeds just 1 °C warming. Yet this is where we reside today! Furthermore, these changes will be nonlinear, unpredictable, and far-reaching, exhibiting teleconnections across the planet (just as El Niño does today).

Climate change is expected to be increasingly nonlinear once a number of thresholds or tipping points are exceeded. Arctic sea ice loss is weakening the Arctic jet, allowing more poleward penetration of warm continental air masses into the Arctic but also triggering more intense polar vortexes. Both the warming Arctic and melting Greenland ice sheet are slowing down the meridional overturning circulation (MOC) due to the influx of surficial freshwater into the North Atlantic. To clarify, the MOC stores and transports heat, freshwater, and carbon around the globe. The slowing down of the MOC is reducing CO2 storage in the deep ocean. In fact, Bertler and coauthors suggest that between 1550 CE and 1800 CE, atmospheric CO 2 decreased by 10 ppmv as more CO2 was absorbed by the deep MOC adjacent to Antarctica. Today, Greenland is losing ice at the fastest rate since the end of the Little Ice Age 350 years ago. In the North Atlantic, just off the coast of Greenland, the cooling MOC loses buoyancy as heat is lost from the surficial water (this surficial water originates as glacial meltwater from the Greenland Ice Cap) to the atmosphere. The MOC current then flows on the deep ocean floor towards the Southern Atlantic, where upwelling returns the nutrientrich MOC to the ocean's surface adjacent to Southern Africa. A slower MOC stores less carbon and hence fewer nutrients, less heat storage, and less freshwater. As a result, the Arctic is becoming warmer and drier, causing more droughts in the boreal forest area of NA. A similar scenario plays out across the Amazon basin, although this is being intensified by a recent acceleration in deforestation as more forest is being cut and burnt to provide immediate acreage for organic soybean cultivation. The slowing MOC is reducing upwelling of cold water off both Antarctica and Southern Africa, which is intensifying Antarctic ice sheet melting and reducing nutrient-rich sources along the edge of both continents, with catastrophic impacts on commercial fishing, and krill and sardine numbers that are impacting whales, penguins, albatrosses, and seals, among others, that thrive in these formally carbon-rich waters.

Just to confuse things, massive melting of the Greenland ice sheet and subsequent intrusion in the North Atlantic of buoyant, freshwater water have in the past been associated with the rapid onset on the Mini Ice Age following the peak of the Medieval Warm Period. The hypothesis was that buoyant, low-salinity freshwater flowing off Greenland forces the surficial North Atlantic drift south, denying Europe warm water and warm oceanic air masses. The rapid cooling of Europe and subsequent droughts and famines created conditions that compromised the health and immune systems of the majority of the European population and thus facilitated the transmission and lethality of plague or Medieval Black Death once it arrived in Europe, courtesy of the Golden Horde's use of plague victims as biological weapons in the siege of Caffa in 1346.

Lenton and coauthors argue that the collapse of both the Amundsen Sea embayment of West Antarctica and the Wilkes Basin of the East Antarctic ice sheet are imminent! Combined, they could trigger a 6-7 meter rise in sea level somewhere within the next 100-1000 years. However, the Greenland ice sheet is doomed and expected to collapse by 2030. Furthermore, the Arctic is expected to become ice-free in summer with 2 °C warming. Elsewhere, a 99% die-off of corals is expected if global temperatures reach 2 °C above pre-industrial levels. The impact on all marine commercial fisheries would be catastrophic, while the ocean's principle tropical marine fish hatchery would be lost. Lenton and coauthors also suggest that the Amazon's tipping point, where negative atmospheric feedbacks would shrink moisture inputs triggering desertification, lies somewhere between 20-40% forest-cover loss. Since 1970, 17% of the Amazon forest has been lost, yet deforestation is currently accelerating after years of decline. The rapid warming of the Arctic has led to pest infestations, fires, and an increase in the number and intensity of boreal forest droughts.

Estimates vary, but Lenton and coauthors suggest that in order to remain below 1.5 °C warming, we must cap CO2 emissions and not emit another 500 gigatons (Gt) of CO2. Sadly, we are already close to this threshold! Permafrost emissions are expected to yield 100 Gt of CO2 and unquantified amounts of methane. While the Amazon is expected to yield 90 Gt of CO2 and the boreal forests another 110 Gt of CO2. However, we are currently emitting 36-40 Gt of CO2 annually, suggesting we will exceed the 500 Gt limit in the 5-7 years!

Although extinctions will be addressed in detail in Chap. 8, localized extinctions or mass mortality events (MMEs) suggest that many regions on earth have reached critical ecological thresholds that could lead to broader-scale global extinctions. An ecological threshold is the condition where a relatively small change or disturbance in the landscape, for instance, an increase in the frequency and duration of droughts, can rapidly change the functioning and structure of an ecosystem. Both Scotland and Alaska were once world famous for their salmon fishing; in January 2017, the Alaskan salmon fishery collapsed, and in 2018, the Scottish salmon fishery also collapsed! For instance, not a single salmon was caught within the Sprey and Nith rivers in 2018. Katherine Mills and coauthors suggest these declines started in the 1960s, but unlike the decline in trout observed in Norway which appears connected to increased lake acidity caused by sulfur emissions from coalburning in the UK power-plants, salmon collapses appear linked to warmer ocean temperatures that are negatively impacting plankton communities.

Declining plankton communities and harmful algal blooms (HABs) appear to have recently triggered mass mortality events among seabirds in the North Pacific and North Atlantic. For instance, between October 2016 and January 2017, 350 dead puffins and other seabirds washed up on the shore of Saint Paul Island. In fact, Piatt and coauthors suggest some 62,000 dead or dying common murres (Uria aalge) washed up on beaches from California to Alaska between the summer of 2015 and the spring 2016, of which two-thirds were adults. In reality, possibly as many as a million birds died because very few (ocean-wandering) dead or dying birds will wash ashore. Subsequently, 22 murre breeding colonies completely failed between 2015 and 2017. This is unprecedented! Timothy Jones and coauthors suggest that a period of elevated sea-surface temperatures in the eastern Bering Sea triggered a mass mortality event in surficial zooplankton and a reduction in prey-fish densities that precipitated a mass mortal-ity event or mass die-off among fish, bird, and mammal species in 2014-2017. Piatt and coauthors nicknamed this record-breaking, super-heated pool of surface water in the eastern Bering Sea, a "Blob." The Blob was a product of surficial heating that began in 2013, continued through the 2015 El Nino, and lasted through to 2016-all a product of global warming. The Blob stretched along the NW US coastline for 16oo km and exhibited anomalous sea-surface temperatures 3-6 °C above normal. According to Jessie Yeung, in September 2019, another Blob was discovered off Washington State and yet another Blob east of New Zealand some 1 million square km in size and exhibiting SST 5 °C warmer than normal.

It appears that mass mortality events triggered by climate events are on the rise and are not restricted to fish or birds! Mass die-offs of saiga antelope in Kazakhstan occurred in 1981 with 70,000 dead, again in 1988 with 200,000 dead, and again in 2015 with 200,000 dead! Postmortem analysis in 2015 concluded that Pasteurella multocida, that harmlessly infects most antelopes, erupted to kill thousands. Recent work by Kock and coauthors suggests that these mass die-offs occurred following heat waves of greater than 37 °C, with in excess of 80% humidity suggesting climate triggered a threshold event among the bacteria allowing them to become vascular and lethal.

In 2019, over 100 gray whales washed up dead along the western seaboard of the USA. This represents the largest mass die-off of gray whales recorded. Many that were found were emaciated! In one case, a whale had resorted to eating eel grass, and yet eel grass offers nothing nutritionally to whales. Several dead whales have been found hundreds of miles from traditional feeding areas and migration routes suggesting the whales were desperately searching for food. Together, these facts suggest climate change and warming sea temperatures are changing the location and supply of whale prey creatures such as amphipods, small, bottom-dwelling oceanic shrimps.

Puffins, saiga antelope, and gray whales mass die-offs can be tied to some form of environmental or biotic perturbation triggered by climate change. Other mass die-offs occur over longer time periods and hence are more difficult to establish a cause-and-effect. Nevertheless, Christmas Bird Counts, Birding Bird Surveys, Big Day Counts, Backyard Counts sponsored by the likes of the British Trust for Ornithology and the RSPB in the UK, and state DNRs, Audubon Society, and the Cornell Lab for Ornithology in the USA, suggest ramped, slow climate change is responsible for population crashes among house sparrows in the UK and evening grosbeaks in the North America. Interestingly, the house sparrow in NA appears to be expanding, yet its mother-population in Europe is struggling. The RSPB offers hints and plans for house sparrow nest boxes to its membership to help reduce their mortalities.

It is possible that the 2019/2020 bushfires in Australia have reached a tipping point. Barker and Price show that within eucalypt forests, crown fires are twice as likely after a previous crown fire than an understory fire. Similarly, understory fires are more frequent if preceded by an understory fire rather than a crown fire. These relationships mirror fires observed in NA, suggesting there are fire thresholds! According to Bowman and coauthors, the mature alpine ash forest of Australia is critically threatened by fires. Since 2002, approximately 85% of the alpine forest has been burnt, and in many areas suffered repeat burns. These repeat burns have killed 97% of all alpine ash seedlings. Analysis, in 2009 by Leonard and coauthors, of the Kilmore-Murrindindi fire complex that burnt ~250,000 ha in Victoria, Australia, suggests that the number and size of unburnt patches of forest, that act as floral and faunal forest refuges, are determined by topographic and vegetational micro-climates yet fuel load and broader climate variables can override these effects. Their work suggests that the excessive heating that Australia suffered before and during the 2019/2020 fire season will reduce the number of surviving refuges. As a consequence, across Australia, a divergent floral landscape is beginning to replace the mature forests.

In contrast, today some 3200 pairs of Dartford warbler establish territories, which barely bred on the UK mainland in the 1960s, with only 22 pairs breeding in 1966. Gibbons and Wotton suggest a long-run of mild winters since 1963, the product of global warming, lowered the climate-driven mortality threshold of Dartford warblers that exist at the outer limit of their range in the UK.

According to Nowosad and Stepinski, approximately 15% of the earth's surface between 1992 and 2015 was modified through either direct human action or climate-induced changes, much of this from natural forest to agriculture. But much of this occurred as forest fragments left after previous episodes of deforestation (identified at the scale of 9 km × 9 km or 81 km 2 blocks) were converted to agricultural land-use. Their work suggests recent deforestation activities mirror deforestation practices observed across the American Midwest in 1830 -1920s, southwestern Australia in the 1950s, and the UK in 1914 -1945 , which left less than 5% of England forested. Nowosad and Stepinski found globally that deforestation rapidly accelerated once when more than 50% of the land surface of each 81 km 2 block was deforested. Across the tropics, deforestation driven by the forest frontier phenomenon (see Climate chapter) where inverse urban-rural migration seeded by tropical forest transition policies of the 1960s-1970s in South America and income inequality (see Extinction chapter) drive the slash-and-burning of tropical forest adjacent to newly built roads, then in-fill between roads, and finally precipitate further road-extension into the forest as the land gives out. As Miller notes, once penetration into the forest occurs, subsequent forest degradation makes it comparatively easy to finish the job of forest elimination.

It seems humans despise heterogeneous landscapes of farms and scattered forest fragments! Lovejoy and Nobre support these contentions and suggest that the surviving Amazon rainforest has reached a critical tipping point that once breached will initiate an irreversible dieback process, whereby less and less rainfall will fall on the Amazon. At the moment, 75% of all rain falling on the Amazon is returned to the atmosphere above the forest through transpiration-the Amazon is hot and humid! Incredibly, the forest recycles this moisture five to six times before air masses move this moisture up the Andes. As more of the Amazon is deforested, less moisture will be recycled. According to Lovejoy and Nobre, currently 17% of the entire Amazon basin has been deforested and rainfall across the Amazon is already declining. As deforestation continues, this will lead to the development of a savannah-like landscape of scattered trees and huge expanses of grass replacing the once giant tropical rainforest. The impact of carbon sequestering will be catastrophic. This will accelerate climate change. But this is not the only issue! The drying and opening of the Amazon rainforest and population by peoples has significantly increased the fires that erupt during its moderate dry season. Although the 2019 fire season was not the worst-2005 was two-and-half times bigger in spatial extent and number of fires-Global Fire Database reports that the Amazon is burning at rates far in excess of fires observed in the 1960s, 1970s, and 1980s.

It appears that the Amazon and Congo are about to cross a catastrophic tipping point; within the next decade, according to Hubau and coauthors, both the Amazon and Congo rainforests will no longer act as a "carbon sinks." If this happens, this will greatly accelerate global climate change. Previous work suggested that the Congo was a stable, net carbon sink, but their work suggests that the Congo and other tropical African forests are in decline. They also suggest that forest uptake of carbon peaked in the 1990s and has been in steady decline since.

Today, vast amounts of chemical fertilizer are necessary to keep crop production levels at or near levels attained 20-30 years ago even in Iowa, Cambridgeshire, the Netherlands, Ukraine, paddy fields of India, or Mexico. However, with rigorous management, organic farming can attain and sometimes exceed 80% production levels of non-organic commercial farming.

Joel Simon writing on the crisis in El Campo graphically illustrated the impact of political land reform and use of fertilizer as bribes in Mexico on rural-urban, rural-international migrations, and drug cartels. Before the Spanish conquest, the mountains of Oaxaca were heavily forested and sustained a corn culture and economy. Today, the earth no longer gives! The land cannot support corn nor peoples. The mountains of Oaxaca are among the most heavily eroded landscapes on earth, 70% of the state's arable land is ruined, and many of its peoples have fled into the barrios of Mexico City, or moved on to the USA, or been absorbed into Mexico's deadly drug war.

And yet Mexico's Green Revolution that began in the 1950s promised such a different outcome. Simon found that the Green Revolution reached the lush hills and mountains of Oaxaca in the mid-1970s. Government officials donated several bags of chemical fertilizer each year to each farmer, in return for their loyalty at each election. Initially, the corn stalks grew larger and more quickly, but corn crops failed if chemical fertilizer was not applied each year. Simply put, the additional artificial fertilizer stripped the remaining natural fertility of the soils away as the corn grew bigger and faster. At this point, the farmers and their corn culture were tied inextricably to chemical fertilizer! The soil's natural fertility threshold had been exceeded. In fact, more fertilizer was needed each year to maintain corn yields. Yet, critically, the free program was canceled and instead replaced by Banrural, a government farm bank that offered credit lines for fertilizer. But each year, more fertilizer was needed, so by the late 1990s, according to Simon, 80-90% of the working-aged males in communities like San Juan Mixtepec in Oaxaca were working in the USA to pay farm debts incurred in trying to maintain their traditional corn culture. Today, labor migration from Mexico to the USA is difficult and sometimes deadly, and so more Oaxacans are left mired in debt in Mexico. ◄

HIV and African Americans in the southern United States: Sexual networks and social context

A multispecies overkill simulation of the end-Pleistocene megafaunal mass extinction

Does mammal community composition control recruitment in neotropical forests? Evidence from Panama

Positive severity feedback between consecutive fires in dry eucalypt forests of southern Australia

We're doomed': Mayer Hilland on the climate reality no one else will dare mention. The Guardian

Cold conditions in Antarctica during the little ice age-Implications for abrupt climate change mechanisms

Abrupt fire regime change may cause landscape-wide loss of mature obligate seeder forests

Empirical evidence for the effect of airline travel on inter-regional influenza spread in the United States

Landscape sensitivity and change

The artificial production of carbon dioxide and its influence on temperature

Collapse: How societies choose to fail or succeed

The loss of diversity

Response time of wetland biodiversity to road construction on adjacent lands

Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the younger Dryas cooling

The ecological road-effect zone of a Massachusetts (USA) suburban highway

The Dartford warbler in the United Kingdom in 1994

Global warming and fluvial geomorphology

Process interactions, temporal scales and the development of hillslope gully systems: Howgill fells

Asynchronous carbon sink saturation in African and Amazonian tropical forests

Forecast and control of epidemics in a globalized world

Drastic population fluctuations explain the rapid extinction of the passenger pigeon

Global warming of 1.5°C. (IPCC)

IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (IPCC). Geneva: Intergovernmental Panel on Climate Change

Sustained reworking of hydraulic mining sediment in California: GK Gilbert's sediment wave model reconsidered

Controls of number of bird species on montane islands in the Great Basin

Unusual mortality of tufted puffins (Fratercula cirrhata) in the eastern Bering Sea

Population variability and extinction in the avifauna of a tropical land bridge island

The concentration and isotopic abundances of carbon dioxide in the atmosphere

On the available evidence for the temperature dependence of soil organic carbon

Large increases in flood magnitudes in response to modest changes in climate

Saigas on the brink: Multidisciplinary analysis of the factors influencing mass mortality events

Climate tipping points-Too risky to bet against

Amazon tipping point: Last chance for action

An overview of the flora of the Sky Islands, southeastern Arizona: diversity, affinities, and insularity. Biodiversity and management of the Madrean Archipelago: the Sky Islands of southwestern United States and northwestern Mexico

Climate change and human health: Present and future risks

2016: One of the warmest two years on record

History, origin and extent of soil erosion on Easter Island (Rapa Nui)

Geographers find tipping point in deforestation

Climate and ecosystem linkages explain widespread declines in North American Atlantic salmon populations

Factors in the emergence of infectious diseases

An AL epidemiologist sent the first warning of the Wuhan virus

Stochastic, empirically-informed model of landscape dynamics and its application to deforestation scenarios

Climatic and hydrologic extremes in early Chinese history: Possible causes and dates

Airborne Nitrogen Dioxide plummets over China

The origin of human pathogens: Evaluating the role of agriculture and domestic animals in the evolution of human disease

Phillipps' field guide to the mammals of Borneo and their ecology

Changes, perturbations, and responses in geomorphic systems

Principles of geomorphic disturbance and recovery in response to storms

Extreme mortality and reproductive failure of common murres resulting from the Northeast Pacific marine heatwave of

The song of the dodo. Island Biogeography in an Age of Extinction

The chimp and the river

Permafrost dynamics in the 20th and 21st centuries along the east Siberian transect

Geomorphic thresholds and complex response of drainage systems

Endangered Mexico: An environment on the edge

Alpine proglacial suspended sediment dynamics in warm and cool ablation seasons: Implications for global warming

Human influence on California fire regimes

Air travel and the spread of influenza: Important caveats

Massive crater under Greenland's ice points to climate-altering impact in the time of humans

Chapter 4 · Thresholds

Terraces of Douglas Creek, northwestern Colorado: An example of episodic erosion

Direct evidence of extensive diversity of HIV-1 in Kinshasa by 1960

A blob of hot water in the Pacific Ocean killed a million seabirds, scientists say. CNN update