2025 Was Another Exceptionally Hot Year

This year will be the second warmest on record, surpassed only by 2024. It continues a recent trend of exceptional and unexplained warming. The last three years have been, by far, the warmest on record.

Each of the past three years has recorded a temperature more than 1.5°C warmer than pre-industrial times, putting the world, at least temporarily, in violation of the international goal to limit warming below that level, according to the Copernicus Climate Change Service.

The recent acceleration in warming, which has exceeded climate model predictions, has intrigued scientists. Experts have studied a range of potential factors, from a recent volcanic eruption in the South Pacific to a decline in sunlight-blocking pollution.

Building on recent research on these questions, climate scientist Zeke Hausfather examined four potential drivers of the warming wave in a recent analysis, published in Carbon File.

The 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, the recent increase in solar production. Although the timing fits well with the recent warming spike, Hausfather estimates that these factors can explain less than half of the temperature rise.

A third potential factor is the formation of a powerful El Niño in late 2023, when warm waters build up in the eastern Pacific, causing temperatures to rise around the world. But while El Niño can explain the exceptional heat of 2024, Hausfather shows, it cannot explain the rise in temperatures of early 2023.

Finally, there has recently been a dramatic drop in sulfur dioxide emissions. The pollutant, emitted by coal-fired power plants, blocks sunlight, thereby cooling the planet. Its decline has fueled warming. Over the past 18 years, sulfur dioxide emissions have fallen by 40 percent as countries, including China, have cleaned up pollution from coal burning. Then, in 2020, a new international rule reduced sulfur dioxide from cargo ships, leading to a sharp drop in pollution from shipping.

Scientists have focused on shipping to explain the sudden burst of warming. While research has generally shown that reduced pollution from shipping has only a modest impact on temperatures, a study by James Hansen, former chief climatologist at NASA, found that reduced pollution from shipping would explain almost all of the recent exceptional warming.

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

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