New Frontiers in the Study of Pleistocene-to-Modern Records of Climate Change
Aradhna Tripati, University of California, Los Angeles
We have made numerous inroads in the study of Earth’s climate history since the 1970s, including discovering that present-day levels of greenhouse gases are unprecedented based on the analysis of the chemistry of ice cores and sediment, that climate and CO2 have covaried through time, consistent with greenhouse gas theory, support for the orbital theory of climate change, and the discovery that abrupt climate changes are possible. Yet I will argue the field of paleoclimate is poised to undergo an intellectual period of growth that is not unlike planetary science and astronomy because of new tools that have the potential to illuminate the past more clearly than at any time in the history of our field. New technology is enabling exploration and discovery, resolving longstanding controversies, and allowing for the development of new hypotheses.
For example, a primary goal of paleoclimate research over several decades has been to establish the magnitude of temperature change in different regions, and new tracers are allowing us to finally accurately determine Pleistocene climate variations. I have been adapting a new isotopic tracer, the carbonate ‘clumped’ isotope thermometer, based on the measurement of molecules containing 13C-18O bonds, for the study of Last Glacial Maximum paleoclimate. We are using this tool to study regional changes in hydrology, one of the largest uncer- tainties in observations of paleoclimate, and in simulations of both past and future climate change. Clumped isotope thermometry can be particularly powerful when combined with outputs from isotope-enabled models to probe how atmospheric processes and terrestrial hydrology respond to changing climate forcing.
There are a number of questions pertaining to mid-latitude climate dynamics that we are working to address. For example, we would like to understand why there were large lakes present in areas that are now relatively arid. Through the combination of observations and models, we are able to distinguish the importance of atmospheric rivers, jet stream changes, monsoonal activity, and evaporative controls on water budgets in different regions of the Western US. Another area we are working in involves using ancient soil deposits from different regions of the US, China, and Europe to develop terrestrial maps of paleotemperature and the isotopic composition of precipitation for the Northern Hemisphere, in order to investigate the role of stationary wave forcing and albedo in changing atmospheric circulation.
Tropical paleoclimate dynamics are another area where there are several key questions that need to be addressed. We have been work- ing to reconcile the distribution of tropical glaciers during the last ice age with nearby ocean temperature reconstructions, one of the most longstanding problems in paleoclimatology. This issue is one that seemed to require seemingly impossible lapse rates and has been puzzled over by researchers in a range of disciplines, including paleoclimatologists, oceanographers, glaciologists, terrestrial ecologists, atmospheric dynamicists, and climate modellers - all of whom have published on the topic. Despite decades of study, this problem has not yet been resolved with a dynamically reasonable explanation. I have undertaken a cross-disciplinary effort that brings together the latest developments in geochemistry and atmospheric dynamics to provide an integrated model of changing tropical ocean-atmosphere inter- actions andprovides an explanation for this longstanding problem. Wehave used clumped isotopes to confidently provide newconstraints on ocean temperatures in the warmest open ocean region (the Pacific warm pool), and have shown this region has warmed by a greater amount than previously thought, an innovative result that has recently been validated using two independent thermodynamically-based methodologies. These data allow us to accurately constrain average vertical temperature profiles through the combination of these results with the distribution of glaciers. This work compellingly demonstrated the mean vertical temperature profile of the region is not described by a moist adiabat, consistent with our analysis of extensive modern datasets, and in turn led us to hypothesize that atmospheric mixing is needed to accurately predict tropical glacier distributions during both the 20th century and the Pleistocene. An area for future study involves directly constraining past zonal temperature gradients in the tropical Pacific to resolve an ongoing controversy over the nature ocean-atmosphere interactions in the region.