Paleoclimate perspectives on polar ice sheet and sea-level sensitivity to global warming

Professor Tim Naish

Antarctic Research Centre, Victoria University of Wellington

Near- and long-term future projections of global mean sea-level rise are hampered by a lack of understanding of the potential dynamic contribution of the polar ice sheets. As the Intergovernmental Panel on Climate Change’s Working Group 1 nears the completion of the 5th Assessment Report a major challenge continues to be placing an upper bound in sea-level projections for 2100. The so called “deterministic” approach sums observed and model-projected trends in the known contributions (e.g. ice sheet and glacier surface mass balance, ocean thermal expansion and ground water storage), and implies an upper bound of ~80-100cm by 2100. The “semi-empirical” approach scales past observed sea-level change to mean surface temperature, using this relationship with future temperature scenarios to imply a significantly higher upper bound of up to ~2m by 2100. The discrepancy between the two approaches in part relates to the poorly understood contribution of ice dynamics – that is the rate of flow of ice sheets into the ocean. An ensemble of Antarctic ice sheet models produces highly divergent results, primarily because of uncertainties around the mass changes in the East Antarctic Ice Sheet with some models showing increased precipitation driving a positive mass balance overall, even with loss of the marine-based West Antarctic Ice Sheet. Current best estimates suggest a 10-20cm dynamic ice sheet contribution by 2100.

Of concern is that marine based ice sheets are highly sensitive to increases in ocean temperature at their margins and rapid disintegration may ensue if the ice sheet is grounded in deep sub-glacial basins behind the present day calving line. Recent studies show the highest rates of ice sheet thinning and retreat are occurring at locations around the West Antarctic Ice Sheet where the surface ocean has warmed.

Geological paleoclimate records allow the equilibrium sensitivity of polar ice volume and global sea-level change to be reconstructed and assessed during past warm climates and deglaciations, that may be representative of our future climate trajectory. In this talk I will focus on ice sheet responses to climate forcing, and rates of sea-level rise during: (1) The mid-Pliocene warm period ~3 million years ago when the world was 2-3°C warmer, and atmospheric carbon dioxide concentrations were 400ppm (close to present). (2) The Last Interglacial Period ~ 125,000 years ago when the world was 1-2°C warmer. (3) The warming period from the Last Glacial Maximum ~20,000 years ago to our present interglacial. All three past times provide natural experiments with insights into the future response of the polar ice sheets to global warming.

Biography

Professor Tim Naish is Director of the Antarctic Research Centre at Victoria University of Wellington and Principal Scientist at the New Zealand Crown Research Institute, GNS Science. He is a paleoclimatologist focused on reconstructing past ice sheet and global sea-level changes relevant to future projections. He has participated in 9 expeditions to Antarctica and helped found ANDRILL, an international Antarctic Geological Drilling Program. He is currently a Lead Author on the Intergovernmental Panel of Climate Change’s 5th Assessment Report, and Director of the Joint Antarctic Research Institute (JARI). Tim and his team at the Antarctic Research Centre have a developed a strong track record of communication of climate change and Antarctic issues and their impact on new Zealand through education and outreach to the public, practitioners and policy makers. He received the New Zealand Antarctic Medal and the New Zealand Science and Technology Medal for fundamental contributions to Antarctic climate change research.