How Much Will Reduction in Shipping Emissions Stoke Global Warming?

A controversial new research paper claims that a major reduction in emissions of SO₂ (sulfur dioxide) since 2020, due to a ban on the use of high-sulfur fuels by ships, could result in additional global warming of 0.16 degrees Celsius (0.29 degrees Fahrenheit) for seven years – over and above that from other sources. The paper was published by a team of NASA scientists.

This example of the law of unintended consequences, if correct, would boost warming from human CO₂, as well as that caused by water vapor in the stratosphere resulting from the massive underwater eruption of the Hunga Tonga–Hunga Haʻapai volcano in 2022. The eruption, as I described in a previous post, is likely to raise global temperatures by 0.035 degrees Celsius (0.063 degrees Fahrenheit) during the next few years.

It’s been known for some time that SO₂, including that emanating from ship engines, reacts with water vapor in the air to produce aerosols. Sulfate aerosol particles linger in the atmosphere, reflecting incoming sunlight and also acting as condensation nuclei for the formation of reflective clouds. Both effects cause global cooling.

In fact, it was the incorporation of sulfate aerosols into climate models that enabled the models to successfully reproduce the cooling observed between 1945 and about 1975, a feature that had previously eluded modelers.   

On January 1, 2020, new IMO (International Maritime Organization) regulations lowered the maximum allowable sulfur content in international shipping fuels to 0.5%, a significant reduction from the previous 3.5%. This air pollution control measure has reduced cloud formation and the associated reflection of shortwave solar radiation, both reductions having inadvertently increased global warming.

As would be expected, the strongest effects show up in the world’s most traveled shipping lanes: the North Atlantic, the Caribbean and the South China Sea. The figure on the left below depicts the researchers’ calculated contribution from reduced cloud fraction to additional radiative forcing resulting from the SO₂ reduction. The figure on the right shows by how much the concentration of condensation nuclei in low maritime clouds has fallen since the regulations took effect.

The cloud fraction contribution is 0.11 watts per square meter. The other contributions, from a reduction in cloud water content and the drop in reflection of solar radiation, add up to a total of 0.2 watts per square meter extra radiative forcing averaged over the global ocean, arising from the new shipping regulations, the NASA scientists say.

The effect is concentrated in the Northern Hemisphere since there is relatively little shipping traffic in the Southern Hemisphere. The researchers calculate the boost to radiative forcing to be 0.32 watts per square meter in the Northern Hemisphere, but only 0.1 watts per square meter in the Southern Hemisphere. The hemispheric difference in their calculations of absorbed shortwave solar radiation (near the earth’s surface) can be seen in the following figure, to the right of the dotted line.

According to the paper, the additional radiative forcing of 0.2 watts per square meter since 2020 corresponds to added global warming of 0.16 degrees Celsius (0.29 degrees Fahrenheit) over seven years. Such an increase implies a warming rate of 0.24 degrees Celsius (0.43 degrees Fahrenheit) per decade from reduced SO₂ emissions alone, which is more than double the average warming rate since 1880 and 20% higher than the mean warming rate since 1980 of approximately 0.19 degrees Celsius (0.34 degrees Fahrenheit) per decade.

The researchers remark that the forcing increase of 0.2 watts per square meter is a staggering 80% of the measured gain in forcing from other sources since 2020, the net planetary heat uptake since then being 0.25 watts per square meter.

However, these controversial claims have been heavily criticized, and not just by climate change skeptics. Climate scientist and modeler Zeke Hausfather points out that total warming will be less than the estimated 0.16 degrees Celsius (0.29 degrees Fahrenheit), because the new shipping regulations have only a minimal effect on land, which covers 29% of the earth’s surface.

And, states Hausfather, the researchers’ energy balance model “does not reflect real-world heat uptake by the ocean, and no actual climate model has equilibration times anywhere near that fast.” Hausfather’s own 2023 estimate of additional warming due to the use of low-sulfur shipping fuels was a modest 0.045 degrees Celsius (0.081 degrees Fahrenheit) after 30 years, as shown in the figure below.

Further criticism of the paper’s methodology comes from Laura Wilcox, associate professor at the National Centre for Atmospheric Science at the University of Reading. Wilcox told media that the paper makes some “very bold statements about temperature changes … which seem difficult to justify on the basis of the evidence.” She also has concerns about the mathematics of the researchers' calculations, including the possibility that the effect of sulfur emissions is double-counted.

Next: Philippines Court Ruling Deals Deathblow to Success of GMO Golden Rice