Clouds play an important role in keeping the Earth both warm and cool -- some types of clouds shade the planet from sunlight while others help retain heat. Greenhouse gases are changing the amount of heat that is trapped in the atmosphere and a new study finds that as the planet warms clouds will become thinner, which could reduce the amount of sunlight that is reflected away from the rapidly warming planet.
The study, led by Mike Zelinka, a researcher at Lawrence Livermore National Laboratory in California, tested 30 new computer models to simulate Earth’s changing climate. When the concentration of atmospheric carbon dioxide is doubled, the average global temperature increases by 3.9°C. This value is considerably higher than previous calculations made -- which were closer to 3.2°C. Beyond that, the researchers also found that with increasing global temperatures, low clouds tend to hold less water and at the same time decrease in extension around the globe.
The net cloud radiative effect. Credit: Zelinka et al.
These thinner clouds are not as efficient at reflecting sunlight, so they allow an increasing amount of radiation to reach the surface. Meanwhile, the amount of infrared radiation that is absorbed by the surface and emitted back into the atmosphere can be absorbed by greenhouse gases, which further increases air temperatures. With this process occurring across the entire atmosphere, it could certainly have an amplifying effect on future global temperatures.
According to Dr. Zelinka, the computer models helped the research team analyze how intense warming temperatures would be if there was an abrupt increase in carbon dioxide concentrations. The models show more climate sensitivity -- the amount of warming on the Earth’s surface that occurs if carbon dioxide concentrations are doubled compared to pre-industrial levels — than initially thought.
The study suggests the way temperatures respond to an abrupt increase in carbon dioxide has significantly increased in the latest generation of global climate models (GCMs). But the authors also suggest that more testing is needed against the observations, in order to better understand the physics behind the effect of clouds on the energy balance of sunlight.