The role that clouds play in climate change
It has been known since the 1970s that clouds play an important climate role and are central to the challenges of accurately modelling and predicting climate and climate change. “You can’t quantify how much you need to reduce greenhouse gas emissions to limit global warming below a target, unless you know how sensitive the Earth’s temperature is to a given rise in greenhouse gas (GHG) concentration,” says Sandrine Bony, coordinator of the EUREC4A project, which was funded by the European Research Council. With world-first experiments, EUREC4A explored both the role of convection and circulation in the response of clouds to climate change, alongside the role of convective aggregation.
The complexity of cloud feedback
EUREC4A was inspired by the scientific discussions surrounding the World Climate Research Programme Grand Challenge on Clouds, circulation and climate sensitivity, which Bony has co-coordinated for over a decade. Climate models had suggested that clouds’ response to warming significantly controls the Earth’s temperature sensitivity to changes in GHG concentrations. But how shallow small cumulus, such as those found in the tropical trade-wind regions, respond was especially uncertain. An explanation for the strongest response was suggested by the ‘mixing-desiccation hypothesis’, which proposed that as clouds rise they force drier air down (‘mixing’). This dries (‘desiccates’) the lower cloud layer, decreasing overall cloud cover and so limiting sunlight reflection. “Despite this behaviour being fundamental to understanding climate change, the difficulties of direct observation meant that the hypothesis had never been tested in situ and the modelling had thrown up significant uncertainty,” explains Bony who was working for the French National Centre for Scientific Research, the project host.
First field experiments refute the hypothesis
The field experiments took place over the tropical western Atlantic Ocean, near Barbados, in early 2020. Four aircraft, four vessels and many air and water drones were deployed to observe clouds, the atmosphere and upper ocean, on scales from the microscopic to the synoptic. The team measured, for the first time, cloud area at the base of the cloud layer with a ‘horizontally pointing’ aircraft-borne lidar and radar. As another first, over 800 dropsondes measured the slow vertical mesoscale air motion – allowing estimates of convective mass flux and so vertical mixing. The team found that ‘mixing’ doesn’t desiccate the lower cloud layer, and so doesn’t decrease the amount of cloud near the cloud base. In fact, tight coupling between clouds and convective motions, alongside mesoscale atmospheric circulations, counter the effect of mixing on humidity. "Clouds are more dynamically driven by small-scale circulations than thermodynamically driven by humidity variations. As the climate models that predict the strongest amplification of global warming by shallow cumuli typically predict the opposite, it suggests their assumptions about cloud responses were wrong,” adds Bony.
Cloud clustering
Shallow clouds over the warm subtropical oceans exhibit a wide variety of spatial arrangements, but these had never been characterised before. EUREC4A developed a classification of the most prominent forms, first from visual analysis of satellite images, then using satellite radiances. Deep clouds can also accumulate and form aggregates. EUREC4A revealed that these interact with sea surface temperature and radiation, impacting climate at the planetary scale, as well as influencing the width of tropical rain belts and precipitation extremes. “Our findings have stimulated a new field of research on the physical processes underlying different patterns, while providing a benchmark for evaluating numerical models,” remarks Bony. The team now wants to better understand the physical processes that drive the mesoscale organisation of convection (shallow and deep), and its potential change under global warming. “Many modelling studies are addressing this question but lack observational data, this is now my research priority,” concludes Bony.
Keywords
EUREC4A, climate change, clouds, radiation, convection, precipitation, Earth, sensitivity, trade-wind, cumulus, greenhouse gas
