2025 Was Another Exceptionally Hot Year

This year will conclude as the second hottest on record, surpassed only by 2024. It continues a recent trend of exceptional, unexplained warming. The last three years have been, by a wide margin, the hottest ever recorded.

Each of the last three years has measured more than 1.5 degrees C warmer than preindustrial times, putting the world at least temporarily in breach of an international goal to limit warming below that level, according to the Copernicus Climate Change Service.

The recent jump in warming, which exceeded the predictions of climate models, has puzzled scientists. Experts have explored a range of potential factors, from a recent volcanic eruption in the South Pacific to a drop in sunlight-blocking pollution.

Drawing on recent research into these questions, climate scientist Zeke Hausfather looked at four potential drivers of the warming surge for a recent analysis, published in Carbon Brief.

First was the 2022 eruption of Hunga Tonga–Hunga Ha‘apai, an underwater volcano in the South Pacific, which sent a massive plume of heat-trapping water vapor into the upper atmosphere. And second was the recent uptick in solar output. Though the timing aligns well with the recent spike in warming, Hausfather finds these factors can explain less than half of the jump in temperature.

A third potential factor was the formation in late 2023 of a powerful El Niño, when warm waters pooled in the eastern Pacific, driving up temperatures worldwide. But while El Niño may explain the exceptional warmth in 2024, Hausfather shows, it cannot account for the jump in temperatures in early 2023.

Lastly was the recent dramatic drop in emissions of sulfur dioxide. The pollutant, which issues from coal power plants, blocks sunlight, thus cooling the planet. Its decline has fueled warming. In the past 18 years, sulfur dioxide emissions have declined by 40 percent as countries, namely China, cleaned up pollution from burning coal. Then in 2020, a new international rule cut sulfur dioxide from cargo ships, leading to a steep drop in pollution from shipping.

Scientists have focused on shipping as an explanation for the sudden warming surge. While research has generally found that cuts to shipping pollution have only had a modest impact on temperatures, one study, from James Hansen, formerly the chief climate scientist at NASA, found that the drop in shipping pollution would explain nearly all of the recent, exceptional warming.

Hausfather says that, together, all four factors may explain the recent surge. But even if they can, he says, questions remain: Is the surge temporary, or is it a sign that warming is accelerating?

ALSO ON YALE E360

How Climate Risks Are Putting Home Insurance Out of Reach