Low-Sulfur Fuel Rules May Have Contributed to Record Ocean Temps

Ship tracks
Ship tracks off the coast of Spain (ESA / Copernicus)

Published Aug 3, 2023 11:41 PM by The Maritime Executive

If efforts to reduce carbon emissions fail, there may be a backup plan: geoengineering, the controversial science of modifying the climate by artificial means. The options include fertilizing the ocean's phytoplankton with iron, installing space-based mirrors or speeding up the weathering of CO2-absorbing minerals at a vast scale. While most are untested and potentially risky, one of them - manmade cloud seeding with SOx - has been in testing since the Age of Steam, and shipping is giving researchers the means to evaluate it.  

SOx emissions "seed" cloud droplets and make the air more reflective for as long as the droplets last. Bright white "ship tracks" seeded by sulfur dioxide from stack exhaust are a common demonstration of this effect, and can be seen in satellite photos of the North Atlantic. This added reflectivity has a localized cooling effect, which has long been recognized. Recent research suggests that the SOx from the stack exhaust may cause a cooling effect even when the "track" is too thin to be visible to the naked eye, and it may have a more substantial impact on temperature than previously thought. 

Shipping has historically been a large-scale SOx emitter because of its high-sulfur fuel, but the HFO tap was turned off in 2020 by the IMO. This handed climate researchers a readymade, global-scale experiment to test out SOx geoengineering - by examining what happens when the SOx stops. The early results suggest that IMO2020 had a big effect, and may even have caused part of the extreme ocean warming observed this year, according to a recent report in Science.

Last week, the North Atlantic posted its highest average temperature on record, just shy of 77 degrees F. There is still a month to go in the summer season and the record will likely broken again before the year is out, according to the National Oceanic and Atmospheric Administration (NOAA). According to Tianle Yuan, an atmospheric physicist at NASA’s Goddard Space Flight Center, the disappearance of air pollutants from HFO might be partly to blame for the surge in water temperature in this region. With less sulfur in today's fuel, there is less SOx in exhaust; with less SOx, less reflective clouds form, leading to less cooling and more heating. 

In a paper published last year, Yuan and his colleagues found that the IMO2020 fuel change cut cloud tracks in key shipping lanes by half. In a follow-up paper currently in review, they estimate that this has a substantial effect on warming, concentrated in areas with heavy shipping activity like the North Atlantic. The reduction in reflective cloud cover over this region, plus an unrelated reduction in reflective airborne dust levels, “can account for most of the warming observed” in the North Atlantic, Yuan told Science.

This has implications for future geoengineers. If reducing SOx causes more warming, the reverse should also be true, researchers say. “It suggests pretty strongly that if you wanted to do it on purpose, you could,” atmospheric scientist Michael Diamond told Science.