Modelling ice-sheets as climate forcing and feedback
Professor Jonathan Gregory, University of Reading and Met Office Hadley Centre, UK
Abstract
Ice-sheets are very sensitive to climate change through its effect on their surface mass balance i.e. mostly snowfall minus surface melting, and through changes in basal melting at marine margins having an effect of the dynamics of grounded ice. Ice-sheet mass changes are a matter of practical concern because of their influence on sea level; the effect of anthropogenic climate change on the ice-sheets could produce changes in sea-level of many metres over future centuries, modulated regionally by the consequent changes in the geoid. Ice-sheets influence climate through other effects than sea-level: absorbed solar radiation is reduced by high albedo, surface temperature is cooled by raised elevation, precipitation is enhanced upwind and suppressed downwind of an ice-sheet, tropospheric circulation is altered by ice-sheet topography in ways which may affect climate remote from the ice-sheet, and freshwater input may influence ocean circulation. If ice-sheets are treated as a boundary condition for climate, changes in ice-sheets are a forcing of climate change, but if ice-sheets and climate are regarded as a coupled system, ice-sheet changes give feedbacks on climate change. In either case, it is valuable to quantify the effects. The coupled approach is essential to achieve a full understanding of the enormous and complex changes which take place during glacial cycles, and is needed for accurate projections of some aspects of anthropogenic climate change. Coupled AOGCM--ice-sheet models offer the most physically comprehensive treatment. This is a relatively new endeavour. There are many technical challenges and no doubt much to be learned scientifically.
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Professor Jonathan Gregory, University of Reading and Met Office Hadley Centre, UK
Professor Jonathan Gregory, University of Reading and Met Office Hadley Centre, UK
Jonathan Gregory joined the Met Office Hadley Centre soon after it opened in 1990, following a PhD in particle physics and a year at the Climatic Research Unit of the University of East Anglia. Since 2003 he has been a senior climate research scientist of the National Centre for Atmospheric Science at the University of Reading and a Met Office Fellow in climate change research at the Hadley Centre, and since 2006 a professor in the Department of Meteorology. He was a coordinating lead author of the sea level chapter of the Third Assessment Report of Working Group I of the Intergovernmental Panel on Climate Change, a lead author of the ocean observations and projections chapters of the Fourth Assessment Report, and a lead author of sea level chapter of the Fifth Assessment Report. In 2009 he was awarded an Advanced Grant by the European Research Council for research on sea-level change. His recent interests also include climate sensitivity and radiative forcing, land ice response to past and future climate change, ocean heat uptake and changes in the Atlantic Ocean meridional overturning circulation. He is an author of 150 publications in climate science.
Towards improving uncertainty in future evolution of the Arctic sea ice and associated impacts
Dr Julienne Stroeve, University of Colorado, USA
Abstract
The modern satellite passive microwave record provides consistent estimates of Arctic sea ice extent since late 1970s. These data document downward linear trends in Arctic sea ice extent for all months, but with the largest trend for September at -86,000 km2 yr-1 between 1979 and 2014. Less clear is how the thickness has changed over time as long-term consistent data sets are not available. However, based on the data that is available, it is clear that the Arctic sea ice has thinned over the last few decades, making the Arctic more vulnerable to melting out each summer.
All climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) show that Arctic sea ice extent will continue to decline, the eventual outcome being an essentially seasonally-ice Arctic Ocean. While the CMIP5 models better capture the observed mean state of the Arctic sea ice extent than the CMIP3 models did, the models remain poor at representing the spatial pattern of the ice thickness and general atmospheric circulation pattern, which bears on the timing of when seasonally ice-free conditions may happen. Given the influence the ice cover plays on the overlying atmosphere, with feedbacks on lower latitudes, improving model performance is essential to making good decisions within an adaptive framework.
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Dr Julienne Stroeve, University of Colorado, USA
Dr Julienne Stroeve, University of Colorado, USA
"Julienne C Stroeve received a PhD in geography from the University of Colorado Boulder, in 1996, for her work in understanding Greenland climate variability. Subsequently she has been a research scientist at the National Snow and Ice Data Center (NSIDC) within the Cooperative Institute for Research in Environmental Sciences (CIRES). Her Arctic research interests include atmosphere-sea ice interactions, sea ice predictability, climate change and associated impacts. She has conducted several Arctic field campaigns. Recent research is focused on understanding rapid Arctic change and what this will mean for the rest of the planet. Dr Stroeve’s work has been featured in numerous magazines, news reports, radio shows, and TV documentaries. She has given keynote addresses around the world on Arctic climate issues and briefed former Vice President Al Gore. Dr Stroeve has published more than 50 articles on peer-reviewed journals and contributed to several national and international reports on climate change."