Picture (above): Rainfall. Credit: Inge Maria (Unsplash).
It has been estimated that as the Earth warms the atmosphere will be able to hold around 7% more moisture for each degree of global warming. As a result, it is expected extreme precipitation events will vary in frequency but become more intense.
Observations from 1951-2015 show globally heavy precipitation has indeed increased over much of the land as expected but it is unclear how much greenhouse gases have been responsible for this increase. This is because the complicating influences of aerosols and the role of natural variation have the potential to disguise the greenhouse gas signal.
A group of international researchers from Korea, Canada, Spain and Australia’s ARC Centre of Excellence for Climate Extremes have used the latest version of the combined climate models (CMIP6) that will be used in the next IPCC report, to disentangle the global warming influence from these other factors.
The researchers also took the opportunity to examine whether the increased equilibrium climate sensitivity that has been reported in these CMIP6 models led to an overestimation of precipitation when compared against the outputs of previous IPCC model groups with lower sensitivities (CMIP3 and CMIP5).
The results showed that CMIP6 models did overestimate precipitation, but the patterns of increase were consistent across all three model groups. When adjusted for this overestimation and compared with observations the influence of human produced greenhouse gases on heavy precipitation was apparent over much of the Northern Hemisphere.
However, in the Southern Hemisphere the influence of greenhouse gases on precipitation could only be partially detected with deeper analysis because of a combination of the uncertainties in the models for these regions and the high variability in precipitation. In the tropics the combination of these two influences was greater than any greenhouse gas signal.
For the Northern Hemisphere, the observed increases in extreme precipitation over the global land, Northern Hemisphere extratropics, western and eastern Eurasia, and global ‘dry’ and ‘wet’ regions, were largely explained by the influence of greenhouse gases. By comparison, aerosols and natural variability showed a much smaller influence with inconsistent trends. In general, the researchers found that single day extreme precipitation events had increased in both frequency and intensity and that longer events over five days saw similar increases. By disentangling the influence of greenhouses gases on intense precipitation events, researchers will now be able to better forecast how these may change and allow policymakers to be better prepared for future flooding events.
Paper: Paik, S., Min, S.‐K., Zhang, X., Donat, M. G., King, A. D., & Sun, Q.. ( 2020). Determining the anthropogenic greenhouse gas contribution to the observed intensification of extreme precipitation. Geophysical Research Letters, 47, e2019GL086875. https://doi.org/10.1029/2019GL086875