Global Warming-induced global water cycle changes pose a significant challenge to global ecosystems and human society. However, estimating historical water cycle change is complex, with around 80 percent of global rainfall and evaporation happening over the ocean.
In a new study, UNSW Sydney scientists estimated how much ocean freshwater has moved from the equator to the poles since 1970. They found that between two and four times more freshwater has moved than climate models predicted. The findings offer insights into how the global water cycle is amplifying as a whole.
The sixth generation of climate modeling (called the Sixth Climate Model Intercomparison Project, or ‘CMIP6’) is used in this study.
The study shows that at least two times more freshwater has shifted from warm to cold regions of the earth. This indicates broader changes to the global water cycle.
The lead author of the study Dr. Taimoor Sohail, a mathematician and postdoctoral research associate at UNSW Science, said, “We already knew from previous work that the global water cycle was intensifying. We didn’t know by how much.”
“The movement of freshwater from warm to cold areas forms the lion’s share of water transport. Our findings paint a picture of the larger changes happening in the global water cycle.”
For the study, scientists analyzed observations from three historical data sets covering 1970-2014. They tracked salinity trends in the warm, salty fraction of the ocean and quantified the observed net poleward transport of freshwater in the Earth system.
During this period, scientists found that an extra 46,000-77,000 cubic kilometers of freshwater were transported from warm to cold ocean regions, i.e., from the equator to the poles. That’s around 18-30 centimeters of freshwater from tropical and sub-tropical areas or roughly 123,000 times the water in Sydney Harbour. This rate is not replicated in the current generation of climate models.
Scientists then compared their outcomes with 20 different climate models. All models underestimated the actual change in warm-cold freshwater transfer.
Dr. Sohail says the findings could mean we’re underestimating the impacts of climate change on rainfall.
“Findings like ours are how we improve these models.”
“Each new generation of modeling adapts past models with real data, finding areas that we can improve upon in future models. This is a natural evolution in climate modeling.”
- Sohail, T., Zika, J.D., Irving, D.B. et al. Observed poleward freshwater transport since 1970. Nature 602, 617–622 (2022). DOi: 10.1038/s41586-021-04370-w