Researchers with their heads in the clouds

Words: Peter Taylor-Whiffen


Global warming has been a known reality for decades – but how much temperatures will rise – and when – is still an open question of enormous ecological and economic importance. Greater understanding of the role clouds play in warming and cooling can help to reduce uncertainty in climate modelling.


Imperial’s Dr Paulo Ceppi and UEA’s Dr Peer Nowack have combined 20 years of satellite observations of cloud formation with current climate models. “Climate models alone can differ by more than a factor of two between the warmest and coolest global warming projections,” says Ceppi. “And the main reason for that difference comes from clouds: how they change and how they affect what we call the ‘radiation budget’ – essentially a measure of how much sunlight is absorbed versus how much infrared radiation is emitted to space. Using global satellite observation data alongside the climate models shows how much global warming can be expected in any given emissions scenario.”


The researchers analysed satellite observations of the radiation budget, combining data on how much sunlight was absorbed and how much infrared is emitted from Earth, as well as how much clouds interfered with these flows of energy. “These observations provide us with highly valuable insights into how clouds affect the radiation budget in any region across the globe,” says Nowack. “In a statistical learning approach, we combine that information with data of meteorological variables like surface temperature, humidity and winds, and use these variables as proxies for small-scale processes that drive cloud formation. From there, we were able to predict how clouds will change if we continue to emit greenhouse gases into the atmosphere.”


“Climate models mostly suggest clouds amplify warming, but with a broad range of possibilities,” says Nowack. “Our observation-based approach implies that we can now reduce this large model uncertainty by making better use of the latest satellite data. Our new method also allows us to estimate the amplifying effect of clouds on global warming for the first time using a single, self-consistent approach. In the past, when such large-scale observational constraints were calculated, they were estimates based on certain locations and specific cloud types only. We thus anticipate that this will be an exciting starting point for much more research to come.”


The duo’s research means scientists will be able to gauge with unprecedented precision how much carbon dioxide can be emitted from Earth before certain global warming thresholds are reached – and that could have a significant impact on environmental policy. But there’s more work to be done. “While our observations tallied generally with existing models, there were some discrepancies,” says Ceppi. “Clouds have two effects on climate – one is through reflected sunlight and the other is via their own greenhouse effect, where they act like an insulating blanket. We found model errors in clouds’ solar and infrared effects – for instance observations suggest a weaker greenhouse effect of high clouds, compared with models. That was a surprise. We have the funding for further research and are looking forward to pursuing this.”

Dr Paulo Ceppi is a Lecturer in Climate Science at Imperial, and Dr Peer Nowack is a Lecturer in Atmospheric Chemistry and Data Science at UEA.