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Quantifying aerosol-cloud-climate effects by regime

Final Report Summary - QUAERERE (Quantifying aerosol-cloud-climate effects by regime)

Predictions of anthropogenic climate change are highly uncertain. They are hampered by
the huge uncertainty of climate forcing, which is dominated by the uncertainty in anthropogenic aerosol-cloud-climate effects. QUAERERE (Latin for researching) aimed at a reliable, observations-based, global quantification of these effects. The approach was to develop new satellite data and analyse these statistically, in combination with climate modelling across scales. To do so, the problem was decomposed into individual processes on the one hand, and the analysis was performed for specific cloud regimes. We found that a new way is necessary to define cloud regimes - previous, widely-used approaches are mis-leading. For this, a new satellite product needed to be created, namely the cloud-base height. A new climatology of cloud condensation nuclei concentrations allowed to better quantify the aerosol-cloud forcing. A particular achievement was the first retrieval of ice crystal number concentrations from satellite data. This is sensitive to aerosol perturbations for cold clouds under strong updraught conditions. QUAERERE further examined how cloud fraction and cloud water content responds to perturbations in cloud droplet concentrations. Cloud fraction increases, implying more than a doubling of the initial effect. Overall, the project contributed substantially to clarify the role of aerosol forcing in climate change, correcting a mis-perception in the scientific debate that it might be very small. This was achieved by the satellite assessments, but also by a combination of observations and modelling, including a constraint on the simulated forcing exploiting the strong aerosol trends since the 1990s over Europe.