We can all agree on this. For many people more or less involved in studying, communicating, or simply trying to understand the climate crisis, the news of the warmest January on record — combined with the search for reasons why global warming has "suddenly" accelerated — has come as quite a blow. "Can't catch a break" might be the expression that best sums up the situation.

But why doesn't global warming seem to want to slow down this year, despite the end of the meteorological phenomenon known as the Southern Oscillation, known as El Niño, which defined the 2023–2024 two-year period?

Science was "surprised" by the January 2025 record

It was "normal" (or rather, predictable) that over those two years the surface temperature of the Pacific Ocean would be higher, and it was equally "normal" that the combination with the temperature rise caused by greenhouse gas emissions could prove explosive. What is not normal is that 2025 began without any slowdown in the trend of the preceding months. To be clear, nobody expected temperatures to suddenly return to average, but at the very least, people anticipated a break in the streak of records that has accompanied us in recent months. And yet — no. January 2025 was the warmest January on record, with the global average temperature reaching 13.23 degrees Celsius (°C), that is 0.79°C above the 1991–2020 average and 1.75°C above the pre-industrial average (spanning 1850 to 1900). The scientists at the Copernicus climate change service, the European service that monitors climate change, themselves said they were "surprised".

The answer to this scenario — which even institutional science had not foreseen (though some research had already begun exploring the question) — comes from a study published on 3 February in the scientific journal Environment: Science and Policy for Sustainable Development, conducted by, among others, the eminent — and that is precisely the right word — James Hansen. In brief: if the global average temperature was far higher than expected during the El Niño two-year period, and above all if there are currently no signs of the planet's thermometer retreating, the cause can be attributed to the reduction in sulphur dioxide (SOx) emissions from maritime shipping. A reduction brought about by the entry into force in 2020 of the regulation adopted by the International Maritime Organization (IMO), which lowered the limit on sulphur content in the fuel oil powering ships from 3.5 to 0.5 per cent.

What does sulphur have to do with Earth's albedo?

A paradox. These emissions, in fact, had two effects: on one hand, they contributed to increased atmospheric pollution (harmful to human health); on the other, they stimulated the formation of those tiny particles or droplets that float in the air and give rise to clouds (aerosols). Clouds that in turn acted as a reflector for solar rays. These emissions therefore, in their own way, limited the absorption of heat by the Earth's surface and oceans. The Earth, without these pollutants — and thus without a clear, soft, persistent cloud cover — has taken on darker hues that absorb more solar radiation and intensify global warming. It is as though curbing sulphur pollution has reduced the "albedo effect" of the clouds in the sky.

Ships were — and remain — the primary source of aerosols over the northern Atlantic and Pacific Oceans. This is why, according to the authors of the study Global warming has accelerated: are the United Nations and the public well-informed?, global warming amplified by the reduction in aerosols from maritime shipping will not slow down despite the onset of La Niña. A condition that could keep the global average temperature around 1.5°C in the coming years.

Let's not make excuses — there's pollution and then there's pollution

An important clarification is needed here: not all aerosols have "positive" effects on the climate. There are light-coloured particles — such as those produced by sulphur — that reflect incoming solar radiation in the atmosphere, contributing to cooling. But there are dark particles, such as soot, produced by the combustion of coal, oil and gas, or generated by large wildfires, the use of wood-burning stoves, or other industrial activities — these absorb solar radiation and further warm the atmosphere.

What does all of this mean? That, in the short term, we will face an increase in the frequency and intensity of extreme weather events — such as floods, heatwaves, but also tornadoes, storms that can develop into cyclones or typhoons. And ultimately, it could have far more rapid effects than those projected in the reports of the Intergovernmental Panel on Climate Change (IPCC) on the Atlantic Meridional Overturning Circulation (AMOC), driven by a massive influx of fresh water from melting glaciers into the North Atlantic. If sea level rise were to accelerate and, simultaneously, the current were to suddenly halt, a "point of no return" could be reached. All of this is unacceptable — hence the urgent need to increase knowledge and awareness (as the study's title itself suggests).

It-can-be-done

There is, therefore, reason for hope. This scenario confirms that when the international community acts decisively and in a coordinated manner, it can have concrete effects on the atmosphere and the health of the planet. It happened with the banning of chlorofluorocarbons to heal the ozone hole; it has happened with the reduction of sulphur emissions that cause harm to human health. Now it is time to do the same to rein in the dizzying rise in temperatures caused by fossil fuels.