Climate Change: A French Researcher Has Discovered What’s Behind the Atlantic’s Overheating

The unprecedented acceleration of global warming since 2020 has raised many questions. On land and in the oceans alike, temperatures have risen at an exceptional pace over the past six years.

For some scientists, climate warming driven by greenhouse gas emissions alone is not enough to explain the extraordinary overheating of surface waters in the Atlantic. Another phenomenon is thought to have played a significant role in the emergence of this marine heatwave: the reduction in pollution from large commercial ships.

80 percent less sulfur over the oceans

From 2020 to 2024, the North Atlantic Ocean experienced uninterrupted record temperatures, with each month warmer than the same month the previous year. In 2020, a new regulation was enacted by the International Maritime Organization, a specialized agency of the United Nations. Under this rule, the sulfur content in marine fuels was reduced from 3.5 percent to 0.5 percent.

Globally, sulfur emissions over the oceans fell by 80 percent within three years. Known as IMO2020, this regulation on ship pollution has been the subject of many studies since it came into force. Scientists do not all agree on its true impact, and a new study, this time led by a French researcher, casts doubt on its actual influence on the climate.

Thibault Guinaldo, a researcher at Météo France specializing in the analysis of climate extremes and an expert on marine heatwaves, published a study showing that IMO2020 did have an effect on the 2023 marine heatwave, but a very limited one.

According to his findings, the anti pollution regulation leads to:
• ‘a weak additional radiative forcing (0.06 to 0.13 W m⁻²)’;
• ‘a measurable but modest global warming (0.05 °C), far below the total warming observed (+0.27 °C over the 2015–2024 period)’.

‘The effect exists. It is robust. But it remains secondary,’ he says. ‘IMO2020 can only explain a very small fraction of the anomaly observed during the 2023 marine heatwave. It is neither a major trigger nor a sufficient condition to explain the intensity and persistence of the event.’

‘This kind of interpretation tends to obscure what really matters,’ he adds, namely ‘the central role of internal climate variability; the prior state of the ocean, particularly stratification; and above all, anthropogenic climate change.’

Sulfur will not save us from global warming

This, he explains, ‘acts on only one lever of anthropogenic climate change, the radiative balance. It does not address ocean acidification, biodiversity collapse, or the major impacts on ecosystems and human health’.

The radiative balance refers to the difference between the energy Earth receives and the energy it emits at the surface and into the atmosphere. ‘When this balance is neutral, meaning the amount of energy lost equals the amount received, the planet’s temperature remains stable, at around 15 degrees,’ explains the French space agency.

The sudden disappearance of sulfur pollution clouds disrupted Earth’s radiative balance, leading to an excess of heat. However, according to the conclusions of the Météo France researcher, this excess appears very small compared with the one caused by greenhouse gas emissions from human activity.

The disappearance of pollution clouds and their link to marine heatwaves has often been cited to justify the use of geoengineering. For some scientists and industrial players, reintroducing sulfur into the atmosphere would be enough to slow global warming. Yet even slight variations in atmospheric sulfur levels can create imbalances.

The study published by the French Academy of Sciences stresses that climate geoengineering ‘cannot under any circumstances replace policies aimed at reducing greenhouse gas emissions and preserving the carbon storage capacity of ecosystems, nor efforts to strengthen adaptation to climate change and the essential risk management needed to limit losses and damage’.