Moisture Isotopes in the Biosphere and Atmosphere
Background and Justification
The International Atomic Energy Agency (IAEA), in cooperation with the WMO, has long been operating the Global Network of Isotopes in Precipitation (GNIP), which has provided global data to understand and simulate the water cycle under present and past climates. Recently, the IAEA initiated efforts to improve the availability of isotope data on other water cycle components in an effort to supplement GNIP data and integrate isotope applications in hydrological cycle, carbon cycle, and climate research. The group for Moisture Isotopes in the Biosphere and Atmosphere (IAEA-MIBA) was constituted and includes a group of scientists with diverse research interests ranging from local ecosystems to global scales. The motivation for this effort stems from the scarcity of experimental data on stable isotopes in biospheric and atmospheric moisture. Routine measurements of stable oxygen and hydrogen isotopes are crucial to the advancement of hydrological and climate research at the local, regional and global scales. The product of this effort will help in providing an alternative to our present dependency on model output for some key variables, and further advance our understanding of:
Regional scale hydrological budgets: Partitioning evapotranspiration fluxes into surface evaporation and vegetation transpiration; distinguishing evapotranspiration vs advection; quantifying local and regional water re-cycling; improving closure of continental, regional and watershed isotope mass balance.
The partitioning of annual carbon fluxes. The measured variables in the MIBA network drive the 18O composition of atmospheric CO2. The measurement of 18O in CO2 offers both an additional constraint and smaller uncertainties for CO2 flux partitioning between the ocean and the terrestrial biosphere at the global scale; and between assimilation and respiration in land ecosystems.
The development of new global change indicators. 18O and 2H in atmospheric vapour can be used as indicators for regional to global-scale reductions in evaporation perhaps in response to changes in global dimming and brightening.
Ecosystem functioning. Distinguishing productivity responses due to soil moisture stress vs. atmospheric humidity stress; ecosystem-specific effects on the 18O of atmospheric CO2; ecosystem-specific patterns of water use (melt water, permafrost), and allowing the combined use of oxygen and carbon isotopes.
Interpretations of 13C and 18O analyses in organic matter: Improvement of tree-ring-based climate reconstructions; explaining the cause of genetic variation in Ci/Ca (net photosynthesis vs. stomatal conductance).
The validation of general circulation models, Particularly those weather prediction models that aim to couple carbon and water fluxes using stable isotopes to improve simulations of the water and carbon cycles.
Past global responses to climate change. Biospheric productivity forces a difference between the oxygen isotopes of ocean water and atmospheric O2 (the Dole effect). The measurements from the MIBA network will allow further elucidation of the current Dole effect, and allow us to constrain estimates of past global productivity using H2O and O2 trapped in ice cores.
