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There is growing evidence that reductions of fuel sulfur content regulated by the International Maritime Organization (IMO) has lead to a 'termination shock' that has contributed as much as 80% of all planetary heat uptake since 2020

Summary​

The International Maritime Organization's (IMO) 2020 regulations reducing sulfur content in shipping fuels from 3.5% to 0.5% have inadvertently caused a 'termination shock' effect, removing sulfate aerosols that previously provided cooling through marine cloud brightening and light scattering. This abrupt reduction in atmospheric particulates contributed 0.2 – 0.27 W/m² of additional radiative forcing. Recent estimates suggest these changes may account for 80% of observed planetary heat uptake since 2020, demonstrating how sudden reductions in anthropogenic aerosols can accelerate global warming. The episode serves as a real-world example of risks of rapid cessation of solar geoengineering.

The sulfur dioxide (SO₂) emissions from maritime shipping have effectively been a multi-decade experiment in SAI geoengineering, but 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. In the context of Stratospheric Aerosol Injection (SAI) and geoengineering, termination shock is defined as the rapid and substantial rise in global temperatures following the cessation of SAI deployment. The light scattering effect and Marine Cloud Brightening (MCB) resulting from sulfate aerosols formed from sulfur dioxide (SO₂) emissions in shipping fuel increase the Earth's albedo above oceans and have contributed to a net cooling effect on global temperatures 1.

In an attempt to reduce the public health impact of sulfur dioxide pollution, the International Maritime Organization (IMO) tightened sulfur regulations to a maximum of 0.5% sulfur content in shipping fuels in 2020 2. Following this, ship tracks, which are a strong indicator of local aerosols, reduced globally by 50% 3. Estimates on how much this reduction in aerosol coverage has contributed to recent radiative forcing are up to 0.2 W/m² to 0.27 W/m² of positive radiative forcing 3 2. To put this number in context, the radiative forcing from anthropogenic greenhouse gas emissions is estimated to be around +4.1 to +4.6 W/m², and global temperatures are estimated to increase by around 1.2°C for every +1 W/m² of radiative forcing 1.

The termination shock associated with these sulfur regulations cannot be overstated, with as much as 80% of all planetary heat uptake since 2020 being attributed to the changes resulting from the IMO sulfur regulations 2. The long-term addition of aerosols into the atmosphere and the abrupt cessation is a strong example of the effectiveness of solar geoengineering techniques, whilst also being a strong cautionary tale of the risks of termination shock.

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Footnotes​

  1. 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), kgad008. https://doi.org/10.1093/oxfclm/kgad008 ↩ ↩2

  2. Yuan, T., Song, H., Oreopoulos, L., Wood, R., Wang, C., Platnick, S., Meyer, K., & Wilcox, E. (2024). Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming. Communications Earth & Environment, 5, 281. https://doi.org/10.1038/s43247-024-01442-3 ↩ ↩2 ↩3

  3. Yuan, T., Song, H., Wood, R., Wang, C., Oreopoulos, L., Platnick, S. E., von Hippel, S., Meyer, K., Light, S., & Wilcox, E. (2022). Global reduction in ship-tracks from sulfur regulations for shipping fuel. Science Advances, 8(29), eabn7988. https://doi.org/10.1126/sciadv.abn7988 ↩ ↩2