Modelling ice-sheets as climate forcing and feedback
Professor Jonathan Gregory, University of Reading and Met Office Hadley Centre, UK
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.
Towards improving uncertainty in future evolution of the Arctic sea ice and associated impacts
Dr Julienne Stroeve, University of Colorado, USA
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.