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Maritime Shipping sulfur emissions have effectively been a multi-decade experiment in sulfate aerosol geoengineering, with some key differences to stratospheric aerosol injection (SAI)

Summary​

Maritime shipping's use of high-sulfur bunker oil (2.5–3.5% sulfur) inadvertently created sulfate aerosols through SO₂ emissions, mimicking some effects of geoengineering. The 2020 IMO regulations reduced fuel sulfur content from 3.5% to 0.5%, to improve air quality and public health. This abrupt reduction appears to have created a "termination shock" with measurable warming impacts post 2020. Shipping sulfur emissions mask warming primarily through marine cloud brightening but also via light scattering. The event demonstrates the climate risks of rapid aerosol reductions and complex trade-offs between air quality improvements and unintended warming. Current estimates show shipping aerosols contributed -0.76 W/m² of radiative forcing.

The sulfur dioxide (SO₂) emissions from maritime shipping have effectively been a multi-decade experiment in SAI geoengineering, except for a few key differences.

Sulfur Content of Maritime Shipping Oil​

Sulfur is present in crude oil when it is extracted from the ground, which is why crude oil often carries the classification of "sweet," with a sulfur content of less than 0.5%, and "sour," with a sulfur content greater than 0.5% 1. Through the refining process known as fractional distillation, crude oil is separated into its various components such as gasoline, diesel, and kerosene, leaving most of the sulfur behind. What remains is a thick black substance known as "bunker oil" or "heavy fuel oil," which is traditionally used by the shipping industry for its relatively low price. The sulfur content of this bunker oil is typically between 2.5% and 3.5% 2. For comparison, the maximum regulated sulfur content for automotive gasoline in California is 0.0016% 3.

International Maritime Organization (IMO) Sulfur Regulations​

In 2020, the IMO introduced new regulations to reduce the maximum allowable sulfur content of shipping fuel from 3.5% down to 0.5% 4. The burning of high-sulfur-content fuels was already regulated for ships near coastlines and in certain oceans, but this new regulation includes use in open waters of all the world's oceans. These regulations were introduced to protect public health, with estimates of premature deaths resulting from shipping fuel emissions being as high as 90,000 per year.

Key Differences Between Sulfate Aerosols from Shipping Fuels and SAI​

Sulfur emissions from shipping fuel appear as an analog for SAI due to their wide dispersal area. Satellite mapping of global atmospheric sulfate shows clear bands known as "ship tracks" along established shipping routes where. However, besides being an unintentional byproduct of international shipping, these bands are temporary as the sulfur is only released into the lower atmosphere (troposphere). This is the main difference from SAI, which is an intended release sulfur into the stratosphere. In the lower atmosphere, precipitation quickly removes sulfur from the air, which limits the overall effectiveness of sulfur aerosols as a cooling agent. Additionally, the majority of the cooling effect of sulfate aerosols from shipping fuels is due to Marine Cloud Brightening (MCB), which is related to cloud nucleation from aerosols, not the light scattering effect of aerosols seen in SAI 2. However, there is evidence of substantial cooling in shipping corridors even when visible ship tracks are not present, signifying a non trivial cooling effect from sulfate light scattering 5 6.

The Albedo Effect of Sulfur from Shipping Fuels​

When shipping fuel is burnt, it releases sulfur dioxide (SO₂) into the atmosphere, which then reacts to form sulfate aerosols. These aerosols can increase the Earth's albedo and help in reducing global temperatures. These aerosols are effective as they provide aerosol coverage over open ocean, which has high absorption rates for solar radiation. The degree of negative radiative forcing from sulfur aerosols is heavily disputed, with a wide variety of estimates. The IPCC uses a conservative estimate of global anthropogenic aerosol forcing (including all aerosol types) of around -1.00 W/m² 7, whereas recent estimates for sulfate shipping aerosols have been as much as -0.76 W/m² 5. As a comparison, the radiative forcing from anthropogenic greenhouse emissions is estimated to be around +4.1 to +4.6 W/m², making consideration of sulfate aerosols from shipping an important factor in future climate change predictions 5.

Sulfur Regulations and Termination Shock​

There are concerns and a growing body of evidence that indicates that the abrupt addition of tighter sulfur regulations has effectively caused a "termination shock", resulting in a rapid rise in global temperatures since 2020 2.

Sources​

Footnotes​

  1. Yashchenko, I. G., & Polishchuk, Y. M. (2019). Classification approach to assay of crude oils with different physicochemical properties and quality parameters. Petroleum Chemistry, 59, 1161–1168. ↩

  2. Yuan, T., Song, H., Oreopoulos, L., et al. (2024). Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming. Communications Earth & Environment, 5, Article 281. https://doi.org/10.1038/s43247-024-01442-3 ↩ ↩2 ↩3

  3. California Air Resources Board. (2020). Rule 431: Sulfur content of fuels (Original work published 1976). https://ww2.arb.ca.gov/sites/default/files/classic/technology-clearinghouse/rules/RuleID4734.pdf ↩

  4. International Maritime Organization. (2020). IMO 2020 sulfur oxide emission regulation. https://wwwcdn.imo.org/localresources/en/MediaCentre/HotTopics/Documents/2020%20sulphur%20limit%20FAQ%202019.pdf ↩

  5. Hansen, J. E., Sato, M., Simons, L., Nazarenko, L. S., Sangha, I., Kharecha, P., Zachos, J. C., von Schuckmann, K., Loeb, N. G., Osman, M. B., Jin, Q., Tselioudis, G., Jeong, E., Lacis, A., Ruedy, R., Russell, G., Cao, J., & Li, J. (2023). Global warming in the pipeline. Oxford Open Climate Change, 3(1), Article kgad008. https://doi.org/10.1093/oxfclm/kgad008 ↩ ↩2 ↩3

  6. Lack, D. A., Corbett, J. J., Onasch, T. B., Lerner, B. M., Massoli, P., Quinn, P. K., Bates, T. S., Covert, D. S., Coffman, D. J., Sierau, B., Herndon, S. C., Allan, J. D., Baynard, T., Lovejoy, E. R., Ravishankara, A. R., & Williams, E. J. (2009). Particulate emissions from commercial shipping: Chemical, physical, and optical properties. Journal of Geophysical Research: Atmospheres, 114(D00F04). https://doi.org/10.1029/2008JD011300 ↩

  7. Partanen, A.-I., Laakso, A., Schmidt, A., Kokkola, H., Kuokkanen, T., Pietikäinen, J.-P., Kerminen, V.-M., Lehtinen, K. E. J., Laakso, L., & Korhonen, H. (2013). Climate and air quality trade-offs in altering ship fuel sulfur content. Atmospheric Chemistry and Physics, 13(23), 12059–12071. https://doi.org/10.5194/acp-13-12059-2013 ↩