The science of why lockdown barely affected global temperatures
Climate and chemistry
Sulphur dioxide (SO₂) gas is mainly produced in industrial processes that burn coal. In the atmosphere, it reacts to form white sulphate aerosols. These particles offset some of the heating caused by greenhouse gases such as CO₂ by reflecting sunlight back into space, in a process known as global dimming. If SO₂ were the only pollutant whose emissions fell, we would expect Earth’s temperature to increase.
Soot, otherwise known as black carbon, is also made when burning dirty fuels, and emitted in large quantities from older cars. Since soot is black, it absorbs sunlight and heats the atmosphere. Cars and aeroplanes also emit lots of nitrogen oxides (NOₓ), gases thatmake ozonein the lower atmosphere where it acts as a greenhouse gas that warms the planet. Satellite images in March and April showed huge reductions in NOₓ over Europe as national lockdowns came into force.
The different gases and aerosols we emit either contribute to global heating or global dimming. So determining how lockdown affected global temperatures is a matter of finding out which effect dominated.
We ran a seriesof computer model simulations of the atmosphere during lockdown, versus what we would have expected if the pandemic had never happened. We fed into the model the best estimates of how much emissions of SO₂, black carbon and NOₓ fell from industry, transport and aircraft for the period between mid-February and mid-June.
Our model simulations showed that reductions of these different pollutants only had a small and temporary influence on the climate, overall, in part due to their opposing effects. This may sound like a dull conclusion, but it has important lessons.
Which sectors were affected most was hugely important. The largest emissions reductions were in transport, where NOₓ and black carbon emissions are particularly high. This largely offset any heating that would otherwise have occurred from the drop in SO₂ caused by the slowdown in heavy industry.
The global average temperature saw little change, but there were regional variations. For example, the Middle East was cooler since less black carbon in the air meant the highly reflective desert sand could send more solar energy back out to space. Other regions, such as eastern China, saw more heating overall, as they had some of the largest reductions in industrial SO₂ emissions. These differences in heating patterns could affect weather systems, such asmonsoon cycles.
What we’ve described here are model simulations – they’re not perfect, but they’re our best method for investigating global atmospheric changes. Simulating the effects of all these different pollutants is difficult. In fact, the struggle to simulate how aerosols affect the climate is one reason we cannot predict exactlyhow hot the climatewill get.
The lockdown offered an invaluable test for our theories about how pollutants affect the climate. From this, we’ll be able to improve our models and make better predictions. We’ll also know better how to plan a strategy that reduces emissions from different sectors without inviting a sudden and sharp increase in global heating.
The post-pandemic climate
The long-term effects of the pandemic on our climate will be determined more by what happens tolong-lived greenhouse gases, such as CO₂ and methane. These remain in the atmosphere forcenturies and decadesrespectively, compared to a few days to weeks for NOₓ, SO₂ and black carbon. CO₂ emissions dropped during lockdown, but not enough to stop levelsin the atmospheregrowing. Global heating won’t stopuntil emissions reach zero.
It may seem daunting that the near shutdown of society didn’t cause abig enough reductionin emissions to stop climate change. But this just shows the limits of doing less of the stuff we normally do, instead of changing how our economies and infrastructure are powered. While lockdown measures have brought temporary reductions in emissions, there are better ways of doing this that cause less harm to society and people.
Only a decisive shift from fossil fuels will stabilize global temperatures. That’s why the decisionsgovernments taketo revive economic growth after COVID-19 will be pivotal. The 2008 financial crisis caused a similar slowdown, butemissions soon reboundedas a direct result of economic rescue packages which invested heavily in fossil fuels. We cannot afford to make the same mistake again.
This article is republished fromThe ConversationbyScott Archer-Nicholls, Postdoctoral Research Associate in Atmospheric Science,University of CambridgeandJames Weber, PhD Candidate in Atmospheric Chemistry, Pembroke College,University of Cambridgeunder a Creative Commons license. Read theoriginal article.
Story byThe Conversation
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