A significant reduction in carbon dioxide emissions worldwide is one bright spot in the climate change battle as travel is curbed during the COVID-19 pandemic, but the drop in air pollution also caused warmer global temperatures for a few months in 2020.
These contradictory and puzzling findings were among those drawn by a recently issued study conducted by the U.S. National Center for Atmospheric Research (NCAR), a federally-funded research and development center based in Colorado.
Triggering these findings were national lockdowns and business restrictions in many countries that fought to stem the spread of COVID-19, but also wound-up warming the Earth.
"There was a big decline in emissions from the most polluting industries, and that had immediate, short-term effects on temperatures," said NCAR scientist Andrew Gettelman, lead author of the study published in Geophysical Research Letters.
"Pollution cools the planet, so it makes sense that pollution reductions would warm the planet."
Gettelman said the long-term impact of the COVID-19 pandemic might be to slightly slow climate change in the long-term due to reduced carbon dioxide (CO2) emissions. CO2 lingers in the atmosphere for decades and has a more gradual effect on climate.
The study also highlights the little-known effects airborne aerosols have in blocking sunlight. Aerosols are suspensions of fine solid or liquid particles (such as cough particles) in the atmosphere or in a gas.
Aerosols tend to brighten clouds and reflect heat from the Sun back into space. On the other hand, CO2 and other greenhouse gases have the opposite effect. They boost temperatures near the planet's surface by trapping heat and preventing it being dissipated into space.
When emissions of aerosols declined in the spring of 2020 as the pandemic took hold, more of the Sun's warmth reached the surface. The warming hit 0.37 degrees Celsius (0.7 degrees Fahrenheit) over most of the United States and Russia.
Temperatures over other parts of planet's land surface in the spring of 2020 were some 0.1 to 0.3 degrees Celsius (0.2 to 0.5 degrees Fahrenheit) warmer than would have been expected with prevailing weather conditions.
The study notes one of the major challenges in projecting the extent of future climate change is estimating the extent of future aerosol emissions. The influence of different types of aerosols on clouds and temperature also has to be taken into account.
The European Union's Copernicus Climate Change Service (C3S) in early January said 2020 tied with 2016 as the world's warmest year on record. It also said the second decade of this century (2011 to 2020) was the hottest on the books.
C3S said temperatures globally in 2020 were an average of 1.25 degrees Celsius higher than in pre-industrial times. The last six years (2014 to 2020) were also the world's hottest on record.
C3S said 2020 saw the highest temperature ever reliably recorded. This unnerving record occurred in August in the United States when a California heat wave pushed the temperature at Death Valley in the Mojave Desert to 54.4 degrees Celsius.
