Abrupt CO2 Change and the Southern Hemisphere Westerlies: Testing the Upwelling Hypothesis

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Carbon dioxide (CO2) is the primary atmospheric greenhouse gas that plays a key role both in current climate change as well as in past natural climate variations such as the ice-age cycle. Understanding the natural causes of CO2 change is an important goal of climate research, with applications in improving climate projections for the future. At the coldest point of the last ice age (around 20,000 years ago) the atmospheric concentration of CO2 was around 90 parts per million (ppm) below the pre-industrial level. Most researchers think that during the ice age this missing carbon was stored in the deep ocean. Around 18,000 years ago, the CO2 concentration in the atmosphere started rising, marking the end of the last ice age. The cause for the CO2 rise is not well understood and heavily debated. A popular hypothesis is that the Southern Hemisphere westerly winds shifted southwards, causing CO2 release from the ocean due to increased ocean upwelling in the Southern Ocean. However, climate models disagree on whether this mechanism actually works. Recent research on Antarctic ice cores has provided evidence for repeated shifts in the position of these westerly winds during the last ice age in response to so-called Dansgaard-Oeschger events. This project will use ice-core samples to measure past changes in atmospheric CO2 at very high resolution during these natural shifts in the Southern Hemisphere westerlies. This will allow a direct test of the upwelling hypothesis for the rise in CO2. Currently, the Southern Hemisphere westerlies are again shifting southwards due to stratospheric ozone depletion and global warming. This may reduce carbon uptake by the world's oceans, exacerbating warming from anthropogenic emissions. These recent trends provide great impetus to better understand the relationship between CO2 and the westerly winds. This project work aims to produce a benchmark CO2 record that will be widely used by climate researchers. The project contributes to development of the STEM (Science, Technology, Engineering and Mathematics) workforce by training a postdoctoral researcher. An ongoing science outreach project will visit around 20 middle school classrooms annually to talk about polar and climate research. The project will measure in decadal temporal resolution the evolution of atmospheric CO2 in the WAIS Divide ice core during key climatic events (Heinrich events 2-5 and Dansgaard-Oeschger events 3-8 and 12) where ice-core markers suggest shifts occurred in the Southern Hemisphere westerlies. Existing ice-core CO2 records imply minor variations during these times, but they lack resolution to be conclusive on this issue. The project includes improvements to the CO2 analytical setup that will enhance sample throughput and analytical precision. The proposed number of samples is 720 (240 unique depths in triplicate) in the interval 27,000 to 40,000 years before present to complement the existing low-resolution CO2 record. The ice-core analyses are integrated with modeling and data-analysis to better understand the Antarctic water isotopic evidence for shifts in the Southern Hemisphere westerly winds and their link to CO2 variations, using existing output from isotope-enabled general circulation models and reanalysis data. In particular, the proposed work investigates the possibility of large wind-driven upwelling events in the Atlantic sector around Antarctica associated with Heinrich events--a previously unrecognized mode of climate variability. This project uses existing ice samples and does not require Antarctic field work.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.