How did COVID-19 lockdowns affect the climate?

In this article, Met Office Research Fellow Chris Jones discusses the study and what it tells us about limiting global temperature rise.

The COVID-19 pandemic has had a massive impact on our lives, and how we go about day-to-day business. It has directly affected millions of people and put our infrastructure under huge pressure. In this new study we look to see if the changes to our daily lives over the last 12 months have had an effect on the climate. On a global scale, the new study by Jones et al. (2021) (which is also the subject of a Research Spotlight in EOS) finds very little changes are detectable, but the way that we rebuild our economies does offer an unprecedented opportunity to “build back better” and shows potential pathways to meet long-term climate goals.

Many nations, including the UK, responded to the COVID-19 pandemic by restricting travel and other activities during 2020, and into 2021. This caused a temporary reduction in global greenhouse gas emissions and local air pollution.

Empty roads in the UK during the COVID-19 pandemic

Atmospheric concentrations of greenhouse gases have been increasing since the mid-1800s. Once in the atmosphere, these gases form a blanket around the planet trapping heat from the sun and increasing global temperatures. Aerosols, which are tiny particles suspended in the atmosphere, can also affect the Earth’s climate. Pollution from cars and factories is a major source of man-made aerosols but they can also be produced naturally. Aerosols play a key role in Earth’s energy balance through influencing the amount of energy from the sun that is either absorbed in the atmosphere or reflected back into space.  

To see if the temporary reduction in greenhouse gases and aerosol pollutants during the pandemic had any effect on our climate, we looked at results from 12 Earth system models to see how they responded to these emissions reductions. To assemble these results, a huge international research effort was coordinated at very short notice, involving 50 scientists from 30 different research organizations around the world. These research groups ran 12 climate models and performed over 300 experiments to assess the implications of 2020’s unusual emissions. 

Figure 1. The map shows the average change in aerosols across the Earth system models. AOD refers to aerosol optical depth. Green shading shows regions with reduced aerosol amounts in the atmosphere, and purple shows regions with increased aerosols.

As seen in Figure 1, the results show a consensus that aerosol amounts were reduced, especially over southern and eastern Asia, for 2020. This led to increases in solar radiation reaching the surface in that region. However, we could not detect any associated impact on temperature or rainfall in that region or globally.

The results are publicly available through a climate model archive known as the Earth System Grid. The climate research community will benefit from these experiments for many years. We recommend that more analyses on regional scales and analysis of extreme weather and air quality would be useful to further understand the impact of emission reductions due to COVID-19 on the climate.

How have emissions changed due to the pandemic?

As the pandemic rapidly spread and affected more and more countries it became clear that it had the potential to disrupt daily life on a global scale and change the way we use energy, burn fuel and emit pollutants into the atmosphere. Several studies analysed activity data – such as that available from Apple or Google – to estimate how this might change greenhouse gas and aerosol emissions. Le Quéré et al. (2020) estimated a drop in carbon dioxide emissions of about 17% during April 2020, and projected this would lead to a decrease of about 7% for the year as a whole. Forster et al. (2020) applied similar analysis to emissions of other greenhouse gases – such as methane, and precursors of ozone and aerosols such as sulphates and nitrogen oxides. All of these saw a peak reduction during April with expected sustained reductions during the rest of 2020. Carbon Brief reported that in the UK, these reductions saw greenhouse gas emissions drop to 51% below 1990 levels – equivalent to temporarily being halfway to meeting the UK’s net zero by 2050 target.

What was the effect of the change in emissions?

How these emissions affect the content of the atmosphere varies for each different gas or aerosol. We know that aerosols only stay in the air for a few days and so the amounts of them can change very quickly. Many places in the world saw big improvements in air quality and visibility due to the reduction in aerosols.

Gases like carbon dioxide have a very long lifetime in the atmosphere and so changes to emissions only affect them very slowly. While a decrease in emissions of 7% is unprecedented, it still means that 93% of our normal emissions went into the atmosphere and carbon dioxide levels continued to build up. A bit like filling a bathtub – we slowed the flow from the taps very slightly, but the water level continues to rise.

How did the climate respond?

Some experiments with the Canadian climate model, CanESM5, showed that the global climate response to emissions reductions like those observed in 2020 is likely to be small (Fyfe et al., 2021). But we know that different models can give different answers, so in our new study we wanted to use as many models as we could to check the robustness of the conclusions. Assuming that the emissions reductions would last for two years before beginning to recover back to previous levels, the models were used to simulate the climate of the five year period from 2020-2024.

Figure 1 shows how the reduced emissions in 2020 led to reduced aerosol amounts, especially over southern and eastern Asia. This also led to small increases in the amount of sunlight reaching the Earth’s surface in that region, as seen in Figure 2, but this was not enough to change the climate. Both for that region, and globally, the annual temperature and rainfall did not change significantly.

Figure 2. Simulated changes in amount of solar radiation reaching the surface (left) and global surface air temperature (right). Each coloured line shows the results from one model and the shaded regions show the spread of results when each model is run multiple times.

Can we build-back “greener”?

To try to understand the effect all of these changes would have on our climate, Forster et al. (2020) used a simplified climate model to look at the conflicting effects. Despite initially very little effect on climate, on longer timescales over many years decreased carbon dioxide emissions will cause a cooling effect. The message being that if we can continue to reduce our emissions then we still have a chance of limiting the level of future warming and the severity of future climate change impacts.

With vaccination programmes bringing the prospect of brighter horizons, now is a good time to consider how we can plan our economic recovery and development in a way which ensures a sustainable future and transition to low-carbon. The goals of the Paris Agreement commit countries to try to limit warming to well below 2°C above pre-industrial levels, while pursuing efforts to limit it to 1.5°C. Reducing emissions to net zero from about the middle of the century is required to limit global warming; this means no longer adding any greenhouse gases to the atmosphere from that date. Many countries have pledged to achieve these net zero goals, but doing so requires far reaching transformation across all parts of society. This includes long-term planning for infrastructure from power generation, to domestic heating, to electric vehicle infrastructure. How we achieve this is up to society, but scientific understanding of the climate system helps with planning possible solutions. The impact of the Covid restrictions has provided a unique opportunity for society to pursue ways forward which could bring permanent changes towards a climate-resilient future.

References

Forster, P. M., Forster, H. I., Evans, M. J., Gidden, M. J., Jones, C. D., Keller, C. A., et al. (2020). Current and future global climate impacts resulting from COVID-19. Nat. Clim. Chang. doi:10.1038/s41558-020-0883-0.

Fyfe, J.C., Kharin, V.V., Swart, N., Flato, G.M., Sigmond, M., Gillett, N. P. (2021). Quantifying the Influence of Short-term Emission Reductions on Climate. Sci. Adv. in press.

Jones, C. D., Hickman, J. E., Rumbold, S. T., Walton, J., Lamboll, R. D., Skeie, R. B., et al. (2021). The Climate Response to Emissions Reductions due to COVID‐19: Initial Results from CovidMIP. Geophys. Res. Lett. doi:10.1029/2020GL091883.

Lamboll, R. D., Jones, C. D., Skeie, R. B., Fiedler, S., Samset, B. H., Gillett, N. P., Rogelj, J., and Forster, P. M. (2020). Modifying emission scenario projections to account for the effects of COVID-19: protocol for Covid-MIP. Geosci. Model Dev. Discuss. doi:10.5194/gmd-2020-373.

Le Quéré, C., Jackson, R. B., Jones, M. W., Smith, A. J. P., Abernethy, S., Andrew, R. M., et al. (2020). Temporary reduction in daily global CO2 emissions during the COVID-19 forced confinement. Nat. Clim. Chang. 10, 647–653. doi:10.1038/s41558-020-0797-x.