New studies suggest a key Paris warming target has been breached

As global temperature records fell month after month last year, and fires and floods grew in synchronous ferocity, climate scientists became increasingly alarmed.

By December, it was clear that the world had sweltered through its hottest year in recorded history, the first to be on average 1.5 degrees hotter than the period before the Industrial Revolution, when we started pumping carbon dioxide into the atmosphere.

A leading climate scientist believes Earth may be even more sensitive to greenhouse gases than previously thought, but there could also be an upside.Credit: Aresna Villanueva

Some took comfort that the horrifying streak of hot weather would end in January, with a La Nina weather pattern that would surely begin to cool the planet.

Instead, when the La Nina began to take hold last month, the global average temperature continued to climb. January 2025 was a staggering 1.75 degrees hotter than the pre-industrial period and 0.79 of a degree above the 1991-2020 average, the United Nations has confirmed.

In the Paris Accord of 2015, the world agreed to try to limit global warming to an average of 1.5 degrees above pre-industrial levels, a threshold that most models suggested was still several years away.

Now, says Bill Hare, Perth-based climate scientist and former lead author for the UN’s chief climate body, the Intergovernmental Panel on Climate Change (IPCC), climate scientists are racing to understand what is happening.

Are the climate models wrong, or has the world slipped past one of the tipping points that might accelerate warming beyond our capacity to control it? Have we already slipped past the 1.5 degree limit?

Complicating the analysis is the fact that the Paris Agreement does not make clear what it means to break through the 1.5 degree barrier. Most understand it to suggest that the barrier has been broken when global temperatures rise above the threshold for a period of 20 years, to allow for exceptionally hot years.

Three papers about the question have been published in quick succession, and have become the subject of intense debate among climate scientists.

One, published in Nature Climate Change by a team led by Emanuele Bevacqua at Germany’s Helmholtz Centre for Environmental Research – UFZ, looked back over climate records to see what happened when earlier heat thresholds were broken.

It showed that when Earth’s temperature rose 0.6 of a degree, 0.7 of a degree, 0.8 of a degree, 0.9 of a degree and 1 degree above the pre-industrial benchmark, the world had begun a 20-year period of temperatures above that level.

More simply put, recent history suggests there is a chance we have already smashed through the 1.5 degree threshold of the Paris Agreement.

Hare warns that though the finding is ominous, more research is needed to confirm it.

A second paper, by climate scientist Alex Cannon, also published in Nature Climate Change, identified June 2024 as the 12th consecutive month of temperatures above the 1.5 degree threshold, and looked into existing climate simulations to understand what this suggested about possible future temperatures.

“The analysis shows that 1.5 degree celsius warming for 12 consecutive months, regardless of recent El Nino conditions, usually occurs after the 1.5 degree celsius Paris Agreement threshold has been reached in archived simulations,” wrote Cannon.

More simply put, this paper also suggested the Paris Agreement’s 1.5 degree target may already have been breached.

The third paper was written by a team led by James Hansen, director of the Program on Climate Science at Columbia University.

Hansen’s paper, published in Environment: Science and Policy for Sustainable Development, explores the causes of recent extreme heat, and what he believes is an acceleration of climate change.

Hansen comes to the conclusion that Earth’s climate system is more sensitive to greenhouse gases in the atmosphere than is properly understood by the IPCC, and that feedback loops − which see climate change causing increased climate change − have already crept into the system.

According to Hansen, human activity is making the planet darker, which means it does not reflect as much heat and light from the sun as it once did, and in turn grows warmer more quickly. This reflectivity is referred to as Earth’s albedo.

Hansen’s research shows that as greenhouse gases cause warming, snow and ice cover shrinks, less sunlight is reflected into space, and warming increases. Warming also tends to shrink cloud cover, which means less light and heat are reflected.

Hansen is also at the forefront of a new area of research into how new rules reducing aerosol pollutants, such as sulphur in the fuel used by the world’s shipping fleet, might be reducing Earth’s albedo.

According to this research, while carbon dioxide and other greenhouse gases are warming the planet, some reflect light and heat back into space. Sulphur in shipping exhaust fumes brightens the clouds above shipping lanes, increasing their reflective capacity and masking some of the impact of climate change. With the sulphur removed, the mask has slipped and warming has stepped up.

According to Hansen and his colleagues the AMOC – the Atlantic Ocean’s main system of currents – will be shut off by fresh water from ice melt flowing into the North Atlantic.

This in turn would lock heat in the waters of the southern hemisphere that are normally cooled by the flow of the AMOC, and cause metres of sea level rise.

This would mark a “point of no return” says Hansen.

His findings contradict those of the IPCC, whose models do not predict an AMOC shutdown this century.

Some leading climate scientists have been sceptical about some of Hansen’s past findings, despite their esteem for his work.

Hare notes that if Hansen is correct, it might mean that the Earth’s climate system may be more sensitive to a reduction in greenhouse gases.

If we act fast on cutting emissions we might get faster-than-expected reductions in temperature.

That might be as close we get to good news in the analysis of the heat that just keeps breaking records.

What does Hare think of Hansen’s broader findings?

“There are not very many physicists who would bet against Jim Hansen without very careful reflection.”

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