Last year raised many major red flags for climate change, perhaps greatest of all being the first year in which global average temperatures exceeded 1.5º C above pre-industrial levels. This is the threshold the majority of countries, including the U.S., vowed not to surpass in the 2015 Paris climate accord. 2025 isn’t looking much better, as January was already the hottest in recorded human history, shocking some scientists who had hoped the La Niña cycle would cool things down a bit. It hasn’t.
These are more than just ominous statistics — this extreme heat has translated into countless examples of real-world “weird weather” including unprecedented heat waves, wildfires, floods and extreme storms across the globe. Virtually nowhere has been untouched by climate change disasters, from California and the American southeast to Spain, Greece and Africa, all of which seems to be getting worse.
Meanwhile, we’re burning record amounts of fossil fuels, accelerating the crisis, all while world leaders like President Donald Trump deny and ignore climate science, even falsely claiming America has a “national energy emergency” which requires more drilling.
Yet if a recent study in the journal Environment led by iconic climate scientist Dr. James Hansen is correct, things are even worse than all of the latest news would make one believe: He claims Earth is about to blow past 2º C above pre-industrial levels. Major media outlets like The Guardian and Inside Climate News are sounding alarm bells about these findings but not everyone agrees with these conclusions.
Some scientists like Dr. Kevin Trenberth, a distinguished scholar at the National Center for Atmospheric Research, say that Hansen neglected to focus on one of the major variables contributing to climate change: water vapor.
"The biggest warming is not in the Northern Hemisphere oceans but in the Southern Hemisphere oceans where aerosols certainly were not responsible for the warming."
“Water vapor is at record high levels, and of course so is the ocean heat,” Trenberth said. Water vapor doesn’t linger in the air, so it doesn’t contribute to long-term climate temperature increases in the same way as carbon dioxide, but it can temporarily raise temperatures and overall amplify the effects of greenhouse gases. “Global integrated water vapor is some 7% higher in 2024 than in 2000. Of course, that is a feedback and depends on higher temperatures (in the right places), but it also very much depends on the warmer oceans,” Trenberth explained.
In particular, Trenberth took issue with the way Hansen “carefully compares things to cause exaggerations,” citing as one example how Hansen studies sea surface temperatures in the Northern Hemisphere for a much smaller region than in the Southern Hemisphere without properly weighting them.
“If he did the latter, he would find that, no, the biggest warming is not in the Northern Hemisphere oceans but in the Southern Hemisphere oceans where aerosols certainly were not responsible for the warming,” Trenberth said. “I am not saying they haven't played a role: they have, but not like Hansen believes.”
Trenberth does not criticize Hansen lightly. The Columbia University professor is a storied scientist, noted for testifying about climate change before Sen. Al Gore, (D-Tenn.), in 1988. This was shortly before Gore ran for president in a campaign that focused primarily on fixing climate change and in the process raised international awareness about the issue for the first time. Yet Hansen has since aroused controversy, such as when he argued in 2000 that non-carbon dioxide greenhouse gases like methane play a large role in climate change. Much research now suggests he was right. He has also been critical of the idea of “tipping points” in climate models, telling Salon last year that the concept is “greatly overused and misused.”
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Hansen was blunt in his response to criticisms of his latest paper: “Sorry, I prefer not to spend significant time responding to scientific illogic.”
Hansen raised the issue of albedo, or the fraction of light that a surface reflects: “Water vapor does not affect the albedo of Earth, so it did not affect our analysis of the magnitude of the aerosol forcing. I pointed out that the contributions to temperature change from doubled CO2 (the CO2 forcing, water vapor feedback, and surface albedo feedback by themselves, together) already provide almost 3º C warming.”
Still others disagree. University of Pennsylvania climate scientist Dr. Michael E. Mann cited an October study in the journal Communications Earth & Environment which found no evidence of any acceleration in global heating. Things are getting hotter, yes, but not surging faster than expected. Both Mann and Dr. Ken Caldeira, an atmospheric scientist at the Carnegie Institution for Science's Department of Global Ecology, pointed to an analysis by University of California, Berkeley climatologist Dr. Zeke Hausfather, who found that the current temperature increases are consistent with pre-existing climate models. Hansen urged skeptics to look at his study and its supplementary material.
“A simple linear fit reveals that the warming rate has been twice as fast since 2010,” Hansen said. “Anyone is free to doubt that it will continue — the real world is the judge, and so far she is on our side.”
"Water vapor does not affect the albedo of Earth, so it did not affect our analysis of the magnitude of the aerosol forcing."
He added, “Models that use the IPCC aerosol formulation yield negligible acceleration. The complete IPCC ‘model fog’ includes results with significant acceleration, but those models are not using the IPCC aerosols!” Hansen believes the prevailing models which incorporate aerosol emissions are inadequate by “producing a fog (range) of model results that the real world is certain to fall within it, so they can never be accused of being wrong. But that approach is hardly helpful to the public, to policymakers, or for the task of understanding the physics of observed climate change.”
Caldeira warned that Trenberth may be making the same mistake as scientists who pointed to a warming trend from 1998 to 2012 as proof that there could be a “hiatus” in climate change, which has been cited as evidence that concerns about climate change were overblown.
“We need to be cautious in over-interpreting short-term fluctuations in the rate of warming,” Caldeira said. Hansen replied that “the leap in temperature is too large to be noise — it has a physical explanation, which we uncover persuasively.”
He added, “If the recent climate change, even the last two centuries, were all that we had to go on, the inferred climate sensitivity would be less certain, but the paleoclimate data confirm our assessment. Our conclusions are well-founded, based on real-world data.”
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Mann, who told Salon that “unfortunately a lot of what Jim says is simply wrong,” argued that “the truth is bad enough — the planet continues to warm at a steady rate and will do so until our carbon emissions reach zero.” His concern about Hansen is that “exaggerating what is happening — particularly when it is used as an argument for very dangerous geoengineering interventions, as Jim does — is a pure gift to polluters and petrostates, who just love this sort of distraction and happily amplify it and weaponize it on social media.”
Hansen says his study “is urgently needed so that young people will have full information at their disposal. I get the impression that the people who try to outlaw even research are like those who shut down progress in nuclear power, which should be our cheapest energy with the smallest carbon footprint.”
It’s not enough to know if climate change is happening — that much is obvious — scientists must also figure out how fast it’s occurring, so we can respond appropriately. But the solution remains the same: we need to stop burning fossil fuels and eroding the environment, especially given its role in capturing carbon.
Regardless of whether one thinks climate change is increasing at the accelerated rate believed by the overwhelming majority of scientists or the hyper-accelerated rate asserted by Hansen, all of the scientists involved in this debate — just like at least 97% of climate scientists — agree on a key point: Human activity is causing temperatures to rise at a perilous rate. As of 2024, the carbon dioxide concentration in Earth’s atmosphere was 424.6 ppm, a jump of more than 50% from levels before our species began relying on fossil fuels less than three centuries earlier.
The increase is, as University of Pennsylvania climate scientist Dr. Michael E. Mann told Salon, is “at least an order of magnitude more rapid than any natural change we know of.”
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