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The scientist who in the 1980s sounded the alarm about climate change says the world is on course to hit 2C above pre-industrial temperatures by 2045, warning the true extent of global warming has been masked by pollution for years.
James Hansen, a former Nasa scientist now at Columbia University, said the record temperatures of the past two years were the first payment owed on a “Faustian bargain”, where air pollution in effect offset global warming.
Scientists have been trying to understand how 2023 and 2024 became the hottest years on record, noting that the end of the last El Niño weather cycle — which drives warmer weather — did not lead to a significant drop in temperatures.
In a paper published in Environment: Science and Policy for Sustainable Development on Tuesday, a team of researchers led by Hansen said aerosol pollution had a cooling effect on the earth by reflecting incoming sunlight back to space, countering global warming driven by greenhouse gas emissions.
But the world was now experiencing the true extent of global warming, the study found, after new shipping regulations came into force in 2020. The regulations limit the amount of sulphur used in shipping fuel, leading to less sulphur aerosol-based pollution in the atmosphere.
“Humanity made a bad deal, a Faustian bargain,” said Hansen. “The Faustian payment to the devil has come due.”
Hansen testified to the US Senate in 1988 about how human activities were responsible for Earth’s warming, in one of the first high-profile warnings about climate change.
Hansen and his fellow authors said aerosols’ cooling effect had been underestimated. In the new paper, the researchers argued aerosols had a much bigger cooling impact when released into relatively pristine air in places that are susceptible to cloud changes.
They predicted that global temperatures will remain at or near record levels this year, a trend that would support their theory about the declining prevalence of shipping aerosols.
“Sea surface temperatures will remain abnormally high, providing fuel for powerful storms and extreme rainfall,” they added.
They said global warming in the next 20 years is likely to be about 0.2-0.3C per decade, with temperatures hitting 2C above pre-industrial levels by 2045.
Under the Paris agreement almost 200 countries agreed to limit global temperature rises to well below 2C and ideally to 1.5C above pre-industrial levels. US President Donald Trump has begun the process of withdrawing Washington from the pact for a second time.
“The 2C target is dead,” said Hansen, citing continued high energy demand from conventional fossil fuels.
Accelerated global warming would drive further melting of polar ice and freshwater flows into the north Atlantic Ocean, which in turn could lead to the shutdown of the Atlantic Meridional Overturning Circulation (Amoc) within a few decades, the researchers said. This is a large system of ocean currents that helps ensure places such as the UK are much warmer than other areas on a similar latitude.
“If Amoc is allowed to shut down, it will lock in major problems including a sea level rise of several metres,” the authors wrote, describing such a scenario as the “point of no return”.
The paper also looked at other so-called climate forcings that can change the planet’s temperatures, but found that issues such as solar radiation and volcano eruptions had a much smaller impact than aerosols linked to shipping and greenhouse gas emissions.
Some scientists have questioned the research. Piers Forster, professor of physical climate change at Leeds university, said the aerosol effect only “contributes a small amount”.
He added: “Other studies . . . find that variability effects such as El Niño have caused the high temperatures over the past two years, and the human induced component of global warming is around 1.3-1.4C, still below the 1.5C Paris benchmark.”
Richard Allan, professor of climate science at Reading university, said the report was “comprehensive” but argued the findings appeared “overly bleak compared to the growing body of scientific research”.
