The year 2025 is set to conclude as the second hottest year on record, surpassed only by 2024. This alarming trend continues a period of exceptional warming that has perplexed climate scientists. According to the Copernicus Climate Change Service, the last three years have marked the hottest on record, with each year measuring more than 1.5 degrees Celsius above preindustrial levels. This temperature rise represents a temporary breach of an international goal to limit warming to below that threshold.
Scientists are grappling with the reasons behind this unexpected spike in temperatures, which has exceeded the predictions of climate models. Various factors are being studied, ranging from volcanic activity to changes in atmospheric pollution. Climate scientist Zeke Hausfather recently analyzed four potential drivers of this warming surge in a publication for Carbon Brief.
One significant factor is the 2022 eruption of the Hunga Tonga–Hunga Ha‘apai volcano in the South Pacific. This underwater volcano released a substantial plume of heat-trapping water vapor into the upper atmosphere, contributing to the global temperature rise.
Another aspect under consideration is the recent increase in solar output. While the timing of this uptick aligns with the warming trend, Hausfather asserts that these factors alone can explain less than half of the overall temperature increase.
The formation of a powerful El Niño event in late 2023 further complicates the analysis. El Niño, characterized by warm waters pooling in the eastern Pacific, typically drives up global temperatures. Although it may account for the exceptional warmth observed in 2024, it does not explain the earlier temperature rise in 2023.
Lastly, a notable drop in emissions of sulfur dioxide—primarily produced by coal-fired power plants—has emerged as a significant contributing factor. This pollutant traditionally helps cool the planet by blocking sunlight. Over the past 18 years, sulfur dioxide emissions have declined by 40 percent, particularly as nations like China have implemented stricter pollution controls. Additionally, a 2020 international agreement to reduce sulfur dioxide emissions from cargo ships has further decreased this pollutant, potentially leading to increased warming.
While research has typically found that reductions in shipping emissions have only a modest impact on global temperatures, a study by James Hansen, the former chief climate scientist at NASA, suggests that the decline in shipping pollution could account for nearly all of the recent exceptional warming.
Together, these four factors may explain the recent temperature surge. Nonetheless, questions linger regarding the nature of this warming. Scientists are left pondering whether this increase is a temporary fluctuation or indicative of an accelerating trend in climate change. As global temperatures continue to rise, the urgency for comprehensive climate action becomes increasingly evident.
