1218. The Importance of Short-term Factors in Rising Temperatures

2025 started with record-breaking high temperatures, and researchers are now actively discussing how to account for the rate of ongoing climate change. An editorial in Nature Geoscience pointed out that quantifying the causes and effects of recent temperature increases could provide important insights into the future.

In January of this year, the world's average surface temperature was 1.75 °C higher than the pre-industrial climate. The unprecedented heat broke the record for the warmest period, which has been repeatedly broken starting in 2023. The 2023 temperature spike was expected in part due to a combination of human-induced climate change and the occurrence of El Niño, a phenomenon characterized by rising global temperatures. However, the magnitude of the spike was surprising, and many climate scientists expected some temperature decline once El Niño ended in late 2024. The continued record temperatures are puzzling, raising the question of whether they are natural variability or an accelerated human-induced warming.

As humans continue to emit carbon dioxide into the atmosphere, the Earth will continue to warm. But this relatively simple picture is complicated by natural oscillations and fluctuations between the interconnected components of the Earth system. Humans affect the climate in many ways, including through the emission of carbon dioxide and other short-lived climate forcing factors such as aerosols. Understanding how human and natural factors combine to produce short-term climate variability can help us better constrain the long-term response to carbon dioxide.

One of the main causes of natural climate variability is the El Niño Southern Oscillation, which consists of three phases: El Niño, La Niña, and neutral. Sea surface temperatures in the central and eastern tropical Pacific Ocean rise during El Niño periods and fall during La Niña periods, which is reflected in the global average temperature. A particularly strong El Niño in 2023, combined with a continued increase in greenhouse gases in the atmosphere, could explain some of the recent temperature spike. However, these factors alone do not appear to be sufficient to fully explain the magnitude of the recent temperature increase or its persistence as it transitions into a La Niña event.

A variety of other factors are thought to be contributing factors. The 2022 eruption of Hunga Tonga–Hunga Ha’apai volcano released approximately 150 million tons of water vapor into the stratosphere, which may have contributed to the heat in 2023. However, some studies suggest that the release of sulfur dioxide associated with the eruption may have had a net cooling effect. The recent increase in solar activity this year as it approached solar maximum may have also contributed slightly to the warming. However, taking all these factors into account, it seems that we have yet to find the source of heat.

The answer may lie in the clouds. The reduction in low cloud cover in the mid-latitudes and tropics of the Northern Hemisphere caused the Earth's albedo to reach its lowest on record in 2023. The reduced cloud cover decreased the reflection of incoming solar radiation, which in turn increased the surface temperature. This warming effect may be enough to explain the high temperatures in 2023. Further research is needed to investigate the role of clouds in the continued warming thereafter. The reasons for the cloud cover reduction are still under debate.

If the change in low cloud cover is simply due to natural variability, the associated warming is expected to subside. Alternatively, the reduction in cloud cover may be related to new international marine fuel regulations implemented in 2020 aimed at reducing sulfur emissions. These emissions act as cloud condensation nuclei to increase the brightness of low-level marine clouds, leading to the formation of long, highly reflective clouds called "ship tracks". Although the new regulations have reduced ship tracks that may have contributed to the recent warming, it is not yet clear whether they can fully explain the observed changes in low-level cloud cover. A third, more worrying explanation for the cloud cover decline is the emergence of a low-cloud feedback, in which low-cloud cover decreases with increasing temperatures, further intensifying warming. How clouds respond to warming remains one of the greatest uncertainties in understanding the climate response to carbon dioxide emissions. A strong low-cloud feedback could lead to more future warming than currently expected.

Pinning down the drivers of the recent anomalous warmth will be invaluable in constraining future trajectories. In particular, we need to identify the factors that caused the observed changes in cloud cover. Now more than ever, it is essential to understand the complex interactions between greenhouse gas-induced warming and near-term climate variability.

(Reference)
Temperature rising. Nat. Geosci. 18, 199 (2025). https://doi.org/10.1038/s41561-025-01663-x

Contributor: IIYAMA Miyuki, Information Program