Among the biggest questions for climate change forecasts is how atmospheric aerosols shape clouds, which can help cool the planet. Now, a new study finds that a promising strategy for understanding how aerosols and clouds interact can overestimate the cooling capacity of pollution-generated clouds by up to 200 percent, researchers reported on Jan. 29.
“Clouds in general and how aerosols interact with climate are a major uncertainty in climate models,” says Franziska Glassmeier, an atmospheric scientist at Delft University of Technology in the Netherlands. Scientists know that aerosols, both natural and by volcanoes, and caused by humans, as well as by pollution, can change the thickness of a cloud, the ability to disperse sunlight, or the amount of rain it produces. But these complicated physical effects are difficult to simulate, so scientists have looked for real-world examples to study these effects.
Enter the tracks of the ships. The leakage produced by huge cargo ships crossing the oceans can form these bright lines of clouds. Tiny exhaust particles act as nuclei of clouds: water vapor condenses on the particles to form droplets of clouds, the aqueous material of the clouds. Boat tracks are “this first example in which we can see this cause and effect,” Glassmeier says. "Put particles in and you'll see how the clouds get brighter." Brighter clouds mean they are reflecting even more sunlight into space.
Visible and measurable by satellite, the tracks offer a potential window into how the largest-scale industrial pollution across the globe may be altering the planet’s cloudy landscape, and perhaps how these clouds can affect the climate. Satellite analyzes of ships' runways involve measuring the density of water droplets in the clouds from the images and calculating how the brightness of the clouds changes over time.
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To assess how well ships ’runways actually represent the overall impact of cloud pollution, Glassmeier and colleagues compared the cooling effect of ships’ runway clouds with that of simulated pollution-derived clouds, as can happen on a city. In particular, the researchers wanted to simulate how the thickness and brightness of clouds, and therefore their cooling effect can evolve over time, as a result of processes such as rain and evaporation.
The problem, the team found, is that the boat’s tracks don’t tell the whole story. Ship tracks are short-lived, because the source of pollution is always on the move. But industrial pollution does not usually occur in a short pulse: instead, there is a constant flow of particles into the atmosphere. And that difference in inputs affects the way natural clouds respond over time.
In both ship tracking studies and industrial pollution simulations, clouds initially illuminate and produce a cooling effect. This is because, in both cases, the addition of abundant aerosol particles to the atmosphere gives the water vapor numerous surfaces on which to condense, creating many small water droplets that form this brightest cloud and reflect incoming radiation.
However, after a few hours, as a boat moves forward, the boat’s track disappears and the pulse of pollution ceases, Glassmeier says. The initial bit of cooling decreases as the pre-existing natural clouds return to their original, uncontaminated state.
But in the case of industrial pollutants, natural clouds do not return to their original state, the simulations show. On the contrary, pollutants hasten the disappearance of clouds. This is because the smaller aerosol-seeded droplets begin to evaporate faster than the larger natural cloud droplets. This increase in evaporation dilutes the original cloud, allowing more heat than if the pollutants never arrived. And that can have an overall effect on the weather, not the cooling, the team says.
“There’s this time-scale effect that needs to be taken into account,” Glassmeier says. Relying solely on boat route data to understand all sources of pollution misses this gradual thinning effect. "It wouldn't pull all the data from the ship's track; we just need to interpret it in a new way." Current climate models tend to omit this slimming effect, she says.
The new study is "really useful in helping to interpret cloud-aerosol relationships in satellite data," says Edward Gryspeerdt, an atmospheric physicist at Imperial College London who did not participate in the study. "It shows that the cloud's response to aerosols is not instantaneous, but evolves over time."
Scientists have been aware that the tracks of ships may not lead to cooling, says Graeme Stephens, an atmospheric scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. by increasing the rate of evaporation at the tops of the clouds, at the same time as precipitation is suppressed, which maintains part of the thickness of the cloud. These two competing responses make it difficult to determine the final destination of a cloud.
But what ships ’tracks can do is act as“ a controlled lab, ”Stephens says. "They offer us a way to examine the influences of aerosols on clouds in a direct and concrete way."