CAREER: Neutrons in Numerical Simulations--Integrating Water Isotopic Data across Past and Present to Improve Future Hydroclimate Projections

William Marsh Rice University, Houston TX

Investigators

Abstract

The heavy isotopes deuterium (D), a form of hydrogen in which the nucleus has a neutron as well a a proton, and oxygen-18 (18O), a form of oxygen with two extra neutrons, are found in trace amounts in all water on Earth. These isotopes are of interest for two reasons: first, the heavier water molecules formed from them, with formulas HDO and H2(18O) instead of H2O, evaporate more sluggishly and condense more readily than ordinary water, and the differences in evaporation and condensation depend on temperature. The relative abundances of the heavier forms of water thus contain valuable information about the climatic conditions encountered by water along its journey through the hydrological cycle. Second, the heavier isotopes are stable, so their record of climatic conditions lasts essentially forever and can be used to track the water cycle over periods from days to millennia. The heavier isotopes of water can be measured in ice cores over multiple ice ages, and isotopic abundances present in rainfall long ago can be determined from tree rings, speleothems (the stalactites and stalagmites found in limestone caves), and even fossil corals. But the climatic record encoded in the heavier water isotopes has not been used to its full potential for a variety of reasons, chief among them the lack of easy access to isotope data in a standardized format, covering both present-day measurements and paleoclimate proxies. Another is a lack of isotope-enabled climate and earth system models that can be used to interpret the isotope record. Coordinated modeling efforts in which isotopic effects are represented consistently across an ensemble of models can be extremely valuable for relating isotopic abundances to specific climatic effects. For example it would be difficult to determine based purely on observations whether depletion of heavy isotopes in ice cores is a consequence of warmer temperatures in the source region where the water evaporated or a consequence of rainout as the water vapor traveled from the source region to the ice sheet. Such distinctions can easily be made in model simulations, but guidance from models is most trustworthy when several models are used and they all handle isotopes in a consistent manner. Work performed here addresses the data access barrier through the creation of the Water Isotopes and Climate Network (WISONet), a research platform that provides seamless access to modern isotope observations and paleoclimate proxy data. Present-day isotope data includes in-situ measurements of rainwater isotopic composition, in-situ measurements of water vapor isotopic composition, and estimates of water vapor isotopes based on satellite data (for instance HDO retrievals from AIRS, the Atmospheric Infrared Sounder). The paleoclimate proxies mentioned above are also included, along with relevant paleoclimate reconstructions covering the last two millenia (the project builds on the earlier Iso2k effort). As for isotope-enabled model simulations, the project supports the Stable Water Isotope Intercomparison Group version 3 (SWING3), a coordinated effort to generate a database of simulations for the period 1000 to 2100 CE using consistently isotope-enabled models (typically the atmospheric component models of earth system models). The resources of WISONet and SWING3 are used to address questions related to the climate sensitivity of the earth (the amount of warming caused by a doubling of carbon dioxide) and the response of the east-west tropical overturning circulation to warming and cooling. Other research focuses on the extent to which water isotopes can be used to infer changes in climate processes, for instance whether they can be used to infer the extent of vertical mixing from the ocean surface to the atmosphere above the planetary boundary layer. The educational component of this CAREER award includes efforts to teach climate and hydrological science to audiences including students in the Houston Independent School District (HISD), the Girl Scouts of San Jacinto Council (GSSJC), students at Rice University, and the general public. The Principal Investigator (PI) works with 11-15 HISD high school teachers to develop lessons related to climate science and the water cycle. Through this award the PI organizes an annual Girl Scout Climate Challenge Event at Rice, attended by over 100 Girl Scouts. As for on-campus activity, the award supports the addition of labs and field trips to the PI's climate science classes. For instance Longhorn Caverns is a venue where students can learn about speleothems and their use as a paleoclimate proxy. Finally, the PI engages with the general public by managing the development of rooftop rainwater collectors which can be deployed and maintained by interested members of the public. The rainwater collectors are designed to collect rainwater samples during intense storms and properly store the samples so that their isotopic composition is preserved for later measurement. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.