The global water cycle underpins every aspect of human society and our environment, therefore shifting patterns of rain and evaporation have profound consequences for agriculture, infrastructure, and habitats. However, quantifying historical water cycle change is challenging, owing to a lack of direct observations - particularly over the ocean, where the majority of global precipitation and evaporation occur.

Observed poleward freshwater transport since 1970, published today in Nature, provides a new estimate of global water cycle change from warm to cold regions while identifying a key inaccuracy in current climate models. 

The research team, which included John Church and Damien Irving, used ocean salinity as a proxy for rainfall in their study. Changes in the ocean’s salinity can be used as a type of rain gauge to detect water cycle changes. 

A/Prof Zika explains: "When fresh water falls as rain on the ocean, it dilutes the sea water and makes it less salty. When water evaporates from the surface, the salt is retained, increasing the salinity". 

The team developed new methods enabling them to precisely link changes in the ocean’s salinity to changes in the part of the water cycle moving fresh water from warmer to colder regions. Their estimates indicate how the broader water cycle is changing in the atmosphere, over land and through the oceans.

The team discovered that an additional estimated 46,000 to 77,000 cubic kilometres of water - the equivalent of 123,000 Sydney Harbours worth of freshwater - have shifted towards the poles since 1970. 

This equates to 7% more rain in wetter areas, and 7% less rain (or more evaporation) in drier areas, and reveals that between two and four times more freshwater has moved than existing climate models anticipated.

The results reveal that the effects of climate change and rising global temperatures on rainfall are more drastic than previously thought. Long term changes to the water cycle are tipped to generate more intense droughts and extreme rainfall events.

The team is optimistic that the methods they are developing will become a mainstay in analysing both observations and climate models. 

"I would say this is the first time that the poleward transport of freshwater - a critical limb of the water cycle - has ever been quantified", said Dr Sohail. "This finding is going to be particularly useful to scientists and climate modelers who will have a new baseline for freshwater transport that can be used well into the future". 

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Taimoor Sohail UNSW
Jan Zika