Recently published data provides persuasive evidence that IMO regulations can work as designed, with measurable global impacts. This comes at a critical moment, as the maritime industry prepares for the MEPC/ES.2 in October, and anticipates the finalisation of the next wave of environmental regulations.
The Indicators of Global Climate Change 2024 study offers validation of regulatory effectiveness. When the IMO introduced the sulphur cap on 1 January 2020, the results were substantial and trackable: Sulphur dioxide (SO₂) emissions from international shipping dropped by 8.4 Tg (or 8.4 million tonnes) —from 10.4 Tg in 2019 to just 2.0 Tg in 2020. This closely matched the IMO’s projected 8.5 Tg decrease.
Compellingly, this change is detectable in the global atmosphere just 4 years later, demonstrating that maritime regulations can deliver impact within relevant timescales.
Sulphur’s climate paradox
Here’s where things become more nuanced. The IMO sulphur regulations actually increased short-term radiative forcing (contributing to net warming) by about +0.1 W/m² from 2020-2024. At first glance, this seems counterproductive for climate goals. But this apparent contradiction reveals well-informed environmental policy at work:
The science: SO₂ from ship exhaust creates aerosol particles that reflect sunlight back into space, providing an accidental cooling effect.
Why this is still climate progress: The alternative—burning sulphur-heavy fuel to maintain artificial cooling—would mean accepting continued air pollution to temporarily conceal an ever-growing CO₂ problem. Real climate progress requires eliminating both greenhouse gases AND harmful pollutants, even when there are short-term trade-offs.
Broader benefits justify interim trade-offs
Health and environmental gains: SO₂ causes acid rain, respiratory diseases, and ecosystem damage. The IMO regulations deliver massive public health benefits, particularly in port cities, preventing thousands of premature deaths annually.
Accelerating real solutions: With the sulphur mask being less effective, shipping’s true warming impact is more visible. This transparency creates stronger incentives for genuine decarbonisation rather than reliance on accidental side-effects of pollution.
This success story provides three crucial insights:
– Regulations work as designed, implementation is key: When properly implemented, IMO environmental regulations can deliver their intended environmental outcomes.
– Policy impact is possible within timeframes that matter: Atmospheric changes are detectable within just four years, demonstrating that maritime environmental policies can operate on timelines relevant to reach global climate targets.
– Long-term goals may require complex short-term trade-offs: The short-term increase in warming from sulphur reduction shows that effective environmental policy sometimes involves navigating nuanced trade-offs rather than simple win-win scenarios.
Well-designed maritime policy doesn’t just make headlines, it alters the planet’s climate trajectory on timelines that matter.
How can we apply the lessons from the sulphur cap to shape mid-term decarbonisation measures with equal rigour and clarity?
Image: Aerosols (which include shipping’s sulphur emissions) provide negative radiative forcing, offsetting some of the warming from greenhouse gases.
Study: https://essd.copernicus.org/articles/17/2641/2025/
