Testing the Tropical Greenhouse Forcing Hypothesis Using a Halmahera Proxy SST Record

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Abstract: Under this award the proponents will test a new hypothesis: tropical ocean temperature variability over the last several hundred thousand years is dominantly driven by changes in greenhouse forcing. This hypothesis will be tested by developing a new proxy sea surface temperature (SST) record from a site in the western equatorial Pacific warm pool lying just east of Halmahera, Indonesia. A high sedimentation rate core (~15 cm/ky) from this site taken by collaborators from Tongji University in Shanghai, China will be targeted for development of a ~30-140 y resolution proxy SST record. SSTs will be reconstructed using the Mg content of the surface-dwelling planktonic foraminifera Globigerinoides rubber. The choice of proxy, resolutions and timescales will allow for three tests of the tropical greenhouse forcing hypothesis. In each case, the Halmahera SST record will be compared to the established greenhouse gas record to determine what fraction of the SST variance can be explained by greenhouse forcing, as opposed to other potential climate forcing mechanisms. The first test uses the near-monotonic rise and mid-rise plateau in atmospheric CO2 over the last deglaciation, the most well dated portion of both the ice core and marine records. The second test uses the four distinct oscillations in atmospheric CO2 that occurred between 65 and 20 thousand years before present during marine isotope stage (MIS) 3. The third test uses orbital scale (23- 100 ky) oscillations in atmospheric CO2 that are well documented in several Antarctic ice cores. Other climate influences will be evaluated as alternative hypotheses.The development of the Halmahera proxy SST record will add to available records of tropical Pacific climate variability on both centennial-to-millennial and orbital timescales. A high resolution (~30-70 y) record of warm pool SSTs during the deglaciation and MIS 3 has direct relevance to testing models of rapid climate change during these time intervals. In addition, if greenhouse forcing is the main control on tropical climate variability, the past relationship between forcing and temperature change provides a direct measure of climate sensitivity. Results from this study will advance independent determinations of climate sensitivity from paleoclimatic data. Broader impacts include collaboration with scientists in China and record of teaching and developing graduate students.