Clouds play an important role in our climate by regulating the amount of solar energy that reaches the surface and the amount of Earth’s energy that is radiated back into space. The more energy the planet traps, the warmer our climate will become. If less energy is collected, the climate will cool. Understanding this energy balance is critical to answering any of the questions raised by climate change.
Cumulus clouds in trade-wind regions cover nearly 20% of our planet, providing a cooling effect. Until recently, it was assumed that global warming would reduce the surface area covered by clouds, thus amplifying the warming. However, a team has now refuted that assumption.
In a major field campaign in 2020, Dr. Raphaela Vogel who is now at Universität Hamburg’s Center for Earth System Research and Sustainability (CEN) and an international team from the Laboratoire de Météorologie Dynamique in Paris and the Max Planck Institute for Meteorology in Hamburg analyzed observational data they and others collected in fields of cumulus clouds near the Atlantic island of Barbados. Their analysis revealed that these clouds’ contribution to climate warming has to be reassessed.
“Trade-wind clouds influence the climate system around the globe, but the data demonstrate behavior differently than previously assumed. Consequently, an extreme rise in Earth’s temperatures is less likely than previously thought,” says Vogel, an atmospheric scientist. “Though this aspect is very important for more accurately projecting future climate scenarios, it definitely doesn’t mean we can back off on climate protection.”
With our new observations and findings, we can now directly test how realistically climate models portray the occurrence of trade-wind clouds. In this regard, a new generation of high-resolution climate models that can simulate the dynamics of clouds around the globe down to scales of one kilometer are particularly promising.
Dr. Raphaela Vogel
Many climate models have simulated a significant decrease in trade-wind clouds, which would mean that much of their cooling function would be lost, causing the atmosphere to warm even more. The new observational data indicates that this is unlikely to occur.
What is certain is that as global warming continues, more water on the ocean’s surface evaporates and moisture near the base of trade wind clouds increases. In contrast, the air masses in the upper part of the clouds are very dry and only become slightly moister. This results in a significant difference in moisture above and below. This is dispelled in the atmosphere when the air masses mix. The previous hypothesis: drier air is transported downward, causing the cloud droplets to evaporate more rapidly and making it more likely that the clouds will dissipate.
The observational data from Barbados now provides the first robust quantification of how prominent vertical mixing is and how it affects moisture and cloud cover overall. As a result, it is the first data to shed light on a critical process for understanding climate change. In summary, increased mixing does not cause the lower layers to dry out or the clouds to dissipate. Rather, the data show that as vertical mixing increases, cloud cover increases.
“That’s good news, because it means that trade-wind clouds are far less sensitive to global warming than has long been assumed,” says Vogel. “With our new observations and findings, we can now directly test how realistically climate models portray the occurrence of trade-wind clouds. In this regard, a new generation of high-resolution climate models that can simulate the dynamics of clouds around the globe down to scales of one kilometer are particularly promising. Thanks to them, future projections will be more accurate and reliable.”
The team members planned the month-long field campaign EUREC4A (2020) around extended flights with two research aircraft outfitted with different instruments and operating at different altitudes, as well as shipboard measurements from the R/V Meteor, a German research vessel managed by the University of Hamburg. A single plane was used to drop hundreds of atmospheric probes from an altitude of nine kilometers. The probes collected atmospheric data on temperature, moisture, pressure, and wind as they fell.
While the ship performed surface-based measurements, the other plane surveyed clouds at their base at an altitude of 800 meters. As a result, an unprecedented database has been created that will aid in understanding the enigmatic role of clouds in the climate system and in more accurately forecasting their role in future climate change.
The height of the clouds determines whether they cool or warm. The trade-wind clouds investigated here have a maximum altitude of two to three kilometers, reflect sunlight, and thus cool the atmosphere. Higher clouds, on the other hand, amplify the greenhouse effect, warming the climate.
