Under usual daily life, the human footprint on the Earth System is vast. As a result, a very large perturbation is required to cause an observable difference from this ‘business-as-usual’ baseline: COVID-19 is providing that perturbation. As of July 2020, as much as half the world’s population has been under some version of sheltering orders7 (Fig. 2a). These orders have substantially reduced human mobility and economic activity (Fig. 2b), with ~70% of the global workforce living in countries that have required closures for all non-essential workplaces and ~90% living in countries with at least some required workplace closures8.
a | The Oxford Government Response Stringency Index7 on six different dates between 1 February and 1 June. b | Percentage of people staying at home, as estimated by mobility data from cell phones91, for five US states. c | Percentage change in carbon dioxide emissions13,92 for the World, China, the USA and Europe. Each day’s value is the percentage departure in 2020 from the respective day-of-year emissions in 2019, accounting for seasonality. d | Percentage change in cumulative carbon dioxide emissions12,93 for January through April 2020 compared with January through April 2019 for the World, China, the USA and Europe. The differences in timing of sheltering and mobility in different areas of the world are a source of information that can be used in understanding causality in the Earth System response. In the case of carbon dioxide emissions, the early onset and subsequent relaxation of sheltering in China is clearly reflected in the timing of reduction and subsequent recovery of emissions in China relative to the USA and Europe.
The scale of this socioeconomic disruption is likely to be detected in the Earth System at local to global scales (Fig. 1). Some responses are direct, while others will result from interactions between humans, ecosystems and climate. The impacts of the socioeconomic disruption are, thus, also likely to vary across timescales: although the direct impacts of the reduction in human mobility will be strongest during the sheltering period, many of the most lasting impacts could result from cascading effects initiated by the economic recession, some of which (such as those induced by changes in public policy, the structure of the economy and/or human behaviour) could persist for decades following the initial economic recovery.
The reduction of human activities, and the efforts to manage their revival, have varied around the world (Fig. 2). Given the variations in the timing, strength and approach to sheltering7, it may be possible to track effects through the components of the Earth System. Likewise, because the large-scale reduction in human activity will necessarily be temporary, it will be possible to observe whether or how Earth System processes return to their previous states after activity returns to something approaching pre-pandemic levels. The event, therefore, provides a unique test bed for probing hypotheses about Earth System sensitivities, feedbacks, boundaries and cascades6,9,10,11, presuming that the observing systems are in place to capture these responses (Box 1).
Path I: Energy, emissions, climate and air quality
Impacts on energy consumption, and associated emissions of greenhouse gases and air pollutants, are likely to cascade across timescales (Fig. 1). In the near-term, reductions in mobility and economic activity have reduced energy use in the commercial, industrial and transportation sectors, and might have increased energy use in the residential sector12,13. These direct impacts will interact with secondary influences from energy markets, such as the severe short-term drop in oil prices in March and April 2020 (ref.14). Further, as with past economic recessions15,16, energy demands — and the mix of energy sources — are likely to evolve over the course of the economic recovery in response to market forces, public preferences and policy interventions17,18. This evolution could have long-term effects on the trajectory of decarbonization if, for example, the economic disruption delays the implementation of ambitious climate policy or results in decreased investments in low-carbon energy systems16. Alternatively, large government stimulus spending could target green investments that overhaul outdated infrastructure and accelerate decarbonization18.
Misunderstandings have arisen with regards to declines in carbon dioxide emissions caused by COVID-19-related disruption, with some interpreting short-term reductions to suggest that austerity of energy consumption could be sufficient to curb the pace of global warming. A reduction in fossil CO2 emissions proportional to the economic decline15 would be dramatic relative to previous declines. For example, the decline in daily CO2 emissions peaked at >20% in the largest economies during the period of sheltering13 (Fig. 2c) and the cumulative reduction in global emissions was ~7% from January through April 2020 (ref.12) (Fig. 2d). However, these daily-scale declines are temporary13 and the rebound in emissions that is already evident13,19 (Fig. 2c) supports the likelihood of a reduction in annual emissions that is smaller than 7%.
Nevertheless, a 5% drop in annual fossil CO2 emissions from 37 billion metric tonnes per year20 would exceed any decline since the end of World War II (ref.13). There is a strong basis that such a reduced atmospheric CO2 growth rate would lead to a reduced ocean carbon sink21 and, thus, also a temporary reduction in the rate of ocean acidification. On the other hand, a 5% decrease would still leave annual 2020 emissions at ~35 billion metric tonnes, comparable to emissions in 2013 (ref.20). Such a decline — and associated changes in the ocean and land carbon sinks — might not be statistically detectable above the year-to-year variations in the natural carbon cycle and, regardless, global atmospheric CO2 concentrations will inevitably rise in 2020, continuing a long-term trend. Progress in understanding the carbon-cycle responses to COVID-19 will, therefore, be challenging and, at a minimum, will require new methods for tracking the unprecedented short-term perturbation in emissions through the Earth System.
Based on past events and fundamental understanding, there are a number of hypotheses of how sheltering-induced changes in atmospheric emissions could influence the climate system more broadly (Fig. 1). On short timescales, reduced air travel decreases the abundance of contrails, which can be detected in the radiation budget (as occurred during the brief cessation of air travel following the 11 September attacks5). The response of atmospheric aerosols to sheltering is likely to vary regionally, with changes in emissions, meteorology and atmospheric chemistry influencing the outcome (Box 2). While reductions in aerosols have occurred in many locations (Fig. 3), they have also been observed to increase in others22, highlighting the important role of secondary chemistry in these assessments. Changes in atmospheric aerosols could further influence cloud and precipitation processes23,24, and might be detectable in the local surface energy budget25. A reduction in scattering aerosols will also cause warmer surface temperatures over emitting regions26 (Fig. 4), potentially manifesting as more frequent and/or intense heatwaves27,28. If aerosol reductions persist across the Northern Hemisphere, this could have short-term impacts on the onset, intensity and/or intraseasonal variability of monsoon rainfall29,30,31, particularly given that both local and remote aerosol emissions can influence variability within the monsoon season31.
Difference in tropospheric NO2 column density (panel a) and aerosol optical depth (panel b) for select months between 2020 and 2019. Aerosol optical depth (AOD) data are from the NASA Visible Infrared Imaging Radiometer Suite; NO2 data are from the NASA Ozone Monitoring Instrument, processed as in ref.94. Year-to-year changes in air quality reflect a complex array of processes in addition to COVID-19 restrictions. For example, strong NO2 decreases over Northeast China coincide with the Wuhan lockdown95, while those over the UK in January–Febuary predate COVID-19 restrictions. Relative to NO2, AOD data show less regional coherency. Confident attribution to COVID-19 restrictions highlights a new challenge to explain these observed spatio-temporal differences and to place them in the context of the longer-term satellite and ground-based observations (Box 2).
NO2 (panel a), SO2 (panel b), PM2.5 (panels c and d) and surface-temperature (panels e and f) changes for the month of January simulated by the Community Multiscale Air Quality/Weather Research and Forecasting (CMAQ-WRF) model in response to domain-wide removal of traffic (left panels) or power-plant (right panels) emissions. Experiments simulate one month using January 2010 emission factors and January 2013 meteorological fields. They are, thus, idealized illustrations of the potential for Earth System models to pose hypotheses, illuminate and constrain key processes, and identify data-gathering priorities; as these simulations predate the COVID-19 pandemic, they should not be considered an attempt to recreate COVID-19 conditions.
On longer timescales, changes in the energy intensity of the economy, the carbon intensity of energy or the pace of deforestation could affect the long-term trajectory of global climate (through the trajectory of greenhouse gas emissions and associated land and ocean carbon-cycle feedbacks). These effects could go in either direction: for example, in the US electricity sector, coal plants will likely shut down at an accelerated pace as a result of the economic slowdown, continuing a long-term decline32. However, in the transportation sector, policy intervention to stimulate the economy might loosen emissions standards33, increasing emissions relative to the pre-pandemic trajectory.
The short-term reductions in pollutant emissions have already resulted in noticeable changes in air quality in some regions (Box 2). If sustained, improved air quality could yield multiple benefits. These include improved crop health34, as air pollution can reduce regional harvests by as much as 30% (ref.35). In addition, ambient air pollution is a significant cause of premature death and disease worldwide36, even from short-term exposure37,38. Several well-documented historical examples illustrate how decreased ambient air pollution can improve human health39. These include effects from short-term reductions in traffic, travel and/or industrial activities associated with events such as the 1996 Atlanta Olympic Games40 and 2008 Beijing Olympics41,42,43,44,45. While associations between air quality and health outcomes are hypothesized in studies of the current pandemic46,47, understanding the role of air quality as an indicator for the epidemic trajectory is an emerging challenge. Further, any health improvements resulting from improved air quality during the pandemic should not be viewed as a ‘benefit’ of the pandemic but, rather, as an accidental side effect of the sheltering that was imposed to protect public health from the virus.
Some of the most lasting impacts of the COVID-19 crisis on climate and air quality could occur via insights into the calculation of critical policy parameters. Two of the most important, and controversial, are the value of mortality risk reduction (sometimes termed the value of a statistical life, or VSL) and the pure rate of time preference (or PRTP), which is one component of the social discount rate and measures willingness to trade off well-being over time. The VSL is important to the analysis of all environmental regulation in the United States and can determine whether environmental regulations as mundane as a labelling requirement for toxic chemicals will pass a cost–benefit test. The PRTP is important in evaluating long-term societal trade-offs — most notably, climate-change regulation — and can be important in calculating an economic value of avoiding climate damages48,49. With a higher PRTP, aggressive mitigation of greenhouse gases becomes less attractive, while a low rate, which places relatively higher value on the well-being of future generations, suggests that far more aggressive regulation of today’s emissions is warranted.
Both the VSL and the PRTP can be difficult to quantify. However, the COVID-19 crisis is making these trade-offs more explicit, as governments, communities and individuals make historic decisions that reflect underlying preferences for current and future consumption and the trade-off between different types of economic activity and individual and collective risk. The diverse responses to the unusual conditions during the pandemic could reveal far more about how different societies manage these trade-offs than has been revealed in the last half-century. As those insights are incorporated into the formal policy-making apparatus, they will have lasting effects on the regulations that impact the long-term trajectory of climate and air quality.
Path II: Poverty, globalization, food and biodiversity
By amplifying underlying inequities in the distribution of resources, the socioeconomic disruption caused by the response to COVID-19 will almost certainly have negative long-term impacts on human health and well-being. In particular, the economic shock is likely to increase the extent and severity of global poverty50, both from direct impacts on health, employment and incomes and through disruptions of supply chains and global trade51. The severe impacts on poverty rates and food security that are already emerging50 are indicative of these disruptions and are a sign of how tightly many of the world’s poorest households are now interwoven into the global economy. The unwinding of these relationships in the wake of restrictions on human mobility and associated economic shocks will provide insight into the role of economic integration in supporting livelihoods around the world. A severe and prolonged deepening of global poverty is also likely to reduce available resources for climate mitigation and adaptation, increasing climate risks and exacerbating climate-related inequities.
The global agriculture sector is a key sentinel for the response of poverty to the pandemic. Primary near-term questions centre around how food security and agriculture-dependent incomes might be affected by unprecedented shocks to local labour supply and global supply chains. A first-order impact has been the income shock associated with widespread sheltering8. Loss of wages in both low-income and high-income countries with limited social safety-nets will drive food insecurity and poverty50.
It is possible that agricultural production in rural areas will proceed largely unaffected, particularly for larger producers of field crops that tend to be heavily mechanized. However, in many locations and for many specialty crops, agriculture still relies heavily on field labour; sufficient labour supply during the key planting and harvest periods is crucial, and there are frequently labour shortages at these critical times. How these pre-existing labour-supply challenges are affected by the scale and scope of sheltering remains to be seen. In the USA, meat-packing plants have become hotbeds of COVID-19, raising the question of whether excessive concentration of this industry might have led to a loss of resilience52. Sheltering-induced return migration from urban to rural areas, as has been widely reported in India, could alleviate agricultural labour shortages in some developing countries. However, mandated sheltering could cause reductions in plantings, which, in combination with the prospect of sheltering during the harvest season, could reduce subsequent harvests.
Such supply-side shocks could combine with general disruption of global trade53 to trigger a cascading series of export bans like those that occurred in 2007–2008 (ref.54), which caused a spike in grain prices and contributed to unrest around the world55. Initial export restrictions are already emerging56. Given that agriculture prices are important for both consumers and producers, such bans tend to hurt rural producers in favour of protecting urban consumers in the exporting countries57. They can also lead to food shortages in import-dependent countries and rapid increases in international commodity prices58, as well as acting to amplify the impacts of climate variability on poverty59. However, global grain stocks are much larger today than they were in 2007, which should help buffer some sheltering-related production shortfalls, should they arise.
Deepening of global poverty is likely to have lasting negative environmental impacts (including deforestation, land degradation, poaching, overfishing and loosening of existing environmental policies), as a larger share of the global population is pushed towards subsistence. For example, after decades of efforts to replace environmental degradation with earnings from ecotourism, the collapse of tourism in the wake of COVID-19 is coinciding with a rapid increase in illegal poaching in southern African parks60. The rapid response is a potential indicator of the importance of the large African tourism industry for the preservation of endangered species. However, further analysis is needed to distinguish the contributions of income and governance/enforcement. Likewise, deforestation in the Brazilian Amazon surged to >2,000 km2 in the first five months of 2020, an increase of ~35% compared to the same period in 2019 (ref.61). Governance appears to be playing a key role in this initial short-term resurgence during the COVID-19 sheltering. Over the longer term, historical drivers62,63 suggest that a prolonged poverty shock is likely to increase deforestation and biodiversity loss. These cascading impacts on ecosystems and biodiversity offer a sobering contrast to the reports of wildlife ‘rebounds’ occurring in response to local sheltering64.
Changes in human behaviour and decision-making induced by the pandemic are also likely to cascade through the globalized Earth System over the long term. For example, although sheltering orders are reducing personal vehicle use, the long-term impacts are less clear and will be determined, in part, by how human behaviours respond to the pandemic. If, for instance, the pandemic causes people to feel more dependent on cars as ‘safe places’, that dependence could act to further reinforce the prominence of the automobile at the expense of public transit. On the other hand, some cities might seek to maintain reductions in traffic by permanently closing some streets and encouraging residents to rely more on walking and bicycles. Another potentially consequential outcome could be a change in the kind of housing and work environments people will prefer in the future. The pandemic favours access to outdoor space and disfavours use of tall buildings with elevators. If these human preferences are sustained for years after the pandemic passes, over the long term, the combination could lead to more sprawling suburbs and fewer residential and office towers, with corresponding consequences for the Earth System.
More broadly, priorities and incentives embedded in government aid and economic stimulus will influence financial investment. For example, rollbacks of environmental restrictions by governments seeking to accelerate economic recovery33 (including fuel standards, mercury, clean water, and oil and gas production on federal lands) could have consequences that outlast the pandemic. Alternatively, efforts to support economic recovery could be directed towards electrification of transportation, along with green jobs that rebuild public transit, housing and critical infrastructure in an environmentally sensitive way18. In the private sector, pandemic-induced changes in perceptions of economic security and human needs could increase investment in technologies or platforms that lower the risk of future pandemics, such as reducing human interactions by introducing more robotics into workplaces. Although the precise trajectory is unknown, the long-term impacts of the pandemic on resource demand and efficiency will be heavily influenced by the response of human behaviour and decision-making, which is likely to vary among and within countries, as has occurred with health practices and policies during the pandemic.
Source: Ecology - nature.com