IMO2020 Regulations Accelerate Global Warming by up to 3 Years in UKESM1

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024EF005011

Authors 

G. Jordan, M. Henry

First published: 14 August 2024

https://doi.org/10.1029/2024EF005011

Abstract

The International Maritime Organization (IMO) introduced new regulations on the sulfur content of shipping emissions in 2020 (IMO2020). Estimates of the climatic impact of this global reduction in anthropogenic sulfate aerosols vary widely. Here, we contribute to narrowing this uncertainty with two sets of climate model simulations using UKESM1. Using fixed sea-surface temperature atmosphere-only simulations, we estimate an IMO2020 global effective radiative forcing of 0.139 ± 0.019 Wm−2 and show that most of this forcing is due to aerosol-induced changes to cloud properties. Using coupled ocean-atmosphere simulations, we note significant changes in cloud top droplet number concentration and size across regions with high shipping traffic density, and—in the North Atlantic and North Pacific—these microphysical changes translate to a decrease in cloud albedo. We show that IMO2020 increases global annual surface temperature on average by 0.046 ± 0.010°C across 2020–2029; approximately 2–3 years of global warming. Furthermore, our model simulations show that IMO2020 helps to explain the exceptional warming in 2023, but other factors are needed to fully account for it. The year 2023 also had an exceptionally large decrease in reflected shortwave radiation at the top-of-atmosphere. Our results show that IMO2020 made that more likely, yet the observations are within the variability of simulations without the reduction in shipping emissions. To better understand the climatic impacts of IMO2020, a model intercomparison project would be valuable whilst the community waits for a more complete observational record.

Key Points

Recent regulations on the sulfur content of ship emissions has accelerated global warming by approximately 2–3 years

Reduced ship emissions induce responses in cloud properties, top-of-atmosphere radiation, and surface temperatures

The regulations contribute to the exceptional warming observed in 2023, yet other factors are needed to fully account for it

Plain Language Summary

In 2020, the International Maritime Organization introduced new regulations decreasing the sulfur content of shipping emissions (IMO2020). Since sulfur is a pollutant, it is expected that IMO2020 will improve air quality and health outcomes. These emissions, however, also lead to the formation of tiny particles in the air which brighten clouds, resulting in more sunlight reflected to space which helps cool the planet. Hence, by reducing sulfur emissions, IMO2020 will lead to planetary warming, yet the magnitude of this effect is hotly debated. In this work, we use a state-of-the-art Earth system model to assess the warming impact of IMO2020. We find that IMO2020 increases the global average temperature by around 0.05°C; the equivalent to 2–3 years of global warming. Thus, IMO2020 helps to explain the exceptional warmth observed in 2023, yet other factors are needed to fully account for it. The year 2023 also had a record decrease in reflected sunlight contributing to the record temperatures, and our results show that IMO2020 made that more likely. Finally, we emphasize that IMO2020 has simply brought forward the warming from reductions in pollutants that are factored in favorable future climate scenarios.

Source: AGU