Reduced sulfur content in shipping fuel associated with increased maritime atmospheric warming

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MAY 30, 2024

Reduced sulfur content in shipping fuel associated with increased maritime atmospheric warming

by Nature Publishing Group

An 80% reduction in sulfur dioxide shipping emissions observed in early 2020 could be associated with substantial atmospheric warming over some ocean regions, according to a modeling study published in Communications Earth & Environment. The sudden decline in emissions was a result of the introduction of the International Maritime Organization's 2020 regulation (IMO 2020), which reduced the maximum sulfur content allowed in shipping fuel from 3.5% to 0.5% to help reduce air pollution.

Fuel oil used for large ships has a significantly higher percentage content of sulfur than fuels used in other vehicles. Burning this fuel produces sulfur dioxide, which reacts with water vapor in the atmosphere to produce sulfate aerosols. These aerosols cool the Earth's surface in two ways: by directly reflecting sunlight back to space; and by affecting cloud cover.

Increasing the number of aerosols increases the number of water droplets that form while reducing their size, both increasing the cloud coverage and forming brighter clouds which reflect more sunlight back to space. Marine cloud brightening is a form of geoengineering where marine clouds are deliberately seeded with aerosols to achieve this effect.

Tianle Yuan and colleagues calculated the effect of IMO 2020 on the atmospheric levels of sulfate aerosols over the ocean and how this affected cloud composition. They found substantial reductions in both the levels of atmospheric aerosols and the cloud droplet number density.

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https://www.nature.com/articles/s43247-024-01442-3
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Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming

Tianle Yuan, Hua Song, Lazaros Oreopoulos, Robert Wood, Huisheng Bian, Katherine Breen, Mian Chin, Hongbin Yu, Donifan Barahona, Kerry Meyer & Steven Platnick

Communications Earth & Environment volume 5, Article number: 281 (2024) Cite this article

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Abstract
Human activities affect the Earth’s climate through modifying the composition of the atmosphere, which then creates radiative forcing that drives climate change. The warming effect of anthropogenic greenhouse gases has been partially balanced by the cooling effect of anthropogenic aerosols. In 2020, fuel regulations abruptly reduced the emission of sulfur dioxide from international shipping by about 80% and created an inadvertent geoengineering termination shock with global impact. Here we estimate the regulation leads to a radiative forcing of
Wm−2 averaged over the global ocean. The amount of radiative forcing could lead to a doubling (or more) of the warming rate in the 2020 s compared with the rate since 1980 with strong spatiotemporal heterogeneity. The warming effect is consistent with the recent observed strong warming in 2023 and expected to make the 2020 s anomalously warm. The forcing is equivalent in magnitude to 80% of the measured increase in planetary heat uptake since 2020. The radiative forcing also has strong hemispheric contrast, which has important implications for precipitation pattern changes. Our result suggests marine cloud brightening may be a viable geoengineering method in temporarily cooling the climate that has its unique challenges due to inherent spatiotemporal heterogeneity.

Introduction
The Earth’s atmosphere has warmed because of human activities increasing the concentration of greenhouse gasses that trap thermal radiative energy in the climate system, creating a positive climate forcing. Human activities have also increased the concentration of aerosol particles that can affect the amount of reflected solar radiation back to space either directly or indirectly by interacting with clouds, which has an overall cooling effect on the climate1. The magnitude of the aerosol cooling effect has significant implications for estimating how sensitive our climate is to greenhouse gas forcing and the amount of expected future warming for a given increase of greenhouse gas concentrations2. The effectiveness of anthropogenic aerosols in cooling the climate also has direct implications for solar radiation modification geoengineering schemes3,4. Such methods aim to produce temporary cooling of the climate through enhanced reflection of solar radiation to space. They are not solutions to greenhouse gas induced global warming and have uncertain and complex additional consequences besides the intended short-term cooling effect4,5,6,7.

Marine cloud brightening (MCB) is a type of solar radiation modification scheme where marine low clouds are seeded with aerosols to become brighter8,9. Examples of small scale, opportunistic MCB experiments were discovered in early satellite observations of ship-tracks, linear features of brighter oceanic clouds because of ship-emitted aerosols10,11. The addition of aerosols from ship emissions results in more cloud droplets, leading to more reflective clouds for a given amount of total In-cloud liquid water, or liquid water path (LWP)12. More recent studies show that aerosols can also change LWP and total cloud fraction (CF), which also greatly affect the amount of solar radiation reflected by clouds2,13,14,15,16.

Aerosols sourced from global shipping industry affect clouds and we can view the shipping emission as a long-running inadvertent MCB experiment. On January 1, 2020, new International Maritime Organization (IMO) regulations on the sulfur content of international shipping fuel took effect. The IMO 2020 regulation (IMO2020) reduced the maximum sulfur content from 3.5% to 0.5%17. While IMO2020 is intended to benefit public health by decreasing aerosol loading, this decrease in aerosols can temporarily accelerate global warming by dimming clouds across the global oceans. IMO2020 took effect in a short period of time and likely has global impact. IMO2020 effectively represents a termination shock for the inadvertent geoengineering experiment through a reverse MCB, i.e., marine cloud dimming through reducing cloud droplet number concentration (Nd) (Fig. 1). Observations of ship-tracks suggest that IMO2020 has reduced the occurrence and modified the properties of ship-tracks across global oceans, demonstrating that a regulation intended to reduce pollution had collateral effects on cloud microphysics18. Analyses of remote sensing data have shown evidence of cloud dimming in the South Atlantic shipping lane19. Outside the South Atlantic, the effect of IMO2020 does not have a distinct spatial structure18,19, which makes direct observation of the impact more challenging.