Collaborative Research: P2C2: Reconstructing Holocene Climate Change in the Southern Hemisphere from Southern Alps Mountain Glaciers and Tree Rings

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A major objective of climate science is to evaluate ongoing planetary warming within the context of past natural variations of the climate system, such as the well-known Little Ice Age and Dark Ages cold periods, and Medieval and Roman warm periods, as well as warmer-than-present conditions in some regions registered during the early Holocene, about 8,000 to 10,000 years ago. These past climatic anomalies are best documented from the Northern Hemisphere; thus, it is not clear whether they constituted global fluctuations in the atmospheric heat budget or if they were regional in nature. This question has fundamental implications for understanding the underlying drivers of natural climate variability, and for determining whether industrial-age warming has yet exceeded the envelope of climate variability registered over the past 10,000 years. This project establishes the record of Holocene climate variations, based on glaciers and temperature-sensitive trees, in the Southern Alps of New Zealand - on the opposite side of the planet from the North Atlantic region. Mountain glaciers are among Earth's most sensitive climate recorders on timescales of decades and longer. Worldwide recession of mountain glaciers is an iconic illustration of global warming. Glacial landforms afford physical evidence of past glacier extent, and this effort will employ state-of-the-art cosmogenic dating techniques on those landforms to establish precise and accurate chronologies of glacier changes during and prior to the period of recorded observations. This work incorporates tree-ring chronologies of temperature-sensitive South Island species that afford continuous records of climatic fluctuations on annual timescales. Altogether, chronologies of glacier extent and tree growth will be used to develop a comprehensive record of past temperature variation in the Southern Alps for comparison to equivalent records in the Northern Hemisphere. The resultant temperature reconstruction will also provide a key metric for evaluating a suite of global climate and earth-system models developed by the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory, some of which are now being used for weather forecasting by the National Weather Service. This effort will afford field-based educational experiences for the next generation of scientists and science journalists. A goal of paleoclimate research is to understand patterns and drivers of natural climate variations during the Holocene. Documentation of natural climate variability on orbital, centennial, and shorter frequencies provides a contextual framework for modeling ongoing and future warming due to the effects of increasing atmospheric CO2. The most recent major natural climatic variations in the North Atlantic regime prior to the ongoing warming were those of the Roman Warm Period (~250 B.C. to ~A.D. 400), Dark Ages Cold Period (~A.D. 450 to ~A.D. 950), Medieval Warm Period (~A.D. 950 to ~A.D. 1150) and the Little Ice Age (~A.D. 1150 to ~A.D. 1850). These late-Holocene temperature variations are documented by highly resolved records of glacier-length variation in the European Alps, as well as independent, annually resolved tree-ring-based regional summer temperature reconstructions. In addition, European glaciers monitored a period of reduced ice extent during the early Holocene at the same time as tree-line altitudes were higher. Both observations signify summers that were warmer than those of today. But it is not yet clear whether these climate signatures were global or regional in scope; thus, the underlying mechanisms remain uncertain. This work will address this problem by developing a precise 10Be surface-exposure chronology of moraines constructed by glaciers during the Holocene in the Southern Alps of New Zealand - on the opposite side of the planet from the European Alps. The researchers will also develop chronology of Holocene ice retraction by 14C dating recently deglaciated organic remains. Investigators employ a well-calibrated glaciological model to convert these moraine chronologies into a glacier-derived record of temperature over the past two millennia. The proposed glacier chronology will be synthesized with an independent and complementary record of regional summer temperatures for the past 2000 years derived from South Island silver pine ring widths. The glacier and tree-ring records will be used as benchmarks to investigate results from the GFDL ESM2M preindustrial climate simulations. On the basis of this glacier, dendro, and modeling approach, the goal of this work is to determine whether Holocene climate variations were global or not, and therefore improve understanding of underlying drivers and the scope of natural climate variability in light of ongoing planetary warming.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.