The Influence of 21st Century Climate Change on the Isotopic Composition of Atmospheric Moisture and How it Relates to Past Hydrological Changes

Abstract:

Increases in greenhouse gas concentrations through the 21^st century are projected to increase global temperatures and change circulation and precipitation patterns globally. However, there remain many uncertainties in how the general circulation of the atmosphere will change and how it will impact regional hydroclimates. In the low and middle latitudes the isotopic composition (δ) of atmospheric moisture could potentially be useful at tracing these changes in precipitation and wind patterns. In this study sea surface temperatures and sea ice conditions from 21^st century climate projections (RCP8.5 scenario) were used to force the isotope-enabled Global Spectral Model (IsoGSM). This ensemble of IsoGSM simulations provides insight as to how and where water isotopologues will change globally as a result of 21^st century climate change. In general, δ values increase in the subtropics and middle latitudes and decrease in the southern tropics. Changes to horizontal winds suggest that the isotopic changes are likely due to changes in the strength of the Hadley Cell, rather than the poleward expansion of the descending branch of the cells. Regionally, the simulations project consistent increases in δ values through the 21^st century over central and southern Africa, the Tibetan Plateau, and the eastern Australia. Decreasing δ values were found over the eastern tropical Pacific and the western margins of South America. A comparison with a present-day IsoGSM simulation reveals similar regional changes in δ values over the last 60 years. The similarities between recent changes and 21^st century projections of δ values suggest that certain hydrological aspects of 21^st century climate change are already taking place in some regions. Central Africa stands out as a region where IsoGSM simulates robust rises in precipitation and vapor δ values for both the 21^st century and the late 20^th century. The recent rise in δ values over central Africa is validated against δ variations from a Lake Bosumtwi sediment archive. The present-day increase in regional δ values could be a precursor of worsening drought conditions as the Earth warms through the 21^st century.