Burning question baffles science
Last year, the global mean temperature rose far beyond what was predicted. Scientists cannot agree as to why – and for some it is deeply troubling
Deadly heat in the American 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 warningabout rapidly rising temperatures driven by humanity’s relentless burning of fossil fuels.
But last year’s sudden spike in global temperatures went far beyond what statistical climate models had predicted, leading one noted climate scientist to warn that the world may be entering “uncharted territory”.
“It’s humbling, and a bit worrying, to admit that no year has confounded climate scientists’ predictive capabilities more than 2023 has,” Gavin Schmidt, director of Nasa’s Goddard Institute for Space Studies, says in an article in the journal Nature.
Now, he and other researchers are trying to explain why 2023 was so hot. Many theories have been proposed, but “as yet, no combination of them has been able to reconcile our theories with what has happened”, Schmidt writes.
Last year’s global average temperature of 14.98 degrees Celsius was about a third of a degree warmer than the previous hottest in 2016, and 1.48 degrees warmer than the late 1800s preindustrial period against which global warming is measured.
While human-caused climate change and the El Nino effect can account for much of that warming, Schmidt and other experts say the extra 0.16 to 0.22 degrees Celsius are harder to account for.
Theories for the increase include a 2020 change in shipping emissions regulations designed to help improve air quality around ports and coastal areas, which may have had the unintended consequence of enabling more sunlight to reach the planet.
The 2022 eruption of the Hunga Tonga-Hunga Ha’apai volcano in the southern Pacific Ocean also shot millions of tonnes of water vapour and ash into the stratosphere, which scientists say helped to trap some heat.
What’s more, a recent uptick in the 11-year solar cycle may have contributed about a 20th of a degree Celsius of warming.
But these factors alone cannot explain what’s happening, Schmidt says. “Even after taking all plausible explanations into account, the divergence between expected and observed annual mean temperatures in 2023 remains about 0.2 degrees C – roughly the gap between the previous and current annual record,” he wrote.
Schmidt says he thinks one of three things could be going on.
It’s possible that 2023 was a “blip” – a perfect storm of natural variables and Earth cycles lining up to create a freakishly hot year. Should that prove to be the case, “it won’t have huge implications for what we’re going to see in the future, because it would have Below from left: the 2022 eruption of the Hunga Tonga-Hunga Ha’apai volcano in the southern Pacific shot millions of tonnes of water vapour and ash into the stratosphere; Antarctic penguins are severely affected by the loss of sea ice; and US President Joe Biden and First Lady Jill Biden visit the aftermath of the fire at Lahaina in Hawaii on August 21, 2023. been just such a rare and unlikely thing that is not going to happen again any time soon”, he says. But he suggests that is unlikely, as those elements “have never lined up to give us a blip this large”.
Another possibility is that scientists have misunderstood the driving forces of climate change.
While greenhouse gases, volcanic eruptions and airborne fine particles are known to affect global temperatures, perhaps the full extent of their effects has been underestimated or miscalibrated. Should that be the case, he says, research and data sets will hopefully catch up soon.
The last explanation is the system itself is changing in ways that are faster and less predictable than previously understood.
“That would be worrying because science is really all about taking information from the past, looking at what’s going on, and making predictions about the future,” Schmidt says. “If we can’t really trust the past, then we have no idea what’s going to happen.”
However, not everyone agrees with his assessment. Michael Mann, presidential distinguished professor in the department of earth and environmental science at the University of Pennsylvania, says the premise that 2023’s warmth cannot be explained – or that it is inconsistent with model simulations – is “simply wrong”.
“The situation is extremely similar to what we saw in the 2014 to 2016 period as we transitioned from several years of La Nina conditions to a major El Nino event, then back to La Niña,” Mann says, referring to opposing weather events caused by changes in the flows of warm and cold water in the Pacific Ocean. He says some recent modelling shows the global temperature spike in 2016 was even more of an outlier than that of 2023.
“The plot shows that the surface warming of the planet is proceeding almost precisely as predicted,” Mann says. “And the models show that the warming will continue apace as long as we continue to burn fossil fuels and generate carbon pollution.”
When asked about this interpretation, Schmidt says it’s true the 2014 to 2016 period was similarly anomalous. But there is a key difference between then and now.
The 2016 temperature spike came on the heels of an El Nino event, with the biggest anomalies in February, March and April of the year following its peak, he says. He says similar patterns occurred after previous El Ninos in 1998 and 1942.
Conversely, last year’s spike arrived in August, September, October and November – before the peak of El Nino – “and that has never happened before”, Schmidt says. “It never happened in the … record that we have. It doesn’t happen in the climate models.”
Alex Hall, a professor of atmospheric and oceanic sciences at the University of California Los Angeles, largely agrees with Schmidt’s assessment that the hypothesised factors alone cannot account for the large temperature anomaly experienced in 2023 and early 2024. He likened it to the emergence of extreme wildfires in the past decade, which was not entirely foreseen.
“What we’ve learned is that there’s an aspect of this that isn’t fully predictable – that we don’t fully understand – and that we are tempting fate here a little bit 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 those are going to have potentially big impacts.”
Hall said the rapid transition from a persistent La Nina to a strong El Nino last summer probably played a role, as did the change in aerosol regulations.
It’s humbling, and a bit worrying, to admit no year has confounded climate scientists’ predictive capabilities more than 2023 GAVIN SCHMIDT, DIRECTOR OF NASA’S GODDARD INSTITUTE FOR SPACE STUDIES, FROM AN ARTICLE IN NATURE
The plot shows that the surface warming of the planet is proceeding almost precisely as predicted MICHAEL MANN, PRESIDENTIAL DISTINGUISHED PROFESSOR AT THE UNIVERSITY OF PENNSYLVANIA
It’s sort of a planetary emergency for us to figure out what’s going on ALEX HALL, PROFESSOR AT UCLA
He also posited that the rapid loss of Antarctic sea ice in 2023 – itself an outcome of the warmer planet and oceans – could have created a 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.
“It’s sort of a planetary emergency for us to figure out what’s going on when we see these types of changes,” Hall says. “There should be large teams … working on it to try to understand it, and we don’t really have those kinds of efforts, so I think there’s lessons, too, for the need for focus on this particular topic.”
While he and other scientists may not agree on just how extraordinary 2023 was – or what was behind its exceptional warmth – they all acknowledged the signs of a planet pushed to its limits.
Mann says: “It’s unfortunate so much has been made of the El Nino-spiked 2023 global temperatures, where in my view, there is nothing surprising, or inconsistent with model predictions, there.
“There are much better, scientifically sound reasons to be concerned about the unfolding climate crisis – particularly the onslaught of devastating weather extremes, heatwaves, wildfires, floods, drought, which by some measures are indeed exceeding model predictions.”
Last year was marked by extreme weather events, with more billion-dollar disasters in the US than any other year, according to the US National Oceanic and Atmospheric Administration (NOAA).
Already this year, January and February continued the global hot streak, marking nine consecutive months of a record-breaking temperatures. Hong Kong recorded its hottest March day on record as temperatures shot up to 31.5 degrees Celsius on March 24.
In his Nature article, Schmidt says the inexplicable elements of the recent warming have revealed an “unprecedented knowledge gap” in today’s climate monitoring, which drives home the need for more nimble data collection that can keep pace with change.
He notes it may take researchers months or even years to unpack all the factors that could have played a part in the sizzling conditions. “We need answers for why 2023 turned out to be the warmest year in possibly the past 100,000 years,” he writes. “And we need them quickly.”
Although El Nino is expected to wane this summer, there is still a 45 per cent chance this year will be warmer than 2023, NOAA says.
But it is a near certainty 2024 will rank among the five hottest years on record – so far.