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Canadian Geese swim in the waters of Lake Ontario in Mississauga, Ont., in January, 2019.

Nathan Denette/The Canadian Press

The Earth’s average surface temperature in 2018 was the fourth highest in nearly 140 years of recordkeeping and a continuation of an unmistakable warming trend, NASA scientists announced Wednesday.

The data means that the five warmest years in recorded history have been the past five, and that 18 of the 19 warmest years have occurred since 2001. The quickly rising temperatures over the past two decades cap a much longer warming trend documented by researchers and correspond with the scientific consensus that climate change is caused by human activity.

“We’re no longer talking about a situation where global warming is something in the future,” said Gavin Schmidt, director of the Goddard Institute for Space Studies, the NASA group that conducted the analysis. “It’s here. It’s now.”

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While this planet has seen hotter days in prehistoric times, and colder ones in the modern era, what sets recent warming apart in the sweep of geologic time is the relatively sudden rise in temperatures and its clear correlation with increasing levels of greenhouse gases like carbon dioxide and methane produced by human activity.

The results of this rapid warming can be seen from the heat waves in Australia and extended droughts to coastal flooding in the United States, 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 U.S. Midwest and Northeast.

The Earth’s temperature in 2018 was more than 1 degree Celsius (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 the line will be crossed, despite the fact that more than 190 nations have signed the Paris climate agreement, which sets targets for reducing greenhouse gas emissions. (The United States is still technically a party to the accord, although President Donald Trump has pledged to withdraw.)

Even an increase of 1.5 degrees will have dire consequences, according to the U.N. science panel on climate change.

Schmidt spoke of these markers not as cliffs that the world would plunge over, however, but as part of a continuing slide toward increasing levels of harm.

“Symbolically, it’s important,” he said.

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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 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, 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.

In related research, two new studies published Wednesday in the journal Nature suggested that melting polar ice caps could lead to even more extreme weather.

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One study, led by Tamsin Edwards, a climate scientist at King’s College London, focused on the effect of Antarctic ice-shelf collapse. It estimated that sea level rise caused by melting in Antarctica would be close to 10 inches, much lower than a 2016 study based on much of the same data that forecast a rise of 5 or 6 feet by 2100.

But even this lower estimate for sea level rise would be enough to trigger an increase in extreme weather events, according to a companion study led by Nicholas Golledge, an associate professor at the Victoria University of Wellington’s Antarctic Research Centre in New Zealand.

For example, Golledge said, an influx of fresh water from Greenland could slow the circulation of the Atlantic Ocean, which, among other things, would lead to warmer waters in the Gulf of Mexico and, subsequently, more hurricanes.

In addition to being the fourth warmest year on record, 2018 was also the fourth costliest for weather disasters in the United States. Last year’s total was $91 billion, less than 2017, when disasters inflicted a record-breaking $306 billion of damage. But it reflects a broader trend of more frequent and intense extreme weather events including hurricanes, floods, droughts and wildfires that scientists say are to be expected with a warming climate. According to NOAA, 2018 was the first time on record when wildfires have exceeded average hurricane costs in the United States.

Schmidt of NASA said that the new global temperature 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.”

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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.

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