Using satellite-based remote sensing observations, a research team from the Universities of Leipzig and Cologne as well as the SRON Netherlands Institute for Space Research have succeeded in measuring the number of dirt particles in the atmosphere on which clouds form. These particles, known as aerosols, cool the climate by acting as cloud condensation nuclei: the more cloud droplets form around these particles, the less sunlight penetrates a cloud. This cools the climate, although this process is outweighed by the much stronger greenhouse effect. The study ‘Optimal choice of proxy for cloud condensation nuclei educes uncertainty in aerosol-cloud-climate forcing’ was published in the journal Science Advances.
“You have to combine simulations with satellite measurements and use aerosol concentrations near the Earth’s surface. All other methods lead to larger errors,” says Professor Dr Johannes Quaas, Professor of Theoretical Meteorology at Leipzig University.
To achieve this, the researchers exploited the difference between the Northern and Southern Hemispheres, as the continents are distributed unevenly: there is far more land and industrial activity in the Northern Hemisphere. Air pollution likewise differs substantially between the Northern and Southern Hemispheres. From this, conclusions can be drawn about the role of air pollution in cloud formation. These new findings make it possible to project more accurately how much the climate will warm in the coming decades.
“Overall, we estimate that the effect of aerosols on clouds has offset around a quarter of the anthropogenic greenhouse effect,” Quaas explains. However, this cooling diminishes as air quality improves, because aerosols are short-lived. They are washed out of the atmosphere after about a week, whereas carbon dioxide, the main cause in human-made climate change, remains in the atmosphere for many centuries.
“In this joint research project, we also showed that the number of cloud droplets relevant to the climate effect can best be estimated by the concentration of dirt particles at the cloud base, i.e. where the cloud forms,” says Dr Christoph Böhm from the University of Cologne’s Institute of Geophysics and Meteorology. He developed the method for determining cloud base height from satellite observations and provided the relevant data for the study. However, Böhm also adds that it remains difficult to capture the necessary three-dimensional distribution of aerosols across large areas. The latest study, which began in 2022, was based on satellite observations from the US space agency NASA and the Dutch research institute SRON, as well as on climate simulations.
“There are still many uncertainties when it comes to aerosols and clouds,” emphasizes Professor Quaas. In the EU-funded research project ‘CleanCloud’, the researchers now aim to explore a range of questions, including how thunderstorms may change as air quality continues to improve and clouds become cleaner.
Media Contact:
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Publication:
https://www.science.org/doi/10.1126/sciadv.aea4828