Humanity is going through a health and a related economic crisis due to COVID-19. The impacts of the measures taken by governments are far and wide. The restrictions imposed on population mobility and commercial activities have resulted in changes in anthropogenic emissions and in the atmospheric chemical composition. These changes were especially pronounced in urban areas and observable in air pollutants as well as in greenhouse gases.
The potential connections between air pollution, the virus and the disease are of great interest to the WMO Global Atmosphere Watch (GAW), as well as the opportunity to observe an unprecedented, transient and complex change in anthropogenic emissions in most parts of the world. The GAW community has initiated studies on the impacts of the crisis on atmospheric composition on the global, regional, national and urban scales. The global and regional studies are based on the global network of satellite observations and numerical modelling and data assimilation, while smaller-scale studies are largely driven by direct analysis of in situ observations.
Carbon dioxide (CO₂) is a long-lived greenhouse gas that accumulates in the atmosphere. When CO₂ sources and sinks are in net balance, concentrations of CO₂ will have a small variability. That was the case over the 14 000 years that preceded the industrial era, which started around 1750 AD. Emissions from burning fossil fuels and changing land uses have led to an increase in CO₂ in the atmosphere from the pre-industrial level of 280 parts per million (ppm) to current levels that are over 410 ppm (this means 410 CO₂ molecules per million of air molecules or 0.041% of all air molecules).
The latest analysis of observations from GAW, supported by the Scientific Advisory Group on Greenhouse Gases and the World Data Centre on Greenhouse Gases, shows that globally averaged surface mole fractions (the measure of concentration) calculated from this in-situ network for CO₂, methane (CH₄) and nitrous oxide (N₂O) reached new highs in 2019, with CO₂ at 410.5±0.2 ppm, CH₄ at 1877±2 parts per billion (ppb) and N₂O at 332.0±0.1 ppb. These values constitute increases over the pre-industrial level of 148%, 260% and 123% respectively [WMO2019]. Concentrations of these main greenhouse gases continued to rise in 2019 and 2020. Global average figures for 2019 will not be available until late 2020, but data from all global locations, including flagship observatories – GAW Global stations Mauna Loa (Hawaii) and Cape Grim (Tasmania) – indicate that levels of CO₂, CH₄ and N₂O continued to increase in 2020 (see Figure 1). More information on greenhouse gas trends are available in the WMO Greenhouse Gas Bulletin and the United in Science report [WMO2020].
|Figure 1: Monthly mean CO₂ mole fraction in ppm at (left) at Cape Grim observatory and (right) at Mauna Loa observatory. The dashed red line represents the monthly mean values, centred on the middle of each month. The black line represents the same, after correction for the average seasonal cycle. Sources: capegrim.csiro.au/ and www.esrl.noaa.gov/gmd/ccgg/trends/mlo.html|
Despite efforts to reduce per capita emissions as agreed in the Kyoto Protocol and the 2015 Paris Climate Agreement, emissions of CO₂ have increased globally year-by-year to about 1% in the last decade [GCP2019]. The additional CO₂ emissions have resulted in an increase in atmospheric CO₂ that is between 2 and 3 ppm per year [WMO2019] in that period. This variability of about 1 ppm in the atmospheric growth rate is almost entirely due to variability in the uptake of CO₂ by ecosystems and oceans. These two sinks together take up roughly 50% of human emissions [GCP2019].
|Figure 2: Average daily emissions from 5 February to 6 May 2020 (red area) and average of the previous years during the same period (grey area). The dark-orange horizontal bars cover the periods of official lockdown while the light-orange bars indicate periods of partial lockdown or general restrictions (e.g. schools closed, personal contact reductions, mobility constraints). Source: ICOS2020b|
The Global Carbon Project [GCP2020] has analysed the reduction of economic activities due to the COVID-19 lockdown in the major economies of the world. They estimated that, during the draconian period of the restrictions, daily emissions may have reduced by up to 17% globally. As the duration and severity of lockdown measures remain unclear, prediction of the total annual emission reduction over 2020 is very uncertain. The GCP estimates this annual reduction at between 4.2% and 7.5%. This is the kind of emission reduction rate needed from year-to-year in the coming 30 years to reach the Paris Agreement target of limiting climate warming to 1.5 °C. This implies that the annual global increase in CO₂ (typically 2 to 3 ppm) will shrink by 4.2%-7.5% (that is by 0.08 to 0.23 ppm and transient up to a factor of two higher), well within the 1 ppm natural interannual variability. A similar conclusion was drawn by CarbonBrief [CB2020] and the Integrated Carbon Observation System [ICOS2020a].
The global atmospheric CO₂ signal is the integration of all-natural and anthropogenic fluxes into and out of the atmosphere that have been well mixed by turbulent mixing and atmospheric transport. The GAW global network of surface stations can resolve global changes of atmospheric CO₂ over a year within 0.1 ppm of precision. Satellite observations cannot yet reach this precision for the global mean. When in situ measurements are made closer to particular sources and sinks, individual signals can be stronger but are also entangled, and in most cases the natural signal shows the highest variability with strong diurnal and seasonal variations, while fossil fuel emissions are relatively consistent. This makes it hard to detect changes on the order of 10% to 20% on timescales of a year or less. In several cities and regions around the world, measurements are now being made of the radioactive isotope Carbon-14 in CO₂ to enable the separation of fossil fuel sources of CO₂ from ecosystem sources and sinks regardless of how variable the latter are. However, these Carbon-14 measurements are rare and it takes a lot of time to analyse discrete samples in the lab. Most high precision CO₂ measurements are performed by continuously measuring in situ instruments in networks that are designed to receive the integrated signal of all sources and sinks.
To determine changes in the fossil fuel signal when there is high natural CO₂ variability requires long time series to generate robust statistics and complex data modelling using data assimilation techniques. Emissions changes of the order of 10% to 20% are hard to quantify with certainty unless one measures within about 10 km of the fossil fuel emission sources. An example of significant changes in emissions that can be measured directly within cities (such as proposed in the WMO IG³IS program framework) is shown by ICOS [ICOS2020b] where reductions in emissions of up to 75% were measured in the city centres of Basel, Florence, Helsinki, Heraklion, London and Pesaro, using so called eddy covariance techniques that directly measure vertical exchange fluxes within a circumference of several kilometres from the measurement point (Figure 2).
Analysis of the available data demonstrates that a reduction of emissions in the order of 4% to 7% globally does not mean that CO₂ in the atmosphere will go down. In fact, CO₂ will continue to accumulate in the atmosphere and concentrations will continue to rise with just a slight decrease in the rate of this increase. Discerning the change will be difficult because of the superimposed and larger natural variability.
Only when the net emission of CO₂ comes close to zero, can one expect the net uptake by ecosystems and ocean to start to slightly reduce CO₂ levels in the atmosphere. But even then, most of the CO₂ already added to the atmosphere will remain there for several centuries and take part in the warming of our climate.
|Figure 3: Mean tropospheric NO2 density, comparison between January and February 2020. Source: NASA2020|
The reduction in economic activity and population mobility has contributed to localized improvements in air quality. The lifetime of air pollutants in the atmosphere is shorter than for greenhouse gases. Therefore, impacts of emission reductions on air pollutants are more localized and can be seen much faster in atmospheric concentrations. Reduced levels of nitrogen dioxide (NO₂) have been observed from satellites during the lockdown in many parts of the world, including for instance China (Figure 4; NASA2020) and Italy (Figure 3, CAMS2020). Yet, air quality is partly determined by emissions and partly by changes in the weather. While stagnation leads to an accumulation of pollutants near to sources, wind, vertical mixing and rain contribute to their dispersion. To disentangle the effects of weather from those of reduced emissions, detailed analyses are required. In some parts of Europe, the detection of a statistically robust trend is yet more challenging, as shown below for some of the capitals in Northwestern Europe. Weather-related episodes of high (between weeks 3 and 4; week 6) and low NO₂ surface concentrations are the main features that can be seen (Figure 5; CAMS2020). Several methods have been developed to try to delineate the effects of weather and emissions changes by estimating what would have been the spring of 2020 under “business as usual” conditions using, in particular, Machine Learning techniques (see [Barré2020]).
Many scientists are investigating the impacts of COVID-19 lockdown measures on air quality as well as the impact of air pollution levels and other environmental factors on the outcome and spread of the disease. A survey among the GAW community collected 86 responses, the majority of which addresses the impact of lockdown measures on pollution and greenhouse gas levels. A dedicated urban scale study is led by the GURME Scientific Advisory Group. This activity is also supported by work on anthropogenic emissions themselves and on the changes that can be deduced from publicly available activity data such as transport/mobility or energy statistics (see for instance for Europe [Guevara2020]).
|Figure 4: Surface concentrations of NO2 over northern Italy, comparison between 31 January and 15 March 2020. Source: CAMS2020; ECMWF||Figure 5: Time series of NO2 surface concentrations in northwestern European capitals [CAMS2020].|
The Copernicus Atmosphere Monitoring Service (CAMS) provides daily analyses of hourly concentrations of regulatory air pollutants. These can serve as “ground truth” to assess quantitatively, and in more details, the changes in concentrations identified by satellites and attributed to the effects of COVID-19 measures across the world. CAMS has created a COVID-19 resource to quickly provide robust data [Peuch2020].
The WMO Research Board has established a task team on COVID-19 in consultation with the World Health Organization (WHO) through the WMO/WHO Joint Office. The Task Team supported the organization of the international virtual Symposium on the Impact of Climatological, Meteorological and Environmental Factors on the COVID-19 Pandemic on 4–6 August and the outcomes of the Symposium were presented to the WMO Executive Council in September.
The temporary emission reductions are no substitute for climate action or air quality policies. Long-term efforts and commitments are required to reach net-zero greenhouse gas emissions and cleaner air.
[CAMS2020] Air quality information confirms reduced activity levels due to lockdown in Italy at 01/10/20.
[CB2020] Evans, S. www.carbonbrief.org/daily-global-co2-emissions-cut-to-2006-levels-during-height-of-coronavirus-crisis, at 09/10/20.
[GCP2019] Friedlingstein, P. et al. Global Carbon Budget 2019. Earth Syst. Sci. Data 11, 1783–1838 (2019). doi. org/10.5194/essd-11-1783-2019
[GCP2020] Le Quéré, C. et al. Temporary reduction in daily global CO₂ emissions during the COVID-19 forced confinement. Nat. Clim. Chang. (2020). doi. org/10.1038/s41558-020-0797-x
[ICOS2020a] Kutsch W. et al. Finding a hair in the swimming pool: The signal of changed fossil emissions in the atmosphere. www.icos-cp.eu/event/917, at 9/10/20.
[ICOS2020b] ICOS study shows clear reduction in urban CO₂ emissions as a result of COVID-19 lockdown. www.icos-cp.eu/event/933, at 9/10/20.
[NASA2020] Airborne Nitrogen Dioxide Plummets Over China earthobservatory.nasa.gov/images/146362/airborne-nitrogen-dioxide-plummets-over-china at 9/10/20
[Peuch2020] Peuch, V.H. et al. Copernicus contributes to coronavirus research. www.ecmwf. int/en/newsletter/164/news/copernicus-contributes-coronavirus-research at 09/10/20 atmosphere. copernicus.eu/european-air-quality-information-support-covid-19-crisis at 09/10/20
[WMO2019] WMO Greenhouse Gas Bulletin (GHG Bulletin) - No. 15: The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2018. library.wmo.int/index.php?lvl=notice_display&id=21620
[WMO2020] United in Science, WMO, United Nations Environment Programme (UNEP), Intergovernmental Panel on Climate Change (IPCC) et al. library.wmo.int/ index.php?lvl=notice_display&id=21761#.X3w_uEBuJjs
[Barré2020] Barré, J., Petetin, H., Colette, A., Guevara, M., Peuch, V.-H., Rouil, L., Engelen, R., Inness, A., Flemming, J., Pérez García-Pando, C., Bowdalo, D., Meleux, F., Geels, C., Christensen, J. H., Gauss, M., Benedictow, A., Tsyro, S., Friese, E., Struzewska, J., Kaminski, J. W., Douros, J., Timmermans, R., Robertson, L., Adani, M., Jorba, O., Joly, M., and Kouznetsov, R.: Estimating lockdown induced European NO2 changes, Atmos. Chem. Phys. Discuss., doi.org/10.5194/ acp-2020-995, in review, 2020. acp.copernicus.org/ preprints/acp-2020-995/
[Guevara2020] Guevara, M., Jorba, O., Soret, A., Petetin, H., Bowdalo, D., Serradell, K., Tena, C., Denier van der Gon, H., Kuenen, J., Peuch, V.-H., and Pérez García-Pando, C.: Time-resolved emission reductions for atmospheric chemistry modelling in Europe during the COVID-19 lockdowns, Atmos. Chem. Phys. Discuss., doi.org/10.5194/acp-2020-686, in review, 2020. acp.copernicus.org/preprints/acp-2020-686/
Alex Vermeulen, Chair of WMO GAW SAG on Greenhouse Gases, ICOS ERIC – Carbon Portal, Lund, Sweden
Jocelyn Turnbull, Co-chair of WMO GAW Steering Committee of Integrated Global Greenhouse Gas Information System and member of WMO GAW SAG on Greenhouse Gases, GNS Science, New Zealand; and University of Colorado, USA
Vincent-Henri Peuch, Co-chair of WMO GAW SAG Applications, Copernicus Atmosphere Monitoring Service, ECMWF
Oksana Tarasova and Claudia Volosciuk, WMO Secretariat