Human influence strengthens the contrast between tropical wet and dry regions


Andrew Schurer, Andrew P. Ballinger, Andrew R. Friedman, Gabriele Hegerl

Environmental Research Letters
Work Package 2



Wet and dry areas in the tropics and subtropics are expected to become wetter and drier, respectively, as the climate warms. This pattern has been seen in observations and predicted in climate model simulations, but the underlying forcings behind it are unclear. This study used detailed rainfall data going back to 1988 and a series of climate simulations from CMIP6 climate models to investigate this expected pattern. The team found that there is a clear and detectable increase in the contrast between wet and dry regions in the tropics and subtropics, with much of this increase due to increasing levels of anthropogenic greenhouse gases in the atmosphere. The climate models, however, were found to consistently underestimate the changes, suggesting their future projections may be inaccurate. These results are extremely useful to communities and policymakers in the tropics and subtropics, helping them anticipate potentially larger climatic changes than predicted in climate models and make better informed decisions to adapt to and mitigate the impacts of these changes, including droughts and floods. A similar method is being developed for use in Europe, to be utilised in later EUCP studies.

The contrast between wet and dry areas in the tropical and subtropical regions is expected to increase in the future as the climate warms, as shown by climate models and current observational data. Satellite and in situ observations from 1988 to 2019 show a pattern of rainfall increasing substantially in the wettest areas and decreasing slightly in dry regions, a pattern also seen in simulations from models in the CMIP6 project. Despite this clear pattern the climate forcings behind it have remained unclear due to disagreements between different climate models, leading to uncertainty in our projections of atmospheric circulation changes, which significantly affect rainfall patterns and potentially severely impacting people and communities. This study addresses this gap in our knowledge, including whether the observed pattern can be attributed to natural or anthropogenic forcings, contributing to our understanding of how to limit uncertainty in our climate projections.

This study found a clear and detectable increase in the contrast between wet and dry regions in the tropics and subtropics when using combined satellite and in situ rainfall observations. This pattern follows expectations from climate model simulations and our background understanding of how rainfall should react to increasing temperatures. The team found that both natural and anthropogenic forcings are detectable in this signal, with the largest contribution probably due to increases in greenhouse gases. Interestingly, the rainfall observations show a larger response to these external drivers than the climate models predict. The drying trend in dry regions is larger than in all simulations, while the trend in wet regions is underestimated by all but 8 of the simulations in this study. This may indicate that our model-based simulations of future climate change in these areas could also be underestimating the expected change.

The data used here came primarily from the Global Precipitation Climatology Project (GPCP) gridded dataset of monthly rainfall, merging satellite observations with rain gauges around the world. The team focussed on the most reliable part of the dataset, using data from January 1988 to December 2019. This dataset is compared with others to get a better idea of overall trends, refining and improving an existing analysis method. The other main part of this study, understanding the mechanisms behind the observed trends, utilised simulations from climate models used in the CMIP6 project. 58 simulations were used to explore the behaviour of many climate models and to help isolate the role of climate forcings, investigating the causes of rainfall changes more clearly than previous work by applying a formal detection and attribution analysis.

Policy relevance
This study describes useful new methods that can also be applied to other regions. The results of this study are also important for communities and policymakers in the tropics and subtropics as they can be used to help produce better-informed policies to adapt to climate change in these regions, realising that previous model-based projections may have underestimated the magnitude of future changes. Drought and flooding are among the costliest natural disasters, costing lives and livelihoods, and a better understanding of future climate change will help local communities adapt to and mitigate the impacts of these damaging events by taking measures to prepare for them.


Climate models predict a strengthening contrast between wet and dry regions in the tropics and subtropics (30°S-30°N), and data from the latest model intercomparison project (CMIP6) support this expectation. Rainfall in ascending regions increases, and in descending regions decreases in climate models, reanalyses, and observational data. This strengthening contrast can be captured by tracking the rainfall change each month in the wettest and driest third of the tropics and subtropics combined. Since wet and dry regions are selected individually every month for each model ensemble member, and the observations, this analysis is largely unaffected by biases in location of precipitation features. Blended satellite and in situ data from 1988-2019 support the CMIP6-model-simulated tendency of sharpening contrasts between wet and dry regions, with rainfall in wet regions increasing substantially opposed by a slight decrease in dry regions. We detect the effect of external forcings on tropical and subtropical observed precipitation in wet and dry regions combined, and attribute this change for the first time to anthropogenic and natural forcings separately. Our results show that most of the observed change has been caused by increasing greenhouse gases. Natural forcings also contribute, following the drop in wet-region precipitation after the 1991 eruption of Mount Pinatubo, while anthropogenic aerosol effects show only weak trends in tropic-wide wet and dry regions consistent with flat global aerosol forcing over the analysis period. The observed response to external forcing is significantly larger (p>0.95) than the multi-model mean simulated response. As expected from climate models, the observed signal strengthens further when focusing on the wet tail of spatial distributions in both models and data.