Linking changes in water cycle to higher temperatures

Washington University in St. Louis (WeatherFarm) – 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.

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.

A study from the Past Global Changes (PAGES) Iso2k project, led by Bronwen Konecky at Washington University in St. Louis, took an important step toward reconstructing the global history of water over the past 2,000 years. Using geologic and biological evidence preserved in natural archives — including 759 different paleoclimate records from globally distributed corals, trees, ice, cave formations, and sediments — they showed the global water cycle changed during periods of higher and lower temperatures in the recent past.

“The global water cycle is intimately linked to global temperature,” said Konecky. “We found that during periods when temperature is changing at a global scale, we also see changes in the way that water moves around the planet.”

The water cycle is complex, and rainfall in particular has geographic variations that are much more drastic than air temperature. This has made it difficult for scientists to evaluate how rainfall has changed over the period.

“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,” Konecky explained. “This was 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 part of the cycle has a certain isotopic 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.

The scientists found 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.

The project collected, collated, and sometimes digitized datasets from hundreds of studies to build a database. They ended up with 759 globally distributed time-series datasets, representing the world’s largest integrated database of water isotope proxy records.

Piecing together signals from many different types of natural archives was like piecing together apples and oranges. They knew that water isotopes record climate signals in specific ways in different natural archives. Carefully assembled, this common thread could help them to compare a tree ring to an ice core.

“Every archive is different,” Konecky said. “Datasets from different archives are generated by different scientific communities with their own terminology, norms, and reference materials. We came up with data description fields for the database that translate each record’s particularities into a common tongue that made it possible to compare variations in one archive to variations in another.”

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 provided 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.

“As the planet warms and cools, it affects the behavior of water as it leaves the oceans and the vigor of its motions through the atmosphere,” Konecky said, noting it’s a rapid adjustment.

The scientists found that global mean surface temperature exerted a coherent influence on the isotopic composition of global precipitation and “meteoric water” (water in lakes, rivers, and ice melts) throughout the past 2,000 years. The changes they 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.

When it comes to the specific impact of these changes on 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 2023 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 said.