Stable isotopes record changes in the water cycle over the past 2000 years

As Earth’s climate shifts, we experience changes in evaporation and precipitation. Worldwide, these alterations in the water cycle are incorporated into the structures of trees, coral beds, glaciers, sediment deposits, stalagmites and stalactites, providing a powerful global record of changes to the hydrosphere and climate.

Water is composed of two hydrogen atoms and one oxygen atom. Hydrogen can exist as ¹H, with an atomic mass unit (amu) of 1, or in a more rare heavy form, ²H. Similarly, oxygen typically has an amu of 16, but can also be present as a heavier ¹⁸O isotope. Evaporation and condensation preferentially act upon the light and heavy isotopes of water, respectively. As a result, water isotope ratios in reservoirs like the ocean and groundwater fluctuate depending on temperature-dependent evaporation and precipitation rates. These isotope ratios are directly incorporated into biological and geological structures, enabling scientists to study climate trends, water circulation patterns, and linkages between Earth processes. 

In an unprecedented effort to consolidate water isotope proxy records for climate, the Past Global Changes (PAGES) Iso2k working group collected 705 records from 471 sites with data records extending to 2000 years ago. Records included tree rings, coral, ice cores, sediment cores, stalagmites and stalactites sampled across the globe. Dr. Anais Orsi of the University of British Columbia’s Dept. of Earth Oceans and Atmospheric Science provided ice core records from Antarctica. All the paleoclimate datasets are compiled and publicly available from the recently published PAGES Iso2k database. 

With the newly consolidated data set, the Iso2k working group evaluated the effect of temperature on isotope ratios in seawater, precipitation, and snowfall over the past 2000 years.  Their initial findings were published this month by Nature Geoscience in the article ‘Globally Coherent water cycle response to temperature change during the past two millennia’. Yet this is only the beginning. Over the next two years, the group plans to conduct another ~10 studies based on the Big Data they have collected, not to mention the countless other studies this publicly available data will inspire.