Evidence, uncertainty, and questions around record warm years 2023/2024

Key messages

• The years 2023/2024 saw record warmth. The unprecedented magnitude of warming, as well as a significantly elevated Earth energy imbalance (EEI), suggest an acceleration of global warming.
• The recent temperature surge cannot be fully explained based on the long-term warming trend and typical year-to-year fluctuations, even though the transition from La Niña to El Niño in 2023/2024 made it more likely. This suggests other factors are at play.
• The elevated EEI is driven by rising greenhouse gas levels, and exacerbated by the reduced reflection of sunlight from the planet, associated with fewer and less reflective clouds over the oceans (linked to declining aerosol emissions), as well as less ice cover.

Since 2023, global surface temperatures have shattered previous records. While the shift from La Niña to El Niño was expected to warm the planet, the intensity, extent, and persistence of the heat have been unprecedented, and temperatures remain elevated into 2025. This coincides with an elevated Earth energy imbalance (EEI): the difference between energy input from absorbed sunlight (shortwave radiation) and output in the form of infrared (longwave) radiation to space. Since 2000, the increasing in EEI has largely been due to the reduction in the sunlight being reflected from the planet (planetary albedo), resulting in a greater heating rate and so an acceleration of global warming. The role that feedback, declining aerosol particulate pollution, internal ocean variability, and other factors play in warming remain debated. Here, we assess how unusual the 2023/2024 level of warmth was in the context of climate variability, the role of elevated EEI in explaining this warmth, and the factors that may explain the elevated EEI, all of which have implications for the rate of climate change over the coming decades.

A large jump in global temperatures in this period was more likely thanks to the transition from a prolonged La Niña phase to an El Niño in 2023/2024. However, the recent surge does not entirely reconcile with the long-term warming trend combined with internal variability, particularly given that the 2023/2024 El Niño was not as strong as previous ones. This emphasises a need to investigate other contributing factors and to scrutinise changes in Earth’s energy budget.

Physically, the substantial warming from 2022 to 2023 was a consequence of how much heat was absorbed by Earth’s surface layers. The EEI during mid-2022 to mid-2023 more than doubled the 2006–2020 average and is at the upper level of what models predicted. Around 15–20% of this contributed to atmospheric and land warming and, to a lesser extent, the melting of ice, while the remainder heated the ocean. This heating is not sufficient to explain the magnitude of sea surface warming, unless it was concentrated in the shallow, upper-most ocean layers or additional heat from beneath the ocean surface was released during the transition to El Niño in 2023 and this added to the larger EEI heating from above (see Insight 2).

EEI increases since 2000 are mainly the result of the greater absorption of sunlight, which is primarily associated with reduced reflectivity over cloudy regions of the ocean. A moderate additional heating from the 11-year solar cycle also contributed to the warming (Figure 1). Effects from volcanoes, wildfires, and reduced Sahara dust in June 2023 are considered small at the global scale. A considerable factor in the elevated EEI and associated warming from 2001–2019 is reductions in sulphate aerosol pollution (Figure 1). Regulations implemented in 2020 reduced sulphur emissions from international shipping by ~80% creating a heating effect as less sunlight was reflected because of sulphate aerosol effects on clouds. This is mostly estimated to have a moderate effect on global surface warming, though there is a potentially large regional change over mid-latitude oceans in the Northern Hemisphere (Figure 1). Land-based anthropogenic aerosol emissions have also declined in recent decades. Aerosol emissions peaked over East Asia in the early 2000s and have since rapidly decreased, significantly contributing to global warming during 2010–2023 as well as to record high sea surface temperatures in the Northeast Pacific. While extra absorbed sunlight associated with declining East Asian aerosol is linked with long-term warming, their contribution to the 2023/2024 level is less obvious (Figure 1). It is thought that reducing aerosol emissions where pollution has already been mitigated somewhat, such as in East Asia or over the still moderately pristine open ocean, will have a larger effect on making clouds reflect less sunlight.

Uncertainties remain around the causes and implications of the record heat since 2023. Aerosol-cloud interactions and cloud feedbacks are difficult to model with precision, and coarse-resolution global models cannot adequately represent ship tracks, adding to the uncertainty of the impact of reduced sulphur emissions from shipping. A more robust quantification of the cloud feedback, including how the shrinking of cloud zones contributes to reduced planetary albedo, can inform to what extent global warming is accelerating because of these effects. EEI observations from the Clouds and the Earth’s Radiant Energy System (CERES) since 2000 are essential for improving models but are at risk as satellites age.

In summary, new insights add to the evidence that a combination of cloud feedback responses to global warming and reduced reflection of sunlight by clouds as aerosol emissions decline have plausibly contributed to the long-term increase in the absorption of sunlight by the planet since 2000. The exact relative importance of these drivers and the role of internal ocean variability in growing EEI have not been established. These are essential for reducing the range in climate sensitivity estimates, but existing analyses already indicate that very low estimates are unlikely. Extra planetary heating and a transition from an extended La Niña to El Niño in 2023 were instrumental in explaining the record global warmth in 2023/2024. Current levels of global temperature are consistent with a continued acceleration of global warming, and surpassing the 1.5°C threshold above pre-industrial conditions seems inevitable. Once again, the evidence underscores how essential rapid and massive cuts in greenhouse gas emissions are for limiting further warming and associated impacts on societies and ecosystems.