Overestimating Cloud Cooling: How Ship Tracks May Incorrectly Inform Climate Models

Climate models help us predict potential changes in the global climate. In one particular application, climate models are used to estimate the effects of aerosol clouds created by pollution. Aerosols, or suspended fine particles, are generated from human-caused air pollution as well as natural sources including volcanoes. Once suspended in the air, aerosols mix with the atmospheric environment and interact with the existing clouds. Climate models have shown that these aerosol-cloud interactions result in cooling, but they are challenging to precisely evaluate due to unpredictable physical effects. A recent study published in Science demonstrated that cooling is actually being overestimated in these models. 

The real world data to model aerosol-cloud interactions has largely come from observations of ship tracks. Ship tracks, produced by cargo ships, are lines of aerosol clouds over expansive ocean spaces. Exhaust pumped from the cargo ships mixes with ocean clouds, where aerosol particles induce a brightening effect due to water vapor condensation. These brighter clouds reflect more sunlight back to the atmosphere, resulting in a cooling effect. This brightening and associated cooling is incorporated in global climate modeling, but increasing evidence shows that this cooling effect may not be consistent across pollution types and dispersal mechanisms.

The first author of the study, Dr. Glassmeier, assessed the validity of using ship tracks to understand global aerosol-cloud interactions using numerical simulations with satellite data. Her team discovered that the short-lived nature of ship tracks obscure the full long-term effects of these interactions. Clouds over industrial sources experience a constant influx of industrial pollutants, changing the cloud response over time. Industrial clouds respond similarly to ship tracks in the short term, generating an initial brightening effect; however, constant pollutant interaction causes clouds to thin over time, allowing more sunlight to reach the Earth’s surface. This effect is exactly the reverse of what is found in ship tracks, leading to an overall warming effect instead of cooling. This research provides insight into how ship tracks may be appropriately incorporated into climate modeling and how they may not tell the whole story of aerosol-cloud interactions. 

Dr. Franziska Glassmeier is an Assistant Professor in the Atmospheric Science Section of the Department Geoscience & Remote Sensing at the Delft University of Technology in the Netherlands. Her research focuses on the physical effects of clouds and interactions in climate modeling. 

Managing Correspondent: Samantha Tracy

Press Article:Ship exhaust studies overestimate cooling from pollution-altered clouds

Journal Article: “Aerosol-cloud-climate cooling overestimated by ship-track data

Photo Credit: Pixabay

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