A International Tour of a File-Scorching 12 months

2020 was effectively tied to 2016 for the hottest year on record, as global warming associated with greenhouse gas emissions showed no signs of subsiding.

Siberia and the Arctic were among the hottest regions. The heat fueled forest fires that pumped even more carbon dioxide into the atmosphere.

Temperatures in the Siberian city of Verkhoyansk reached a record 100 degrees Fahrenheit in June, more than 30 degrees above average.

The heat could also be felt in Europe, which had the hottest year ever and experienced scorching heat waves in September.

The surface cooling of the tropical Pacific Ocean, which began in the second half of the year, could hardly compensate for the heat elsewhere.

In central South America, warming and drought resulted in forest fires that burned a quarter of the vast Pantanal wetland.

In the United States, warming was most significant in the northeast and southwest. The drought spread to half of the country.

This analysis of global temperatures by NASA’s Goddard Institute for Space Studies, released Thursday, found that 2020 was slightly warmer than 2016. However, the difference was insignificant, the institute’s director, Gavin Schmidt, said in an interview.

“In fact, it’s a statistical draw,” he said.

Other analysis released Thursday, one from the National Oceanic and Atmospheric Administration and one from Berkeley Earth, an independent research group in California, found that 2020 was slightly cooler than 2016, as was one that last week from Copernicus Climate Change Service in Europe has been published. However, the difference was small enough not to be statistically significant.

With the 2020 results, the last seven years have been the warmest since the beginning of modern records nearly a century and a half ago, said Dr. Schmidt.

“We are now very, very clear about the underlying long-term trends,” he said. “We understand where they come from. That’s because the greenhouse gases are pumped into the atmosphere. ”

The planet has warmed by more than 1 degree Celsius since the late 19th century, when the spread of industrialization led to rising emissions of carbon dioxide and other greenhouse gases, and the pace has accelerated in recent decades. Since 1980 the average warming has been around 0.18 degrees Celsius per decade.

But the numbers are only a small part of the story. As climate researchers have predicted, the world’s heat waves, storms, and other extreme weather conditions will increase as the planet warms, leading to disasters like droughts, floods, and forest fires. There was no break in the past year, with record fires in Australia and California and severe drought in Central South America and the American Southwest.

Some climate forecasters had thought that the arrival of cooler sea surface temperatures in the Pacific – part of the recurring global climate phenomenon La Niña – would lower temperatures this year. It is difficult to quantify the impact of La Niña, but it is clear that any effect has been dwarfed by the rise in temperature due to emissions.

La Niña didn’t appear until September and intensified a few months later. La Niña’s climate impacts typically peak a few months after hitting the coolest point in the Pacific, so they may be more chilling in 2021.

When La Niña is factored in, “don’t expect a record year,” said Dr. Schmidt in 2021. “But another top 5 year and clearly part of the line of very warm years we have had,” he added

Dr. Schmidt said his team and others had been studying the effects of the coronavirus pandemic on temperatures in 2020. Lockdown orders and the economic slowdown have reduced greenhouse gas emissions by around 10 percent in the US alone, according to a recent report.

Such a reduction has no immediate effect on temperatures, said Dr. Schmidt, and emissions are expected to rise again as the pandemic subsides and the global economy returns to normal.

A bigger short-term effect, he said, could be reducing some traffic-related pollution, particularly tailpipe emissions of nitrogen oxides, as driving decreased during the pandemic.

Nitrogen oxides form aerosols in the atmosphere that reflect some of the sun’s rays that would otherwise hit the surface and be released again as heat. Even a slight reduction in these aerosols would allow more sunlight to reach the surface and generate more heat that is stored in the atmosphere by greenhouse gases.

Dr. Schmidt said efforts were made over the past year to quantify the effect. “The numbers aren’t great,” he said, but they may have helped make 2020 a record year.

“The warming associated with reducing aerosols may be part of the story,” he said.

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