The past three years were exceptionally warm. New research better explains why these years stood out even more in an already warming climate. The main causes appear to be natural variations (such as an active phase of the solar cycle and El Niño in 2024) and reduced sulfur emissions from shipping.
Why were the years 2023-2025 so warm worldwide?
Although the Earth has been warming strongly for years, temperatures in the past three years (2023-2025) were 0.1–0.2°C higher than in the previous ten years. This seems small, but compared to the average warming of 0.25 °C per decade over the last 30 years, such a jump in one year is remarkably large. Figure 1 shows that since 1850, it has only happened twice before that a year stood so far above the trend line.
Earlier, we discussed possible causes of this extra warming: a positive phase of El Niño, which played a role especially in 2024, decreasing air pollution and thus less reflective fine particles in, for example, East Asia but also from shipping. The eruption of the Hunga Tonga volcano in 2022 may also have played a role, as well as variations in forest fires and desert dust. And there has been a decrease in cloud cover for some time, resulting in less sunlight being reflected. As a result, the Earth absorbs more sunlight than it emits as heat radiation. But, as we previously wrote:
In the coming years, more research will certainly be done to better understand the 2023-2024 peak in warming, partly because an important question underlies it: is the warming accelerating?
New research
New research has now provided better insight into the causes of the extra warm years 2023 and 2024 and confirms some earlier suspicions. Although the decrease in cloud cover has contributed significantly to warming in the 21st century, there are other causes for the extra warm years 2023 and 2024. In 2023, these are mainly the active phase of the solar cycle (+0.05 Celsius) and the decrease of sulfur emissions from ships (+ 0.05 Celsius). In 2024, a positive phase of El Niño adds another +0.1 Celsius.
Other possible influences seem limited. The warming effect of extra water vapor in the stratosphere caused by the eruption of the Hunga Tonga volcano in 2022 appears largely offset by more reflection of stratospheric sulfur from the same eruption. Effects of less fine dust in the air in East Asia and effects of variations in forest fires and desert dust are also estimated to be small.
Also, 2025 ended high in the ranking of warmest years, comparable to 2023. The warming from El Niño then disappeared because the climate shifted to La Niña, but the other factors – the continued reduction of sulfur emissions and the phase of the solar cycle – were still present. It is therefore likely that these two causes also contributed to the high temperatures in 2025.
Accelerated increase of energy in the climate
The causes of the extra warm years 2023-2025 are diverse but do not provide a sufficient explanation for the ever faster increase of energy in the climate system as clearly seen in the increase in total heat content of the oceans since 1990.