Unveiling the impact of cloud phase transformations on climate change
Clouds play a crucial role in regulating Earth’s climate, reflecting solar radiation back into space to prevent excessive warming while trapping infrared radiation to maintain a stable climate. However, global warming can alter cloud properties, creating a feedback loop that either amplifies or mitigates temperature changes. “This feedback loop, called the ‘cloud feedback’, is the largest source of uncertainty in model projections of Earth’s climate sensitivity to changes in atmospheric carbon dioxide,” states Casey Wall, coordinator of the CPFC project and assistant professor of Meteorology at Stockholm University. This research, undertaken with the support of the Marie Skłodowska-Curie Actions programme, set out to address this gap in climate science by developing quantitative methods to understand one of the key physical mechanisms that determine the cloud feedback.
Quantifying cloud feedback
The CPFC project focused on quantifying the radiative feedback caused by the transformation of cloud ice particles into liquid droplets as the atmosphere warms. The research revealed that these changes account for approximately 19 % of the overall global cloud feedback in climate models. The conversion from ice to liquid causes clouds to become optically thicker, which makes them reflect more solar radiation back into space. These cloud opacity changes were previously believed to be a potentially powerful but highly uncertain negative feedback mechanism that could reduce the climate’s sensitivity to changes in atmospheric CO2 levels. “The CPFC project found that this feedback mechanism is not as strong as previously suggested. Hence, it provides another line of evidence that cloud feedback will not substantially dampen future climate warming. This has implications for designing policies about future carbon emissions,” explains Wall.
Developing advanced climate-model tools
To achieve these breakthroughs, the project developed innovative satellite and climate-model data products that track cloud altitude, optical thickness and composition — distinguishing between ice and liquid at the cloud top. A key advancement was the development of a new mathematical technique that uses this data to measure the effects of ice-to-liquid conversion on cloud radiative properties. CPFC also enhanced the Cloud Feedback Model Intercomparison Project (CFMIP Observation Simulator Package), known as COSP software, an open-source package widely used in climate modelling. It simulates satellite observations from model-generated clouds, enabling direct comparisons between simulations and real-world satellite data. CPFC extended COSP’s capabilities by incorporating new variables relevant to cloud phase transitions. The project also collaborated with the MODIS satellite science team to create equivalent variables in an observational dataset. Along with its contributions to satellite data products and climate-model software, the project developed analysis techniques that have been made freely available to the scientific community to further explore cloud-climate interactions.
Future directions in climate science
CPFC has made significant strides in unravelling the complexities of cloud feedback and its impact on global climate sensitivity. Building on the project’s success, researchers are now applying its framework to another critical area of climate science. “We are currently adapting the methods developed in CPFC to investigate the radiative forcing of climate change, caused by interactions between particulate aerosol pollution and clouds. This remains one of the major uncertainties in climate science,” adds Wall. Understanding these interactions is crucial for refining climate projections and informing future policy decisions.
Keywords
CPFC, cloud feedback, climate change, global warming, climate models
