This year’s record temperatures have some scientists fearing an acceleration of warming. But not everyone agrees.
December 26, 2023 at 6:30 a.m. EST
In a paper published last month, climate scientist James E. Hansen and a group of colleagues say the rate of global warming is poised to increase by 50 percent in coming decades, with impacts escalating.
According to scientists, an increased amount of thermal energy trapped in the planetary system – known as the planet’s “energy imbalance” – will accelerate warming. “If there’s more energy coming in than going out, you’ll warm up, and if you double that imbalance, you’ll warm up faster,” Hansen said in a telephone interview.
Zeke Hausfather, a climatologist at Berkeley Earth, also called temperatures in recent months “absolutely mind-boggling” and noted: “There is growing evidence that global warming has accelerated over the past 15 years.” .
But not everyone agrees. Michael Mann, a climate scientist at the University of Pennsylvania, believes that no acceleration is yet visible: “The truth is bad enough,” he wrote on his blog. Many other researchers also remain skeptical, saying that although such an increase can be predicted in some climate simulations, they don’t see it clearly in the data from the planet itself. At least not yet.
The Washington Post used a NASA dataset to analyze average global surface temperatures from 1880 to 2023.
The data shows that the rate of warming clearly accelerated around 1970. Scientists have long known that this acceleration is due to a sharp increase in greenhouse gas emissions, combined with efforts by many countries to reduce the amount of sun-reflecting air pollution. But the data is much more uncertain about whether a second acceleration is underway.
Between 1880 and 1969, the planet warmed slowly, at a rate of about 0.04 degrees Celsius (0.07 degrees Fahrenheit) per decade. But starting towards In the early 1970s, warming accelerated, reaching 0.19 °C (0.34 °F) per decade between 1970 and 2023.
This acceleration is not controversial. Before the 1970s and 1980s, humans burned fossil fuels, but they also released enormous amounts of air pollution, or aerosols. Sulfate aerosols are lightly colored particles that have the ability to offset part of the warming caused by fossil fuels. They themselves reflect sunlight back into space, and also influence the formation of reflective clouds.
The more aerosols in the air, the slower the planet will warm: a trade-off Hansen calls a “Faustian bargain.” The idea is that because aerosolized pollutants have dangerous effects on people’s health, societies eventually decide to clean them up, causing dramatic warming.
In the early and mid-20th century, developed countries were so polluted that the planet was slowly warming. “It was the time of London fogs and very extreme pollution in the United States,” said Gabi Hegerl, a climatologist at the University of Edinburgh. A recent study in the Journal of Advances in Modeling Earth Systems, for example, found that in the 1980s these particles offset about 80 percent of global warming.
However, since the 1970s and 1980s, the influence of aerosol pollution has stabilized, in part due to policies such as the 1990 amendments to the United States Clean Air Act. As the figure above shows, at the same time, greenhouse gas emissions have increased. — leaving aerosols unable to keep up. The result is a planet that is warming is much faster today than in the first half of the 20th century.
But the data is murkier when it comes to whether the pace of warming in recent decades has accelerated further — an increase that could accelerate wildfires, floods, heat waves and other impacts around the world. It may still take years of evidence to overcome the statistical hurdles that climate science requires.
“I think we probably need three or four more years” of data, said Chris Smith, a climatologist at the University of Leeds. “It’s just a little too early at the moment.”
Scientists are wary, in part because some came to the opposite conclusion about a decade ago. At the time, some scientists and many political commentators suggested that the rate of climate change has stalled or was slowing down. The case for what some have called a warming “hiatus” has never been particularly strong – and in retrospect it does not appear that the rate of warming has changed appreciably – but they serve as a caution against claims depending on which warming accelerates or slows down.
To understand why things are currently ambiguous, consider the following “trend of trends” figure, based on an analysis by Mark Richardson, a climate scientist at NASA’s Jet Propulsion Laboratory who published a statistical paper last year that found that an acceleration of warming is not a phenomenon. but clearly detectable.
Richardson looked at each 30-year trend in NASA’s temperature record, starting with the period from 1880 to 1909 and ending with the period from 1994 to 2023. Higher values indicate higher rates of global warming. Here we show results from the period between 1941 and 1970, to better understand how the rate of warming changed during the second half of the 20th century and whether it is still changing today:
Although there is a hint of an acceleration in warming at the very end of the report, it is nowhere near as pronounced as the change since 1970. This helps explain why many scientists remain evasive, for now, on acceleration.
“The temperature near Earth is only a thin layer, and it’s easy for temperatures to vary a lot,” Richardson said. For this reason, it takes scientists longer to be sure that a change is outside of what would normally be expected, he said.
But some scientists say the temperature data simply doesn’t yet show an imminent acceleration.
Hansen says recent changes in aerosols will cause a sharp increase in the rate of warming in just the next few years. In 2020, the International Maritime Organization instituted a rule requiring a substantial reduction in the sulfur content of fuel oil. Sulfate aerosol pollution from shipping has plunged.
Much of the current debate over whether warming is accelerating depends on the consequences of these maritime changes, which can potentially affect the amount of heat absorbed across huge swaths of the world’s oceans. Hansen and his co-authors argue that changing ship emissions contributes to a major increase in Earth’s energy imbalance – the extra amount of heat that remains in the Earth system rather than escaping to space. But not all scientists agree that ocean-going ship pollution regulations have had such an outsized impact.
Hansen acknowledges that global surface temperature data alone does not present a completely clear picture of the acceleration. again – but he predicts it will be soon, as temperatures rise much more in the current El Niño.
“There will be no debate by the end of next spring, we will be very far from the trend line,” Hansen said.
Some climate models also predict accelerating warming in coming years as aerosols decrease. “While there is growing evidence of accelerating warming, it is not necessarily ‘worse than we thought,’ as scientists widely expected something like this.” Hausfather said.
Most agree that it is too early to tell whether the second acceleration is underway. “Trying to estimate the underlying rate of warming over a short period of time is very difficult,” said Andrew Dessler, a climate scientist at Texas A&M University.
“Just because you get a trend that looks very rapid doesn’t tell you what the underlying rate of warming is.”
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