Shipping emissions mandate led to spike in global temperatures

From the article:

The summer of 2023 saw a surprising increase in global temperatures, even within the context of the ongoing greenhouse gas-driven warming trend. Many scientists were flummoxed. Their simulations didn’t show this kind of spike.

“Climate scientists were saying this is essentially impossible, that it is bonkers to see such a jump all at once,” said Daniele Visioni, assistant professor in the Department of Earth and Atmospheric Sciences. “People were saying, ‘Climate change is suddenly accelerating.’ We’d never seen something like this.”

Visioni’s paper, “Modeling 2020 Regulatory Changes in International Shipping Emissions Helps Explain Anomalous 2023 Warming,” published Nov. 28 in Earth System Dynamics, gets to the bottom of it.

Mandated reductions in sulfate emissions from international shipping routes in 2020 are partly responsible for the record high temperatures, the researchers found. Reducing the amount of aerosol particles in the atmosphere reduces cloud coverage; thus, clouds’ ability to reflect solar radiation back to space is diminished. The paper’s findings suggest future policy decisions around abrupt reductions in tropospheric aerosols should take into account their surface temperature impact.

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The regulation required ships to use fuel with a sulfur content of no more than 0.5%, down from the previous limit of 3.5%. This reduction led to a more than 80% decrease in total sulfur oxide emissions from shipping.

And while there was some talk of this tradeoff within the shipping industry, he said, there was little attempt to call widespread attention to the potential effect.

“There was no attempt to say we should have all eyes on the shipping corridor,” Visioni said. “In hindsight, it would have been great to study that four years ago before the problem manifested itself.”

The Cornell researchers looked at monthly global temperature anomalies over the period 2020-23, removing the assumed linear contribution from greenhouse gases and seasonality, in order to determine the shipping industry’s impact on temperature anomalies. They found that removing sulfur dioxide from shipping fuel likely increased the planet’s temperature by 0.08 degrees Celsius.