key: cord-0956677-gcxyrr00 authors: Adhikari, Arpita; Sengupta, Joydip; Hussain, Chaudhery Mustansar title: Declining Carbon Emission/Concentration during COVID-19: A critical review on temporary relief date: 2021-11-14 journal: Carbon Trends DOI: 10.1016/j.cartre.2021.100131 sha: d9f2eddcc5989d8964aa1f7aabb252a69b7e023a doc_id: 956677 cord_uid: gcxyrr00 In December 2019 the deadly pandemic COVID-19 traumatized mankind through its lethal impact. To seize the outbreak, nationwide/region-based lockdown strategies were adopted by most of the COVID-19 affected countries. This in turn resulted in restricted transportation via surface, water, and air, as well as significantly reduced working hours of the industry sectors, so on and so forth. The obvious outcome was a sudden discernible decline in atmospheric adulteration. Accordingly, the anthropogenic emissions at the global and regional/local scales were examined during the lockdown period by several researchers using both or either satellite-based and ground-based monitoring. Among several other air-contaminants, carbon has a dominant toxicological profile causing adverse health effects and thereby attracting researches interest in carbon-release probing during the systematic confinement period imposed by the ruling authorities across the globe. The results of those studies indicated a confirmed decline in carbon emission/concentration making the air more breathable for the period. In this review, the studies related to anthropogenic emissions of carbon during the lockdown period are accounted for by compiling the recently reported data from published articles. Carbon emission had been perhaps pre-historically started since the discovery of fire marking the birth of civilization. An accumulation of the ever-increasing rate of carbon emission/concentrations with the tremendous collective energy demands has led mankind to be affected by its detrimental effect on the environment. Carbon emission/concentration is often stated as one of the main causes of global warming 1 and also responsible for causing human health hazards affecting cardiovascular, neurological, respiratory complications even leading to death 2 . Amongst different emission sources, the transportation sector has been identified as the major contributor on top of the industry sector in a recent report 3 based on data of the United States (Fig 1) . The current COVID-19 epidemic started at Wuhan, China 4 in 2019 and spread rapidly across the globe affecting 220 countries 5 . To break the transmission chain, the lockdown was imposed in most of the affected countries around the world, which in turn restricts the vehicle movement 6, 7 and forces most of the industries to remain shut down for the specified period 8, 9 . Consequently, the carbon emission originated from vehicles, and the industry sector experienced a sudden reduction. Such occasional depletion in carbon emission/concentration has been reported by several researchers based on numerous studies across the globe. However, to get the complete perception of carbon emission/concentration reduction after the outbreak of the COVID-19 pandemic, the published reports are reviewed here. For the sake of categorical analysis of the reported literature, the review has been performed based on the studies over land and oceanic subdivisions. The published results from the lands are further stratified according to the coverage area as well as the major countries of the globe. On the account of the encompassed area over the land region utilized for the study to estimate the carbon emission/concentration, the comprehensive assessment is further sub-divided as cumulative studies, i.e. collecting sample data all over the world, and then as regional studies i.e. collecting sample data from a specific country/region. Initially, the cumulative studies will be discussed and then the regional ones. In assessing the trend of carbon emission/concentration during the COVID-19 outbreak, several researchers around the earth followed extensive research over the substantial landmass covering several countries of different continents. Liu et al. 10 12 also conducted a similar study and reported that daily global CO 2 emissions diminished by 17% by early April 2020 compared to the mean 2019 levels (Fig 2) . Sikarwar et al. 13 assessed the global carbon emission considering US, EU-28, China, and India. They found that enforced lockdown caused a temporary reduction in anthropogenic CO 2 emission by 14%, mainly due to a concurring decrease in surface and air traffic. During the COVID-19 pandemic, the power requirements of the industry sector also declined, which in turn influenced the usual carbon emission. Bertram et al. 14 found that the COVID-19-induced economic downturn and the corresponding reduction of electricity demand along with the decrease in coal production led to a notable drop of 6.8% in CO 2 emissions across the global power sectors. Evangeliou et al 15 examined the change in black carbon (BC) emission over Europe utilizing in-situ observations from 17 European stations in a Bayesian inversion framework. They measured the BC emission during lockdowns and compared the data to the same period in the previous 5 years. They found that BC emissions declined by 23 kt with an average of 11% across Europe. Impacts on global climate due to the pandemic were estimated by Forster et al. 16 using data of 123 countries for the period January to June 2020. They also found a sharp fall in CO 2 emissions during the lockdown span. The cumulative studies comprehensively indicate confirmed reduction in carbon emission/concentration during lockdown periods across the globe. The countries around the world differ widely in terms of annual carbon emission/concentrations. According to the report 17 , China, United States, and India are the top three emitters of CO 2 . Moreover, the industries and the transportation sectors are found to be the major contributors (> 80%) of annual CO 2 emissions. Consequently, the effect of the enforced shutdown, to combat the COVID-19 pandemic scenario, on carbon emission/concentration are expected to be more significant in the top three countries as mentioned before. Thus the authors selected this under the purview of the present review. Owing to the rapid economic development of the world's most populous country, China can be anticipated to experience a substantial decline in carbon emission/concentration during the shutdown session. Zhang et al. 18 examined the effect of the COVID-19 pandemic on China's transportation sector in terms of CO 2 emissions (Fig 3) . They reported that the COVID-19 had a greater impact on transportation energy consumption and CO 2 emissions than SARS. Liu et al. 19 also estimated the impact of the COVID-19 outbreak on the CO 2 emission of China. They found that the reduction in CO 2 emission in the first four months of 2020 is 6.9% compared to the same period in 2019. However, in April 2020, the CO 2 emission becomes comparable to the same span of the previous year indicating rapid recovery of China's national economy. Wang et al. 20 Zhou et al. 25 studied the change in carbon monoxide (CO) concentration in 1375 sites of China during lockdown employing surface-based and satellite-based observations. They found that the average CO surface concentration was reduced by 18.7% with a spatial variation of 8-27%. Zhang et al. 26 monitored the air quality in China during the COVID-19 pandemic covering 367 cities from 23 January 2020 to April 22, 2020. Their study revealed that CO concentration dropped by 30% due to the adopted traffic restriction to cease the spread of viral transmission. Xu et al. 27 investigated the quality of air of central China during the pandemic from January to March 2017-2020. They observed a 27.9% decrease in CO concentration in three cities, namely, Wuhan, Jingmen, and Enshi. Xu et al. 28 studied the BC concentration in the megacity Hangzhou, China, and found a 44% reduction. Bai et al. 29 examined the air quality during the lockdown in Shanghai Municipality, China in the period 25 November 2019 to 22 May 2020. They found a steady reduction in CO concentration in the city during the lockdown period. Spatiotemporal variations of air pollutants were studied by Yuan et al. 30 from January to March 2020 at megacity Hangzhou, China. Their result exhibited that CO concentration was reduced by 30% in comparison to the normal period. The United States ranks second in annual carbon emission and accordingly attracts researchers' attention towards the probable decline in carbon emission/concentration during the period of Government imposed shut down to prevent the pandemic from spreading. Carbon emissions from different sectors such as commercial, transport, industrial, residential and, electric power sector during the outbreak of COVID-19 had been reported by Aloal et al. 31 . The study revealed that carbon emission was reduced in most of the sectors during the lockdown. The consistently declining trend of CO concentration was reported from 28 long-term air quality stations across the U.S. from March 15 to April 25, 2020, by Chen et al. 32 . Elshorbany et al. 33 used the remote sensing method to evaluate the concentration of CO in New York, Illinois, Florida, Texas, and California. Their study concluded that CO concentration reduced in most of the places. They also reported that in the presence of other sources of carbon emission excluding vehicles, the trend of reduction in carbon concentration may vary from place to place. Hudda et al. 34 studied the concentration of BC from vehicles in an urban area of the USA where vehicles are the main source of air pollution (Fig 4) . They carried out the study in a mixed commercial-residential neighbourhood in Somerville between March 27 and May 14, 2020. The study revealed that BC concentration was reduced by 22-46% based on the type of roads. India is the second-highest populated country in the world. Being a developing country, the nation has a major emerging economy, the impact of which led to a concerning increase in annual carbon emission and thereby contributing to the climate crisis. Several studies have been reported in recent days to account for the effect of COVID-19 related shutdown on the variability of greenhouse gases, particularly, on carbon emission/concentration. Along with the pan-India-based investigation on the effect of lockdown on carbon emission, several local/city-based studies are also reported. Ambade et al. 35 had followed such a study at Jamshedpur city, India before (January 3 rd to March 23 rd, 2020) and after lockdown (April 1 st to June 14 th , 2020) monitoring the reduction in BC during the lockdown. Their result depicted nearly 80% reductions in BC emissions because of diminished fuel consumption and a sufficient decrease in other emission sources. In another study by the same group 36 , it was revealed that the BC concentration started to rise again as the restrictions were withdrawn during the unlock phases. Ajay et al. 37 studied BC emission from 2015 to May 2020 at a rural location Challakere, located 230 km northwest of Bengaluru, India. They found that the lockdown has a very marginal impact on BC emission at Challakere because the anthropogenic emission from vehicles or industry is very minimal at such a remote site. The same group 38 also studied the trends in carbon emissions in the megacity of Bengaluru, India during the span from 01 January to 25 May 2020. The results depicted a 60% reduction in BC emission due to lockdown. The two studies performed by this group indicated that the reduction in carbon emission is more significant in the urban/city area where anthropogenic emissions are mostly related to the transportation and industry sectors. The BC concentration over Delhi, India from 18 th February to 31 st July, 2020 (Fig 5) was monitored by Goel et al. 39 . Their measurements unfolded a constant reduction in BC concentration during Government adopted restriction phases with a maximum recorded reduction of 78%. Sharma et al. 40 There are some reports from other countries regarding the studies related to carbon emission/concentration during the COVID-19 pandemic and their subsequent effects. Venturi et al. 47 measures the effect of the outbreak of COVID-19 on the level of CO 2 in the urban region of Italy named Florence. Using the eddy covariance technique they measured atmospheric CO 2 concentrations before, during, and after the national lockdown period. A detailed analysis of the obtained data exhibited that CO 2 concentrations decreased 62% during the lockdown phase while enhancement of the same was observed with the resumption of traffic. Numerous environmental aspects were studied by Ali et al. 48 across Pakistan and measured their change during the COVID-19 pandemic using satellite data. They compared the concentration of atmospheric CO during the lockdown in 2020 with the same period in 2019 and found that atmospheric CO concentration significantly decreased over all the megacities. Kanniah et al. 49 studied the effect of COVID-19 at several stations across Malaysia on different atmospheric parameters including CO concentration. They found that in the urban and suburban regions the reduction in CO concentration is much higher than in rural sites. The effect of COVID-19 lockdown on the air pollution level in Korea was examined by Ju et al. 50 and found that the concentration of CO was decreased by 17.33% due to a reduction in domestic sources. There exist very few studies conducted over the sea region regarding the effect of COVID-19 outbreak-stimulated lockdown phases on carbon emission/concentration. Grados et al. 51 illustrated such an estimation of carbon emission over the sea region of Spain (Strait of Gibraltar) during the COVID-19 pandemic situation. Their study revealed that CO 2 emission decreased by a minimum of 10% in comparison to the previous records. However, Al Shehhi et al. 52 reported the effect of COVID-19 in the sea region globally. Their data evidenced a reduction of 7% in CO 2 emissions. Despite the scarcity of over-the-sea observations reported during the imposed restrictions of the COVID-19 pandemic, the fact-findings are ample enough to discernibly portray the distinguishable characteristics of carbon emission over land and sea. The COVID-19 pandemic around the world threatened the social pace of human life, unlike ever before. Moreover, the viral outbreak compelled mankind to encounter unprecedented situations like the complete shutdown of all kinds of social activities at different corners of the earth. The adopted restrictions to break the chain of transmission of the disease stirred up the escalating fear of livelihood loss and economic downturn due to the sudden and repeated approaches of close-down of all sorts of transportation as well as industry sectors in the strictest means. Despite the darkest impacts of the pandemic, such systematic curtailment turned out to be a blessing to the environment by causing a rapid dip in the ever-increasing trend of carbon emission/concentration. Several studies were conducted across the globe on carbon emission/concentration during the lockdown period to estimate the effect of lockdown (Table 1) . The reports over the land and sea regions are reviewed and the findings depicted a clear picture of the steady decline of carbon emission/concentration around the world during the lockdown period. However, the extent of decrease is utterly different over land and sea. In contrast to the over-the-land reports, the rate of reduction is significantly lower over the sea due to the obvious reason of the limited provision of transportation through the sea and the carbon-absorbing phenomenon of the water bodies. Under these circumstances, the only possible source of carbon over the sea region can be the transported portion of it from the nearby land location. Moreover, the change in carbon emission/concentration is more significant in city/urban areas compared to the rural/remote region as both the vehicles and industries are dominant factors for the former one. It was also noticed that initially, the carbon emission/concentration was decreased with the immediate imposition of lockdown, but carbon emission/concentration gradually increases in the unlock phases as the people started to return to normal life. Thus the declining carbon emission/concentration during COVID-19 extends only temporary relief. Though the carbon emission/concentration reduced because of some unprecedented situations, however, it can be inferred that some specific strategies (e.g. work from home, bicycle to work, etc.) can be adopted by the concerned authorities to achieve reduced carbon emission/concentration and provide breathable air to the world. 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