The Climate Crisis Is Messing With Earth’s Water Cycle

Last month, an ongoing drought forced officials in Southern California to limit around six million people to just one day of outdoor water use a week this summer for the first time ever. On the other side of the world, the heaviest rainfall in 60 years put South Africa through its deadliest storm on record.

These are the kinds of contrasting extremes we can expect to see more of in the future, according to a study published in Nature earlier this year.

Sohail, Taimoor, et al. “Observed Poleward Freshwater Transport Since 1970.” Nature, vol. 602, no. 7898, 2022, pp. 617-622., doi:10.1038/s41586-021-04370-w

“The biggest kind of headline from the study is that the water cycle, which is the rainfall and evaporation that happens over the globe, is getting stronger twice as fast as we previously thought,” study lead author lead Dr. Taimoor Sohail, a mathematician and postdoctoral research associate at University of New South Wales (UNSW) Science, tells Treehugger in an interview. “And what it means when the water cycle is getting stronger or intensifying, is basically that wet parts of the world are getting wetter twice as fast as we previously anticipated, and dry parts are getting drier twice as quickly.”

Several scientific studies, including the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Working Group I report on the Physical Science Basis, have found the climate crisis is speeding up the water cycle. This happens in part because hotter surface temperatures in the tropics lead to more evaporation.

“Then you’ll have cloud feedbacks in the overall transport of heat, and overall atmospheric circulation will intensify, which will transport that water towards the poles,” Sohail says.

However, it has been hard for scientists to determine exactly how much the water cycle is changing because around 80% of global rainfall and evaporation occur over the ocean, a UNSW press release explains. One way to get around this is to look at the salt content at different locations throughout the ocean.

“In warmer regions, evaporation removes freshwater from the ocean leaving the salt behind, making the ocean saltier,” study co-author Jan Zika, an associate professor in the UNSW School of Mathematics and Statistics, explains in the press release. “The water cycle takes that freshwater to colder regions where it falls as rain, diluting the ocean and making it less salty.”

The researchers looked at three datasets to determine salinity in different parts of the ocean from 1970 to 2014.

“That has been done before,” Sohail tells Treehugger. “But we came up with a few methodological tricks that basically allow you to track more cleanly how the salinity is changing.”

These new methods enabled the researchers to better match the changes in ocean salinity to rainfall and evaporation patterns in the atmosphere. Using them, they discovered that 46,000 to 77,000 more cubic kilometers (approximately 11,036 to 18,473 cubic miles) of freshwater moved from the tropics towards the poles during the study period than previously believed. That’s two to four times more water moving poleward than climate models anticipated.

The Nature study isn’t the only piece of recent research to use salinity to determine that the water cycle is speeding up. Another study published in Scientific Reports in April looked at satellite measurements of ocean salinity and found that the salty parts of the ocean were getting saltier and the fresher parts were getting fresher, also revealing an accelerated water cycle.

The fact that the water cycle is speeding up has serious consequences.

“We all rely on the water cycle for fresh drinking water out of our taps, for navigation of shipping vessels; we need our rivers, lakes, and other reservoirs of water to survive,” Sohail says. “And so this finding is basically showing that the access to freshwater will become more uneven.”

Specifically, Sohail projects three main consequences:

Increased average rainfall in wet regions.

Decreased average rainfall in dry regions.

An uptick in the number of extreme precipitation events.

There is also a geographical flow to this shift in precipitation, so the dryer parts of the tropics and subtropics will get even drier while cold, wet areas like Northern England will get even soggier.

“Basically, most of that water is being transported polewards,” Sohail says, and the two poles will also see more precipitation, which usually falls as snow.

However, the authors of the Scientific Reports water cycle study thought the increased moisture in the air from a sped-up water cycle could be one reason that more precipitation is actually falling as rain over parts of the poles, which exacerbates the impacts of global warming by melting ice.

Overall, Sohail says that policymakers need to be aware of the changes to the water cycle and the impacts they will have.

“It’s just important to try and plan for and mitigate those impacts,” he says.

The Nature study also has important consequences for the climate science community. That’s because the research team compared their observations to the predictions of the current generation of climate models, referred to as Climate Model Intercomparison Project Phase 6 (CMIP6). These are the models that the IPCC uses to make its projections, for example. The researchers ran 20 different models from CMIP6 and found that they all “severely underestimate” the intensification of the water cycle found in the study.

Sohail hopes that future scientists will build on his team’s work to determine:

If the models can be improved to better reflect the changes in the water cycle.

If further observations and modeling efforts corroborate the study’s findings.

He emphasizes the fact that the models are underestimating changes to the water cycle doesn’t mean they are wrong overall, especially about the global increase in temperatures.

“This study doesn’t mean that all climate models are wrong or that all climate studies have been incorrect up to this day,” he says. “Climate science is very much an iterative and consensus-building process, and this is just one step in that.”

For his part, Sohail plans to keep working on the method used in the study, intending to update it from a one-dimensional to a two-dimensional framework and thereby reflect more of the variability in the ocean. By doing this, he hopes to get a more accurate read on how sea levels are changing.

“We really care how much sea level is changing in New York City versus Sydney versus other parts of the world,” he says, adding, “my aim is now to kind of look at how sea level is changing regionally and use this updated version of the method to do that.”