Today NASA and NOAA has rejected its annual analysis of global temperatures: Last year was tied to 2018 as the sixth hottest so far, but cooler than 2020. Good sign, right? Yes / No. Not at all.
“It’s easy to want to focus on this variability year after year,” said Bridget Siegers, an oceanographer at NASA. “But it’s important to look at the trend: the last eight years have been the eight hottest in history.”
To calculate global temperatures, the two agencies downloaded data from weather stations around the world, plus measurements made by ships and buoys in the ocean. Other groups, such as Berkeley Earth, a nonprofit research organization, do the same with their own slightly different methodology. But the analyzes are almost identical in their findings. As you can see in the chart below, which compares the results of Berkeley Earth, NOAA, NASA and two other groups in Europe, global average temperatures may have been lower in 2021 than 2020, but are still rises.
One of the reasons for lower temperatures in 2021 is probably La Niña, a strip of cold water in the Pacific Ocean. It is the product of strong trade winds that orbit the ocean, pushing the top layer of water toward Asia, causing deeper, colder waters to rush to the surface to fill the void. This in turn affects the atmosphere, for example by changing the jet flow over the United States and leading to more hurricanes in the Atlantic. The sea itself cools things by absorbing heat from the atmosphere.
The Covid-19 pandemic may have had an additional impact, but not in the way you think. Since the world closed in 2020, fewer emissions have gone into the sky, including aerosols, which typically reflect some of the solar energy back into space. “If you remove them, you make the air cleaner, then it’s a slightly warming effect on the climate,” said Gavin Schmid, director of NASA’s Goddard Space Research Institute, during a news conference Thursday. But as economic activity rose again in 2021, so did aerosol pollution, which again contributed to this cooling effect. The drop in temperature in 2021 “may be due to the resumption of activities that produce aerosols in the atmosphere,” Schmid said.
(Based on the pandemic, the decline in carbon dioxide production has had no cooling effect. Human civilization produces so much of the planet’s warming gas each year that it stays in the atmosphere for so long that the pandemic doesn’t even register in an instant.)