“How a 2020 Shipping Regulation Triggered the 2023 Surge in Global Temperatures”

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The summer of 2023 experienced an unexpected rise in global temperatures, even amidst the ongoing trend of warming driven by greenhouse gases. Numerous scientists were baffled, as their simulations did not predict such a surge.

“Climate scientists were indicating that this is essentially improbable, suggesting it is absurd to witness such an increase abruptly,” stated Daniele Visioni, assistant professor in the Department of Earth and Atmospheric Sciences. “Individuals were expressing, ‘Climate change is hastily accelerating.’ This phenomenon has not been witnessed before.”

Visioni’s study, “Modeling 2020 Regulatory Changes in International Shipping Emissions Aids in Explaining Anomalous 2023 Warming,” released on Nov. 28 in Earth System Dynamics, uncovers the underlying reasons.

Mandated decreases in sulfate emissions from international shipping lanes in 2020 are partly accountable for the unprecedented high temperatures, according to the researchers. Diminishing the quantity of aerosol particles in the atmosphere reduces cloud cover, thereby diminishing clouds’ ability to reflect solar radiation back into space. The research findings suggest that future policy decisions regarding sudden reductions in tropospheric aerosols should consider their consequences on surface temperatures.

Previous studies suggested that such alterations would cause a slight rise in global temperatures due to decreased cloud formation; however, Visioni and co-author Ilaria Quaglia, postdoctoral researcher in the Sibley School of Mechanical and Aerospace Engineering (Cornell Engineering), employed Earth system model simulations to validate the importance of the abrupt decline in sulfate emissions from shipping.

The shipping industry’s changes had been under discussion for years, Visioni noted, with the International Maritime Organization (IMO) deciding in 2014 to initiate stricter sulfate emission regulations by 2020. Fuels with lower sulfur content are considerably more expensive, which delayed industry adjustments, he mentioned.

The regulation mandated vessels to utilize fuel with a sulfur composition not exceeding 0.5%, decreased from the previous limit of 3.5%. This adjustment resulted in an over 80% reduction in total sulfur oxide emissions from shipping.

While there was some discourse regarding this tradeoff within the shipping sector, he remarked, there was minimal effort to raise widespread awareness of the potential impact.

“There was no endeavor to indicate that we should closely monitor the shipping corridors,” Visioni remarked. “In hindsight, it would have been beneficial to investigate this four years ago before the issue became apparent.”

The Cornell researchers examined monthly global temperature anomalies from 2020 to 2023, omitting the presumed linear contribution from greenhouse gases and seasonality to ascertain the shipping industry’s effect on temperature anomalies. They determined that the removal of sulfur dioxide from shipping fuel likely elevated the planet’s temperature by 0.08 degrees Celsius.

“The extraordinary heat transformed into an ordinary warm year once that was accounted for,” Visioni explained.

Quantifying the extent to which these polluting aerosols reflect heat away from the Earth, resulting in a significant rise in temperature, is not, as Visioni indicated, a proposition that these pollution-reduction initiatives should be limited.

“Enhancing air quality yields immediate benefits, and everyone will always strive for that. A lesson here is that we frequently make trade-off decisions,” he remarked. “We are lowering air pollution more than was anticipated a decade ago, indicating a need for much more open discussions. It underlines the growing urgency for emission reductions.”

The shipping sector has, similar to many others, shifted towards alternative fuels to satisfy the IMO’s decarbonization objectives, with methanol, hydrogen, ammonia, wind-assisted propulsion systems, and other technologies gaining momentum.

“We must adopt a more aggressive stance on emissions reductions,” Visioni stated. “We need to bridge a gap. However, we should explore other methods to avert planetary warming. Techniques like cloud brightening and geoengineering climate measures – these will not directly reduce emissions, yet they may be necessary to prevent additional warming.”

Reference: Quaglia I, Visioni D. Modeling 2020 regulatory changes in international shipping emissions helps explain anomalous 2023 warming. Earth System Dynamics. 2024;15(6):1527-1541. doi: 10.5194/esd-15-1527-2024

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