NASA scientists announced Wednesday that the Earth’s average surface temperature in 2018 was the fourth highest in nearly 140 years of record-keeping and a continuation of an unmistakable warming trend.
“The five warmest years have, in fact, been the last five years,” said Gavin A. Schmidt, director of the Goddard Institute for Space Studies, the NASA group that conducted the analysis. “We’re no longer talking about a situation where global warming is something in the future. It’s here. It’s now.”
Over all, 18 of the 19 warmest years have occurred since 2001.
The results of this warming, Dr. Schmidt said, can be seen from the heat waves in Australia and extended droughts to coastal flooding in the United States, in disappearing Arctic ice and shrinking glaciers. Scientists have linked climate change to more destructive hurricanes like Michael and Florence last year, and have found links to such phenomena as the polar vortex, which last week delivered bone-chilling blasts to the American Midwest and Northeast.
While this planet has seen hotter days, and colder ones, what sets recent warming apart in the sweep of history is the relative suddenness of the rise in temperatures and its clear correlation with increasing levels of greenhouse gases like carbon dioxide and methane produced by human activity over the same period.
The Earth’s temperature in 2018 was more than 1 degree Celsius, or 1.8 degrees Fahrenheit, above the average temperature of the late 19th century, when humans started pumping large amounts of carbon dioxide into the atmosphere. Scientists say that if the world is to avoid the worst consequences of climate change, global temperatures must not rise by more than 2 degrees Celsius compared with pre-industrial levels.
It appears highly likely, at least from today’s perspective, that that line will be crossed, despite the fact that 190 nations have signed the Paris climate agreement. (The United States is still technically a party to the accord, though President Trump has pledged to withdraw.)
Even an increase of 1.5 degrees will have dire consequences, according to the United Nations science panel on climate change.
Dr. Schmidt spoke of these lines not as cliffs that the world would plunge over, however, but part of a continuing slide toward increasing levels of harm. “Symbolically, it’s important,” he said.
With concerted effort to reduce greenhouse gases worldwide, scientists say, a slide could be slowed or even, eventually, reversed.
The warmest year was 2016, its record-setting temperature amplified by the Pacific Ocean phenomenon known as El Niño. In 2018, the world experienced the opposite phenomenon, a cooling La Niña, with a weak El Niño toward the end of the year.
The effects of the El Niño late last year are likely to be felt in 2019, said Zeke Hausfather, an analyst with Berkeley Earth, an independent climate research group. He said that 2019 would probably be the second-warmest year on record. Last month, Mr. Hausfather issued figures correctly ranking 2018 as the fourth-warmest.
The publication of the NASA temperature data came in tandem with a similar announcement from the National Oceanic and Atmospheric Administration, which uses a slightly different methodology to determine overall changes in the planet’s temperature but also ranked 2018 as the fourth-warmest year.
The two federal agencies, while broadly consistent, have disagreed on the relative rankings for some years; NASA called 2017 the second-warmest year, while NOAA said it was the third, after 2016 and 2015.
Dr. Schmidt of NASA said that the new figures helped to validate the scientific models that have predicted such warming over time.
“People say, ‘How do we know the science is any good? How do we trust the models? They’re so complex!’ ” But, he said: “That’s the essence of science. You think you understand how something works, you make models and you make predictions and see if they come true. Unfortunately, we’re in a situation where we see it’s come true. And while that’s intellectually pleasing, it totally sucks.”
The annual global temperature ranking is usually announced in mid-January, but it was delayed when the government shutdown prevented federal scientists from completing the analysis.
