Tiny particles in the air, so-called aerosols, are created, for example, by exhaust gases and influence cloud formation in the atmosphere. They can intensify both the cooling and warming effects of the clouds. In order to assess these effects, so-called ship-track studies, which examine the effects of ship exhaust gases, were previously considered suitable. However, a new study now indicates that these studies systematically overestimate the cooling effect. This could mean that aerosols intercept global warming less than expected – but the warming caused by greenhouse gases is less than assumed.
Clouds cover large parts of the sky. They cool our planet because they reflect the incident sunlight back into space. Air pollution in the form of aerosols can intensify this cooling effect: additional water accumulates on the tiny particles, which makes the clouds lighter and thus ensures stronger reflection. The cooling effect of air pollution offsets part of the warming effect of greenhouse gases. How much exactly is, however, controversial among climate researchers. Ship-track studies have so far been considered particularly reliable. These track the influence that ship exhaust gases have on cloud formation over the oceans. Since there are few interfering influences on the high seas, this method seemed to provide a well-suited climate model.
Cloud simulations and satellite images
A team led by Franziska Glassmeier from the Technical University of Delft has now checked the results of ship-track studies using a new method. To do this, the researchers developed detailed cloud simulations on the computer, which they compared with real satellite images. Unlike the ship-track studies, which can only include short-term effects of the exhaust gases, the combination of simulation and satellite data provided a more long-term picture. “Our conclusion is that the cooling effect of aerosols on clouds is overestimated if we rely on ship-track data,” says Glassmeier.
The ship tracks are too short-lived to provide a correct estimate of the cloud lightening. “The problem is that the clouds initially get lighter, but after a while they get thinner and thus less bright again. And the ship’s tracks disappear before we can observe this darkening effect,” says Glassmeier. However, your computer model showed that evaporation from the aerosol particles is increased, which in the long term turns the cooling effect into the opposite.
Greenhouse effect less severe than feared?
When researchers have so far estimated how much greenhouse gases warm the atmosphere, they assumed that the man-made exhaust gases cushion part of the warming through their influence on cloud formation. So if the aerosols were to be reduced in the future, so the hypothesis, this mitigating effect would disappear and we would have to expect accelerated global warming. The new results show this assessment in a different light.
The climate researcher Ulrike Lohmann from the Eidgenössische Technische Hochschule Zürich (ETHZ), who was not involved in the study, explains: “If aerosols cool less due to the aerosol-cloud interactions, the warming effect of the greenhouse gases is also less than previously assumed. This means that future warming will be less pronounced, especially if we further reduce the aerosol particles because of their health effects. That would also mean that we have a little more time until we have reached 1.5 or 2 degrees global warming. ”However, Lohmann also points out that the effect described in the study is only one of many influencing factors. “In short, there are still unanswered questions regarding aerosol-cloud interactions and thus the cooling effect of the aerosols.”
Risky climate engineering
The study also has implications in connection with climate engineering, i.e. targeted – so far mostly hypothetical – interventions in the climate system that are intended to mitigate climate change. One example of a climate engineering method is the targeted brightening of clouds by emitting sea salt aerosols, known as marine cloud brightening. “Our results show that the marine cloud lightening is not as easy as it seems, even from a cloud physical point of view. A thoughtless implementation could even lead to a cloud darkening and the opposite of what was intended, ”says Glassmeier. “We certainly need to do a lot more research into the feasibility and risks of such methods. There is still a lot to learn about how these tiny aerosol particles affect clouds and ultimately the climate. “
Source: Franziska Glassmeier (Technical University Delft) et al., Science, doi: 10.1126 / science.abd3980