A study published in Science attributes the significant temperature rise in 2023 to the Earth reflecting less solar energy back into space. Other scientists believe it is a combined effect of greenhouse gas emissions and significant natural climate variability.
This temperature surge is something scientists still cannot fully explain. Between June 2023 and August 2024, each month set a new heat record, and global temperatures have remained at record highs since. As a result, 2024 is expected to be the hottest year on record, surpassing pre-industrial levels by 1.5°C, which is the most ambitious target of the Paris Agreement.
« 2023 and 2024 have been much hotter than most scientists expected, » reports climate scientist Zeke Hausfather from Berkeley Earth. He describes it as an « unusual temperature spike » that cannot be fully explained by human-made greenhouse gas emissions or the natural El Niño phenomenon, which caused a temperature surge, particularly during its peak in December 2023. « We expected temperatures to decrease after the end of El Niño in spring, but that didn’t happen, and that’s a bit surprising, » agrees climate scientist Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies. Lacking a « clear and consensual explanation, » both scientists express discomfort.
These scientists are pondering a deeper question: Is this simply intense natural climate variability, or do recent data indicate the onset of a climate feedback loop that could lead to faster warming than expected?
A study published in Science on December 5th attempts to settle the debate. According to the findings, the sudden temperature rise observed in 2023 and 2024 is primarily due to the Earth becoming less reflective. « In 2023, the planetary albedo may have reached its lowest level since at least 1940, » reports Thomas Rackow of the European Centre for Medium-Range Weather Forecasts, one of the study’s authors. Albedo, the percentage of solar radiation reflected back into space, has been declining since the 1970s due to reductions in the Arctic and Antarctic ice caps.
More than the polar regions, however, it is the reduction of low-altitude clouds that appears to be a major factor. These clouds, located below 2,000 meters, such as stratus clouds, reflect solar radiation, producing a cooling effect. Their decrease is especially evident over the Atlantic, where unprecedented temperatures were recorded last year.
However, the study does not resolve a key question: Why are there fewer low-altitude clouds? Beyond natural variability, a reduction in aerosols, thanks to air pollution control measures and new restrictions on maritime transport, could be at play. These suspended particles play a role in cloud formation, in addition to reflecting some of the Sun’s rays. The most concerning hypothesis is that climate change itself could be reducing the number of low-altitude clouds. « This evaluation, useful as it is, raises as many questions as it answers, » reacts Zeke Hausfather, who was not involved in the study. « Does the reduction in low-altitude clouds indicate a new shift in the climate system? »
Climate scientist Michael Mann from the University of Pennsylvania dismisses the explanation involving the decrease in albedo, calling it a « real factor, but one that has been integrated into models for some time and cannot cause a sudden change. » He points to one culprit: El Niño, as suggested by a study published in October.
According to this study, the combination of a three-year cooling event, known as La Niña, which lowered global temperatures from 2020 to 2022, followed by an El Niño event, could explain the unexpected temperature rise in 2023. This explanation is not convincing to Gavin Schmidt and Zeke Hausfather.
« Internal Variability »
As for the persistence of high temperatures at the end of 2024, Michael Mann links it to the fact that a transition to a new La Niña did not occur in the fall as anticipated. Climate scientist Christophe Cassou adds other factors: a very particular El Niño event, which caused temperatures to rise earlier and last longer than usual, a decrease in aerosols, and an extremely warm Atlantic Ocean. The warming of this ocean basin was amplified by internal climate variability, according to a study co-authored by Cassou. « There was about a one in a hundred chance of this happening; it’s just bad luck, » he says. Michael Mann criticizes the tendency to « overestimate » the warming, which he believes is consistent with models. He draws a parallel with the reaction seen in the early 2010s, when many climatologists embraced the idea of a « pause » in warming, which was simply a case of natural variability.
Aurélien Ribes, a researcher at the National Centre for Meteorological Research, also urges caution in drawing premature conclusions. « The heat spikes observed in 2023 and 2024 are among the most extreme since 1940, but they are not implausible. They can be explained by an intense peak in internal variability and the rise in greenhouse gas emissions. The latter, along with the reduction of aerosols, is accelerating warming in line with models, » he says, echoing the conclusions of a study he co-authored, published in October.
This ongoing debate about global temperatures risks diverting attention « from the most important consequences of climate change in terms of impacts, which are happening at the regional level, » warns Christophe Cassou.
Antarctic Sea Ice Decline
Work published in November showed that, on all continents except Antarctica, regions are experiencing far more extreme heatwaves than expected by models. This acceleration has primarily occurred over the past five years and has most severely affected densely populated areas, such as Northwestern Europe and China. The coldest extremes are also underestimated, though to a lesser extent. « This is likely due to a mix of internal variability and feedback loops, such as the reduction in aerosols and the fact that drier soils heat up more, » explains Kai Kornhuber, the lead author and climatologist at the International Institute for Applied Systems Analysis.
Among other poorly understood processes, Antarctic sea ice reached its lowest extent ever recorded by satellite in 2023 and 2024. Its Arctic counterpart is not faring much better. « Regarding sea ice, we are clearly observing a break and the emergence of a new state with feedback processes, » notes Christophe Cassou, as this melting exacerbates warming. Globally, scientists agree on one point: humans are now in uncharted territory, as the current climate regime is radically different from that of the 20th century.
