Study links changes in global water cycle to higher temperatures

It’s a multibillion-dollar question: What will happen to water as temperatures continue to rise? There will be winners and losers with any change that redistributes where, when and how much water is available for humans to drink and use.

To find answers and make informed predictions, a team of scientists has looked to the past. 

A study published Nov. 2 in Nature Geoscience and co-authored by UB geologist Elizabeth Thomas takes an important step toward reconstructing a global history of water over the past 2,000 years. 

Using geologic and biologic evidence preserved in natural archives — corals, trees, ice, cave formations and sediments — researchers from the Past Global Changes (PAGES) Iso2k project showed that the global water cycle has changed during periods of higher and lower temperatures in the recent past.

“This suggests that the water cycle may also respond in step with today’s rapid warming, with implications for water supply around the world,” says Thomas, associate professor in the Department of Geology, College of Arts and Sciences. “Understanding how the water cycle will respond to changes in Earth’s temperature in the coming years is critical as our society and economy adapt to a rapidly changing climate.”

Reconstructions of past climate change using geologic data have helped to show the far-reaching influence of human activity on temperatures since the Industrial Age, but assembling hydroclimate records for the same timeframe has proved to be much harder.

The water cycle is complex, and rainfall in particular has geographic variations that are much more drastic than air temperature. 

“We decided to start with water isotope records because they reflect holistic signals and because they’re recorded in all kinds of different natural archives,” says Bronwen Konecky, assistant professor of earth, environmental and planetary sciences at Washington University in St. Louis and lead author of the study. “This is a first step toward reconstructing drought or rainfall patterns at the global scale during the past 2,000 years.”

The global water cycle is vast and intertwined. Water evaporates from the surface of the Earth, rises into the atmosphere, cools and condenses into rain or snow in clouds, and falls again to the surface as precipitation. 

Each water molecule that is part of the cycle has a certain isotopic “fingerprint,” or composition, which reflects small variations in the atomic weight of the oxygen and hydrogen atoms that comprise the molecule. So, individual water molecules can be heavier or lighter.

 With this new study, scientists found that when global temperature is higher, rain and other environmental waters become more isotopically heavy. The researchers interpreted these isotopic changes and determined their timeline by synthesizing data from across a wide variety of natural archive sources from the past 2,000 years of Earth history.

The PAGES Iso2k project team — which includes more than 40 researchers from 10 countries — collected, collated and sometimes digitized datasets from hundreds of studies to build the database they used in their analysis. They ended up with 759 paleoclimate records from globally distributed time-series datasets, representing the world’s largest integrated database of water isotope proxy records.

Thomas led the team that compiled water cycle records from organic molecules preserved in lake sediments. 

“The next step, which led to this study, was to examine all the records in the database together to understand the patterns and processes controlling global water cycle change during past periods of warming and cooling,” she says. “We met regularly for years to discuss figures, interpret the results and write the study.”

They organized teleconference sessions at odd hours to accommodate time zones from Hawaii to Japan to Australia to Europe and in between. 

“We even spent one New Year’s Eve working on the database and the analyses that led to this paper,” Konecky says.

Global scale relationships between temperature and the isotopic composition of certain environmental waters, like seawater and glacial ice, have long been recognized as the planet moves in and out of ice age cycles. Local scale relationships with temperature on timescales of minutes to months are also well established.

But this study provides the first evidence that temperature and the isotopic composition of environmental waters go hand in hand at timescales in between these two — that is, over decades to centuries.

The changes scientists observed were driven by global ocean evaporation and condensation processes, with lower values during the period of time known as the Little Ice Age (1450-1850) and higher values after the onset of human-caused climate warming, starting around 1850.

As for how these changes impact future rainfall and water availability, it is too early to predict who will win and who will lose. But this study’s data from the last 2,000 years suggest that more water cycle changes are likely as global temperatures continue to increase. June, July and August of this year were the hottest months on record for our planet.

“The way water behaves when it leaves the oceans and moves around the atmosphere and rains out — that behavior is strongly impacted by changes in atmospheric temperature,” Konecky says.