Reduced sulfur emissions from ships actually accelerated warming

Imagine the tranquility of a bright, sunny morning being disrupted by the loud bellowing of a massive ship’s horn as it departs from the dock, leaving a trace of sulfur in its wake.

As the earth’s warmest year on record, 2023 was characterized by such clamor, as more and more ships took to the seas.

But there’s a twist. This time, nearly 20 percent of the year’s record warmth was not caused by the grumbling engines of these vessels but the reduction in sulfur emissions from the shipping industry that occurred mainly in the northern hemisphere.

Sulfur emissions in ships

Back in 2020, the International Maritime Organization took a significant step to curb pollution. They enforced regulations that deduced the sulfur content in global shipping fuel by about 80 percent.

Here’s the interesting part: while the reduction did indeed lower the amount of sulfur aerosols being puffed into our atmosphere, it paradoxically accelerated global warming.

How does sulfur cool the Earth?

When ships burn fuel, they release sulfur dioxide into the atmosphere.

This sulfur dioxide, when activated by sunlight, initiates a fascinating chemical dance in the atmosphere that results in the formation of sulfur aerosols.

Water droplets love to cling onto these aerosols, forming linear clouds known as “ship tracks”.

Ever noticed how a cloudy day feels cooler than a sunny day?

That’s because these clouds reflect sunlight, thus reducing the amount of heat that reaches the Earth’s surface. So, the cloud count going down implies an inevitable rise in global temperatures.

Lessons for future

The researchers made use of a machine learning approach to navigate a sea of over a million satellite images. They observed a 25 to 50 percent decline in visible ship tracks, and where the cloud count was down, the heat was up.

Visions of what the future holds, based on simulations of ship aerosols’ effects in three climate models, suggest that we may be in for quite a heatwave.

Roughly half of the potential warming from the shipping emission changes materialized in just four years since 2020, according to the new study.

What’s even more disconcerting is that more warming is expected as the climate response continues to unfold.

Ships, sulfur, and temperature

While sulfur emissions do contribute to temperature changes, they are not the sole culprit behind 2023’s record warming.

Rather, changes in sulfur emissions are but one piece of a vast and intricate puzzle consisting of oscillating climate patterns and greenhouse gas concentrations.

Aerosols, thanks to their cooling properties, play the role of a climate veil by hiding a portion of the warming caused by greenhouse gas emissions.

Problem is, while these aerosols can travel long distances and strongly affect the Earth’s climate, they don’t linger very long in the atmosphere. So when their concentrations decrease suddenly, temperatures can spike dramatically.

“Cleaning up air quality faster than limiting greenhouse gas emissions may be accelerating climate change,” according to the study’s authors.

In this era of rapid decarbonization, it is important to understand the potential magnitude of climate response.

Navigating policy implications

This research emphasizes the crucial link between environmental policy and climate science, highlighting the need for a holistic approach to address shipping emissions.

Policymakers should consider not only sulfur emissions but also their broader climatic impacts. Balancing cleaner air with global temperature management is complex, and investing in alternative fuels is essential for sustainable maritime practices.

The findings call for collaborative dialogue among scientists, policymakers, and industry leaders to effectively tackle climate change mitigation.

Sailing into the unknown

This research reveals that changes in ocean temperatures could result from modifications in ocean clouds.

These changes could be triggered incidentally by the sulfur associated with ship exhaust, or through deliberate climate intervention by reintroducing aerosols over the ocean.

However, several uncertainties linger. More precise data on ship positions and emissions, along with improved modeling that accurately represents potential oceanic feedback, could be the lighthouse guiding us towards a clearer understanding of the climate response to reduced sulfur emissions.

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Credit for this discovery goes to the formidable team at DOE’s Pacific Northwest National Laboratory. Under the leadership of Earth scientists Andrew Gettelman and Matthew Christensen.

The study is published in the journal Geophysical Research Letters.

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