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A study published in Environment: Science and Policy for Sustainable Development looks at shipping aerosol emissions and the rate of global warming.
Dr Karsten Haustein, Climate Scientist, Leipzig University, said:
“Jim Hansen and colleagues have revisited the topic of aerosol-induced warming due to reduced shipping emissions (due to regulatory changes in 2020). It’s a more credible attempt than their last – rather disappointing – effort, but there is still much speculation involved. They estimate the global aerosol forcing from reduced shipping aerosols might be as high as 0.5 W/m2, which is far higher than the current estimates of 0.05-0.15 W/m2. They argue that Earth’s radiative imbalance as well as high levels absorbed solar radiation justify such assumption. Accordingly, they argue that Climate Sensitivity (temperature response after CO2 doubling in the atmosphere) might be as high as 4.5 °C.
“Given that Earth’s radiative imbalance has considerably come down in the 2nd half of 2024 (notwithstanding the uncertainties related to measuring the global radiative imbalance), I continue to remain skeptical of their claims. This is particularly true, as some of the extra warming could be traced to other internal factors that have not been discussed. The so-called ‘hiatus’ discussion in the 2010s should be an example of a cautionary tale. This is true all the more as we know with some certainty that CO2 and methane (CH4) forcing has continued to accelerate slightly, such that additional aerosol forcing increase is not necessarily required to explain what has happened in 2023 and 2024.
“They are correct in one aspect though: 2025 will prove whether there is more to the warming story than we thought.”
Prof Richard Allan, Professor of Climate Science, University of Reading, said:
“Multiple lines of evidence are showing that human caused climate change is gathering pace. Heat is continuing to flood into the climate system as atmospheric greenhouse gases continue to rise and the reflective haze of aerosol particle pollution diminishes in some regions following clean air policies. This is causing the warming of the oceans to increase at ever greater rates.
“The comprehensive, extensive and wide-ranging new report argues that masking of global warming by particle pollution has been underestimated and future climate change may be even worse than anticipated. Cleaning up dirty air may be having a larger than expected effect on increasing how much sunlight reaches the ground, which is adding to a more potent greenhouse effect from continued fossil fuel emissions. The arguments presented are not new and although reasonable they appear overly bleak compared to the growing body of scientific research. However, the magnitude of increases in Earth’s heating rate and ocean surface warming, as well as record January global temperatures despite an expected cooling from La Niña, mean that scientists are carefully scrutinising and puzzling over the unfolding changes to Earth’s climate. And the new report emphasises the urgent need to cut greenhouse gas emissions and to properly account for the full economic cost of our actions on the planet and people.”
Prof William Collins, Professor of Climate Processes, University of Reading, said:
“This paper suggests that the cooling effects of aerosols has been underestimated and hence this has hidden more of the warming effect of greenhouse gases than has previously been assessed. This would make the climate sensitivity to carbon dioxide larger than has been assessed. If this is the case then cleaning up aerosol pollution (as has happened with shipping since 2020) will uncover more of the underlying warming from greenhouse gases. Aerosol pollution peaked in the 1980s, when studies have increased the cooling effect of aerosols their calculations give cooler temperatures in the 1980s than we observed. So this paper sits outside most previous assessments of the strength of aerosol cooling.
“There have been several assessments of the recent decline in shipping aerosols. These range from a negligible effect on the record-breaking 2023 temperatures to a small contribution. It will require detailed comparisons with these previous studies to determine why the shipping contribution in this paper is so much more significant.”
‘Global Warming Has Accelerated: Are the United Nations and the Public Well-Informed?’ by James E. Hansen et al. was published in Environment: Science and Policy for Sustainable Development at 14:00 UK time on Tuesday 4 February 2025.
DOI: 10.1080/00139157.2025.2434494
Declared interests
Dr Karsten Haustein: “No conflict of interests.”
Prof Richard Allan: “No competing interests.”
Prof William Collins: “No conflicts.”