New James Hansen paper links accelerating climate change to new aerosol regulations for ships

A new paper claims that average temperatures have shifted permanently above 1.5C as a result of an obscure factor: recent regulations on shipping.

A recent study led by legendary climatologist James Hansen presents compelling evidence that new International Maritime Organization (IMO) regulations on ship emissions have contributed significantly to the rapid acceleration of global warming observed since 2010.

These regulations, aimed at reducing sulfur dioxide (SO2) emissions to improve air quality, have also had an unintended consequence: the loss of cooling effects from sulfate aerosols that previously offset some greenhouse gas (GHG) warming.

Is global warming accelerating?

Hansen’s paper highlights that since 2010, the rate of global warming has increased by more than 50% compared to the 1970–2010 warming rate of 0.18°C per decade. This trend has caused Earth’s temperature to reach levels not seen in at least 120,000 years, comparable to the extreme warm Eemian interglacial period. The rapid warming observed during the recent El Niño event further emphasizes the severity of this trend, with global temperatures increasing by 0.4°C—far exceeding previous El Niño-driven warming events.

How do aerosols influence radiative forcing?

For decades, climate scientists have recognized the dual role of human-made pollutants: while greenhouse gases trap heat and drive warming, aerosols—tiny particles emitted from burning fossil fuels and biofuels—reflect sunlight back into space, cooling the planet. Charlson et al. (1992) initially demonstrated that aerosol cooling was comparable in magnitude to GHG warming, effectively masking the full extent of anthropogenic climate change.

However, this was a "Faustian bargain," as aerosol pollution also contributes to millions of deaths annually due to respiratory and cardiovascular diseases. As a result, regulatory efforts to improve air quality—such as the IMO’s strict sulfur limits on ship fuels—have inadvertently led to a rapid reduction in aerosol-driven cooling, revealing the full force of GHG warming.

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What does this mean for estimations of climate sensitivity?

Hansen’s research challenges previous estimates by the Intergovernmental Panel on Climate Change (IPCC), suggesting that aerosol climate forcing has been underestimated. The paper argues that:

• IPCC’s reliance on climate models led to a systematic underestimation of both aerosol forcing and climate sensitivity.
• The sudden reduction in ship aerosol emissions in 2020 has likely contributed an additional +0.5 W/m² of climate forcing, far exceeding the IPCC’s estimated +0.079 W/m².
• This discrepancy helps explain the unexpected surge in global temperatures in 2023-2024, which had baffled many climate scientists.

Data from the Clouds and the Earth’s Radiant Energy System (CERES) satellite and deep-diving Argo floats provide critical evidence supporting Hansen’s conclusions. Since 2010, Earth’s albedo has declined significantly, meaning the planet is absorbing more solar radiation than before. The additional 1.7 W/m² of absorbed energy since 2010 is equivalent to the heating effect of an atmospheric CO2 increase from 419 ppm to 557 ppm, underscoring the profound impact of aerosol reductions.

Verifying the role of changes to shipping emission

The study reveals a strong geographic correlation between regions where ship traffic historically produced high sulfate emissions and areas experiencing the most dramatic warming. Specifically:

• The North Pacific and North Atlantic—key shipping lanes—have exhibited the largest increases in absorbed solar radiation and sea surface temperature (SST).
• The post-2020 warming in these regions aligns with the timeline of IMO’s sulfur regulations.
• This pattern suggests a direct link between reduced ship emissions and enhanced warming.

What does this mean for the future?

Given these findings, Hansen warns that Earth’s energy imbalance will continue to drive warming unless aggressive action is taken to curb GHG emissions.

The study emphasizes that there is an urgent need for aerosol monitoring, since improved global observations of aerosol-cloud interactions are necessary to refine climate models and better predict future changes.

Consequently, if aerosol cooling was masking more warming than previously thought, we’ll need to raise our climate sensitivity estimates. Indeed, this raises the prospect of solar radiation modification (SRM). Hansen et al. cautiously suggests that purposeful climate interventions, such as stratospheric aerosol injection, may need to be considered to temporarily counteract warming while transitioning to net-zero emissions.

Taking the Faustian bargain

Hansen’s study underscores the precarious nature of balancing air quality improvements with climate stability. The rapid warming seen in 2023-2024 is not merely an anomaly but a reflection of long-hidden warming forces now being unleashed due to aerosol reductions. Without immediate and substantial reductions in GHG emissions, global temperatures could exceed +2°C within the next two decades, pushing the planet toward irreversible climate tipping points.

This research serves as a stark reminder that addressing climate change requires a comprehensive approach—one that considers not only GHG reductions but also the unintended consequences of pollution control measures. Future policies must integrate climate science insights to ensure that efforts to improve public health do not inadvertently accelerate planetary warming.

To read the full paper, click here.