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Research shows how rainfall responded to past climate change

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A study has revealed new insights in patterns of changes in historical tropical rainfall such as monsoon, which will influence the development of future climate models.

The discovery was made by an international research team made up of researchers from The Open University (OU), the British Geological Survey and Brown University

The study shows how the large-scale tropical rainfall pattern differed in response to changes in climate, thousands of years ago.

Regional rainfall differences

The project, led by researchers from the OU’s School of Environment Earth and Ecosystems in the Faculty of Science, Technology, Engineering and Mathematics, Drs Katrina Nilsson-Kerr (OU’s postgraduate student), Pallavi Anand and Philip Holden, is the first detailed rainfall pattern assessment of the tropical landmasses, including new data for the Indian Summer Monsoon region, revealing regional differences in response to past climate change.

Dr Anand, Principal Investigator for the project, said: “How tropical monsoon rainfall will change due to human-induced climate change remains a big unknown. Therefore, it is crucial that we better understand monsoon rainfall pattern in response to climatic changes in the geologic past, to better inform climate models used for predicting its future variability.”

The paper: Dipole patterns in tropical precipitation were pervasive across landmasses throughout Marine Isotope Stage 5, published in Communications Earth and Environment (Nature) uses samples collected by Dr Anand following an International Ocean Drilling Programme (IODP) voyage to the Bay of Bengal in 2014-15 to better understand historical variability of the Indian Monsoon in response to changing climate.

The study presented new reconstructions of the Indian Summer Monsoon precipitation and compiled existing hydroclimate (records inferred to represent rainfall) data to combine with climate model to assess regional differences in rainfall patterns for six regional landmasses during the warm climate state of the last interglacial, between 130 to 70 thousand years ago, comprising warmer and cooler intervals.

Dr Nilsson-Kerr said: “We have a fairly good understanding of how sea surface temperature (SST) varied in the past providing us with fairly robust predicted patterns of SST response to future warming. However, our understanding of how rainfall, particularly in the tropics, responded to past changes in climate and how it will respond to future warming remains inconclusive. We set out in this study to try and synthesize past changes in rainfall, using existing and new data, over a period of climate warming and cooling”

Global rainfall patterns

Their study provided new data from two, previously data sparse, locations of the Bay of Bengal capturing rainfall pattern during the last interglacial interval. Samples of marine microfossils, preserved in the sediment, analysed in this study were ideally situated to capture past changes in the strength of the Indian Summer Monsoon, as continental rainfall feeds the rivers draining into the Bay of Bengal, promoting freshening of seawater. These data were combined with published data from 125 global locations across six regions in the tropics (30N and 30S) to make both regional and global comparison of rainfall patterns. Dr Anand said the study reveals that the tropical rainfall pattern is “incredibly heterogenous and sensitive to climatic changes”.

Dr Holden said: “Despite the complexities of the rainfall patterns, we found remarkable consistency between the data and the climate model, suggesting great potential for both validation of future projections and for understanding past variability.”

The project was funded by the National Environment Research Council.

Next steps

The next steps are to investigate the Indian Summer Monsoon rainfall response during the past geological warm climate states to ground truth some of the findings. Further integrated studies are needed to strengthen knowledge of tropical rainfall pattern in response to different climate states.

Find out more about OU research in the Faculty of Science, Technology, Engineering and Mathematics

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