Why global warming is accelerating and what it means for the future

Recent years have seen temperatures rise more than anticipated, sparking a debate among scientists. While there is broad consensus on the acceleration of global warming, some argue that it is progressing even faster than climate models suggest, whereas others attribute the rise to temporary natural fluctuations.

The implications are significant; depending on which view is correct, the time available to mitigate or adapt to severe impacts could be shorter than previously thought.

“Ultimately, this is a question of how bad climate change is going to be,” says Zeke Hausfather at non-profit organisation Berkeley Earth in California.

Until the 2010s, Earth’s temperature was increasing at a steady pace of approximately 0.18°C per decade, but there was a slight uptick in warming after that period.

In 2023, the year recorded the highest temperatures ever, exceeding expectations with an increase of 0.17°C. This year was marked by catastrophic events, including deadly floods in Libya, intense cyclones in Mozambique and Mexico, and unprecedented wildfires in Canada, Chile, Greece, and Hawaii.

During 2023, James Hansen at Columbia University in New York, who had previously informed the US Congress that human activities were responsible for global warming, published a significant paper. This paper, co-authored with colleagues, argued that the warming rate had accelerated to around 0.32°C per decade post-2010.

The study primarily blamed this on the reduction of aerosol pollution from fossil fuel combustion, a “Faustian bargain” that had previously masked the warming effects of carbon dioxide emissions by reflecting sunlight and forming clouds.

With aerosol pollution being curtailed worldwide, leading to millions of deaths annually, the hidden warming is now becoming evident, thereby accelerating climate change, the paper stated.

China, as the largest emitter of fossil fuels, initiated a “war on pollution” starting with the 2008 Beijing Olympics, subsequently reducing its sulphur aerosol emissions by at least 75%.

Simultaneously, the International Maritime Organization has imposed stricter controls on sulphur emissions from ships. As the air over oceans is generally cleaner than over land, decreasing aerosols from ships leads to fewer clouds and diminished ship tracks—cloud lines trailing vessels.

These measures have resulted in a 40% decrease in global sulphur dioxide emissions since the mid-2000s. “The atmosphere is cleaner, so more solar radiation is coming in,” notes Samantha Burgess of the European Union’s Copernicus Climate Change Service.

This trend continued in 2024, surpassing 2023’s heat and exceeding 1.5°C above pre-industrial levels for the first time, posing a threat to the ambitious Paris Agreement targets. The high temperatures persisted into 2025, causing deadly heatwaves in Europe and destructive cyclones in South-East Asia and Jamaica.

While there is agreement that reduced aerosol emissions have quickened global warming, scientists debate the extent. Hansen and his colleagues reported a rate of 0.32°C per decade, which is higher than the 0.24°C rate estimated by the United Nations Intergovernmental Panel on Climate Change and the 0.29°C average from the latest climate models.

The debate is complicated by natural factors impacting global temperatures. In 2020, a particularly strong solar cycle began, increasing the sunlight reaching Earth. Then, in 2022, a massive underwater volcanic eruption near Tonga released 146 million tonnes of water vapour and some cooling sulphur aerosols into the atmosphere.

Additionally, a strong El Niño event in 2023 and 2024, characterized by weakened trade winds and warm water moving across the Pacific, further elevated global temperatures.

To determine the acceleration of global warming, scientists must isolate these natural influences to identify the emerging temperature trend. Less natural variability would indicate greater acceleration.

Recently, Stefan Rahmstorf from the University of Potsdam and US statistician Grant Foster concluded through statistical analysis that global warming has accelerated to approximately 0.36°C per decade since 2014.

Meanwhile, Michael Mann at the University of Pennsylvania argues that Rahmstorf, Hansen, and others have overestimated aerosol impacts while underestimating natural variability. Mann suggests that their research shows only a modest increase since the 1990s.

“Recent warmth, which is greatly influenced by the 2023-2024 El Niño event, is entirely consistent with standard climate model simulations,” Mann asserts. “There is no need to invoke any ad hoc mechanisms, including a supposed acceleration within the past decade.”

However, unforeseen climate feedback loops may also be contributing to the recent heat. Clouds, which are challenging to model due to their small and scattered nature, remain the most unpredictable factor.

A study led by Helge Goessling at the Alfred Wegener Institute in Germany attributes about 0.2°C of 2023’s warming to a decrease in low-lying clouds. While some cloud loss is due to reduced sulphur pollution, an “emerging low-cloud feedback” may also be responsible.

Traditionally, a temperature inversion formed by cold, moist air over the ocean and warmer, dry air above kept these layers separate. If climate change is disrupting this inversion by warming the cold air, it could lead to descending dry air, reducing moisture and cloud formation, Goessling explains.

“The more you warm up, the more you would be able to dissolve your low-level clouds,” he says. “It’s really most likely that there is a low-cloud feedback taking place.”

Should sulphur reductions mainly drive the acceleration, climate change might decelerate in future decades as sulphur pollution is minimized. However, if climate feedback loops are at play, warming could continue to escalate.

This scenario would suggest an underestimation of climate sensitivity—the degree of warming from increased atmospheric CO2.

“The worst-case scenario would be that this is coming from a cloud feedback that the models aren’t predicting and would mean that we actually have a much more sensitive climate than any of the models are projecting,” says Brian Soden at the University of Miami, Florida.

Current policies suggest the world is on a course for 2.7°C of warming by century’s end, with a margin of error of about plus or minus 1°C. If acceleration continues, Earth could approach 3.7°C of warming, necessitating even greater reductions in carbon emissions to avert severe consequences.

“3.7°C… makes some regions uninhabitable,” Hausfather warns. “2.7°C would still be bad, but many more regions could potentially adapt to that.”

Moreover, fossil fuel emissions are rising, a trend that must be reversed to limit impacts, according to Burgess.

“Global warming is a bit faster, but we’ve also lost time because we’ve not seen the ambitious measures to decarbonise our society,” she states.