Deadly heat in the southwest. Hot tub temperatures in the Atlantic Ocean. Sweltering conditions in Europe, Asia and South America.
That 2023 was Earth’s hottest year on record was in some ways no surprise. For decades, scientists have been sounding the alarm about rapidly rising temperatures caused by humanity’s relentless burning of fossil fuels.
But last year’s sudden rise in global temperatures far exceeded what statistical climate models had predicted, leading a noted climate scientist to warn that the world could be entering “uncharted territory.”
“It is humbling and a little concerning to admit that no year has confounded the predictive abilities of climate scientists more than 2023,” wrote Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, in a recent article in the journal Nature.
Now he and other researchers are trying to explain why 2023 was so anomalously hot. Many theories have been proposed, but “so far no combination of them has been able to reconcile our theories with what happened,” Schmidt wrote.
Last year’s global average temperature of 58.96 degrees was about a third of a degree warmer than the previous warmest year in 2016, and about 2.67 degrees warmer than the pre-industrial period at the end of the 19th century against which warming the earth is measured out.
While human-induced climate change and El Niño can explain much of that warming, Schmidt and other experts say the extra three or four tenths of a degree are harder to explain.
Theories for the increase include a 2020 change in aerosol shipping regulations intended to help improve air quality around ports and coastal areas, which may have had the unintended consequence of allowing more sunlight to reach the planet.
The 2022 eruption of the Hunga Tonga-Hunga Ha’apai volcano also shot millions of tons of water vapor into the stratosphere, which scientists say helped trap some heat. In addition, a recent increase in the eleven-year solar cycle may have provided about a tenth of an additional warning.
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But these factors alone can’t explain what’s happening, Schmidt said.
“Even after taking into account all plausible explanations, the difference between expected and observed annual mean temperatures in 2023 remains around 0.2°C – approximately the difference between the previous and current annual record,” he wrote in his report.
Reached by phone, Schmidt said he thinks three things could be going on.
It’s possible that 2023 was a “blip” — a perfect storm of natural variables and Earth cycles that lined up to create one freakishly hot year. If that turns out to be the case, “it won’t have a huge impact on what we see in the future because it would have been so rare and unlikely that it won’t happen again anytime soon.” ” he said.
However, he indicated that this is unlikely as these elements “have never lined up to give us such a big blip.”
Another possibility is that scientists have misunderstood the driving forces behind climate change. Although greenhouse gases, volcanic eruptions, and aerosols are known to influence global temperatures, the full extent of their effects may have been underestimated or miscalibrated. If that is the case, he said, research and data sets will hopefully catch up quickly.
The final explanation he offered is that the system itself is changing – and changing in ways that are faster and less predictable than previously thought.
“That would be concerning, because science is really about taking information from the past, looking at what’s going on and making predictions about the future,” Schmidt said. “If we can’t really trust the past, we have no idea what’s going to happen.”
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However, not everyone agrees with his assessment. Michael Mann, presidential professor in the Department of Earth and Environmental Sciences at the University of Pennsylvania, said the premise that the 2023 heat cannot be explained — or that it is inconsistent with model simulations — is “simply wrong.”
“The situation is very similar to what we saw during the 2014-2016 period when we went from several years of La Niña conditions to a major El Niño event and then back to La Niña,” Mann said in an email .
In fact, he said some recent models shows that the global temperature peak in 2016 was even more of an outlier than that of 2023.
“The plot shows that the warming of the planet’s surface is happening almost exactly as predicted,” Mann said. “And the models show that warming will continue rapidly as long as we continue to burn fossil fuels and cause carbon pollution.”
When asked about this interpretation, Schmidt said it is true that the period 2014 to 2016 was also anomalous. But there is an important difference between then and now, he said.
The 2016 temperature spike followed an El Niño event, with the largest anomalies in February, March and April of the year after the peak, he said. He noted that similar patterns occurred after previous El Niños in 1998 and 1942.
Conversely, last year’s peak came in August, September, October and November – before the peak of El Niño – “and that has never happened before,” Schmidt said. “It’s never happened in the temperature record we have. It doesn’t happen in the climate models.”
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Alex Hall, a professor of atmospheric and oceanic sciences at UCLA, said he largely agrees with Schmidt’s assessment that the hypothesized factors alone cannot explain the large temperature anomaly in 2023 and early 2024. He compared this to the rise of megafires, or extreme wildfires, in the past decade, which were not entirely anticipated.
“What we’ve learned is that there’s an aspect that’s not completely predictable — that we don’t fully understand — and that we’re kind of tempting fate here by continuing to interfere with the climate system,” Hall said. . “It’s going to do things that we don’t understand, that we don’t anticipate, and they’re going to have potentially major consequences.”
Hall said the rapid transition from a prolonged La Niña to a strong El Niño last summer likely played a role, as did the change in aerosol regulations.
He also suggested that the rapid loss of Antarctic sea ice in 2023 – itself a result of the warmer planet and oceans – could have created a kind of feedback loop that contributed to more warming. Ice and snow are reflective, so when they melt it can result in a darker ocean that absorbs more heat and sunlight. (According to the National Oceanic and Atmospheric Administration, Antarctica’s sea ice coverage fell to a record low in 2023.)
“It’s kind of a planetary emergency for us to figure out what’s going on when we see these kinds of changes,” Hall said. “It would take large teams of people working to try to understand it, and we don’t really have that kind of effort, so I think there are also lessons for the need to focus on this particular topic.”
While he and other scientists may not agree on how extraordinary 2023 was — or what was behind its exceptional warmth — they all recognized the clear signs of a planet being pushed to its limits.
“I think it’s unfortunate that so much has been made about El Niño-induced global temperatures in 2023, when in my opinion there is nothing surprising or inconsistent with model predictions,” Mann said. “There are far better, scientifically sound reasons to be concerned about the unfolding climate crisis – especially the onslaught of devastating weather extremes, heat waves, wildfires, floods and droughts, which by some measures are indeed exceeding model predictions.”
According to NOAA, last year was marked by extreme weather events, with more multi-billion dollar disasters in the United States than any other year. These included the Lahaina wildfire in Hawaii in August; Hurricane Idalia in Florida that same month; and severe flooding in New York in September.
This year, January and February have already continued the global hot streak, marking nine consecutive months of record temperatures.
In his Nature paper, Schmidt said the unexplained elements of recent warming have revealed an “unprecedented knowledge gap” in current climate monitoring, highlighting the need for more fluid data collection that can keep up with the pace of change.
He noted that it could take investigators months or even years to unravel all the factors that could have played a role in the sweltering conditions.
“We need answers as to why 2023 turned out to be the hottest year in possibly the last 100,000 years,” he wrote. “And we need them quickly.”
Although El Niño is expected to wane this summer, there is still a 45% chance that this year will be warmer than 2023, according to NOAA.
However, it is almost certain that 2024 will be among the five warmest years on record – yet.
This story originally appeared in the Los Angeles Times.