+1.5°C and a global alert level for the oceans

The year 2024 will go down as a milestone in our planet’s climate history. For the first time, the Earth’s average global temperature has exceeded the critical threshold of 1.5°C above pre-industrial levels. The ocean, the backbone of the climate system, has already absorbed over 90% of the excess heat generated by our human activities. It is now in a state of overheating, with record temperatures set for 2024 both at the surface and in its deepest parts.

By Laurie Henry

Cover photo: The Southern Ocean is experiencing one of the fastest rates of warming. © Chao Ban

A recent study led by Professor Lijing Cheng of the Institute of Atmospheric Physics (Chinese Academy of Sciences), in collaboration with 54 international scientists, reveals in the journal Advances in Atmospheric Sciences that the oceans have absorbed record amounts of heat, exceeding thresholds observed since modern measurements began in 1950.

The consequences are already manifesting themselves in an intensification of extreme weather events, accelerated degradation of marine ecosystems and global climate upheavals. At a time when global temperatures have exceeded those of the pre-industrial era by +1.5°C, these new data underline the urgency of rethinking our relationship with fossil fuels and the environment. The results of this study will be used to update ocean monitoring indicators.

Warmer oceans at the surface and at depth

Between 2023 and 2024, the amount of heat stored in the first 2,000 meters of the oceans increased by an unprecedented 16 zettajoules. This energy, equivalent to 140 times the world’s electricity production in 2023, illustrates the capacity of the oceans to store most of the excess heat caused by global warming. At the same time, the average surface temperature of the oceans rose by 0.07°C compared with the previous year.

These figures confirm an alarming acceleration observed over several decades, attributed to the steady increase in greenhouse gas emissions. According to scientists, these data point to systemic climate change with global implications. “ If you want to know what’s happening to the climate, the answer lies in the ocean,” said Professor John Abraham of the University of St. Thomas, co-author of the study.

Temporal evolution of the mean temperature anomaly measured at the ocean surface since 1950 © Cheng et al., 2025

This rise in ocean temperature has direct consequences for climate cycles and ecosystems. The growing intensity of heat exchange between the ocean and atmosphere amplifies extreme events such as cyclones, floods and prolonged droughts. The heat stored by the oceans also encourages the formation of marine heat waves, disrupting sensitive marine ecosystems. These disturbances affect marine biodiversity on a large scale, with forced migration of species and reduced ecosystem resilience.

But ocean warming is not occurring uniformly on a global scale, with significant regional variations amplifying local climate challenges. The North Atlantic recorded surface temperatures above 20.87°C on average, marking a significant increase on the 1991-2020 baseline, as did the Mediterranean. Similarly, the Indian Ocean saw record-breaking heat increases, particularly in its southern areas, where temperatures far exceeded seasonal averages. In the North Pacific, the coastal regions of Japan and the United States saw particularly marked temperature differences, largely due to persistent marine heat waves.

Geographical distribution of the anomaly in the heat content stored by the oceans between the surface and 2000 meters depth, highlighting strong regional variations. © Cheng et al., 2025

By contrast, the tropical regions of the Pacific Ocean showed a more limited increase due to the influence of the La Niña cycle at the start of the year, followed by the appearance of an El Niño event later in 2024. In the Southern Ocean, mean temperatures have also risen, impacting the dynamics of circumpolar currents and accelerating ice melt. These regional variations reflect not only the effects of natural climate oscillations, but also the growing influence of human activities on oceanic climate variability.

Human and environmental consequences

Rising ocean temperatures are therefore acting as a catalyst for major environmental upheavals, primarily affecting already fragile marine ecosystems. Coral reefs, for example, which are essential to biodiversity and coastal communities, are experiencing large-scale bleaching, particularly on the Great Barrier Reef in Australia, as a direct consequence of marine heat waves that reduce the capacity of corals to regenerate and compromise critical habitats for a multitude of marine species.

High temperatures also promote the appearance of dead zones, deprived of oxygen, which directly threaten fish populations and affect the livelihoods of millions of people dependent on fishing. Impacts are not limited to the oceans, as coastal ecosystems are also suffering from increased erosion and saline intrusion exacerbated by rising sea levels, exacerbating the vulnerabilities of human communities.

Temporal evolution of ocean heat content in the upper 2,000 meters, since 1950 (from the Institute of Atmospheric Physics). The blue and red colors indicate whether a particular year was colder or warmer (respectively) than the 1981-2010 reference period. This period serves as a basis for scientists to compare conditions with reference conditions. © Cheng et al., 2025

These environmental disturbances have a direct impact on extreme weather events. Increased ocean heat releases more water vapour into the atmosphere, increasing precipitation and fuelling more intense storms. In 2024, these dynamics amplified climate catastrophes around the world: historic forest fires in Los Angeles, devastating floods in Nigeria and prolonged heat waves in Europe. Researcher Kevin Trenberth explains: “This intensification of hydrological cycles leads to prolonged droughts in some regions and extreme floods in others, highlighting a growing imbalance. These disasters have cost billions of dollars in economic and human losses, and continue to highlight the interconnection between natural and human systems”.

Exceeding the 1.5°C threshold, a global warning

In 2024, the planet crossed the symbolic threshold of 1.5°C above pre-industrial levels for the first time, a limit set 10 years ago by the Paris Agreement, which must not be exceeded to avoid the most catastrophic climate impacts. Even if this threshold was not reached over the 365 days of the year in 2024, this level nevertheless confirms the acceleration of global warming. This +1.5°C anomaly, partly attributed to natural events such as El Niño, above all highlights the cumulative effect of man-made greenhouse gas emissions.

As Copernicus Director Carlo Buontempo explains : “this situation does not represent a point of no return, but rather an urgent warning to take decisive action to reverse the trajectory. Each additional fraction of a degree increases the risk of major disruptions, from extreme weather events to the irreversible loss of critical ecosystems”.

Monthly variations in global surface air temperature (°C) relative to the average of the pre-industrial reference period (1850-1900) from 1940 to 2024 (one color per decade) © Copernicus C3S / ECMWF

At the same time, atmospheric concentrations of carbon dioxide (CO₂) reached a record 422 ppm in 2024, a level more than 50% higher than in the pre-industrial era (280 ppm). This steady build-up of CO₂, accompanied by rising levels of other greenhouse gases (such as methane), reinforces the global warming phenomenon by trapping more heat in the atmosphere, and creates a vicious circle such as in polar zones where high temperatures accelerate the thawing of permafrost by releasing methane, a potent greenhouse gas that in turn exacerbates warming. This new study therefore confirms the urgent need for a global energy transition to reduce emissions and prevent even more profound and irreversible climate and environmental changes.

Source : L. Cheng et al., “Sea Surface Temperatures and Deeper Water Temperatures Reached a New Record High in 2024”, Advances in Atmospheric Sciences (2025)