The discoveries of accelerating ice loss from Antarctica and Greenland and the importance of ice sheet dynamics in their mass balances rest on measurements by a suite of satellite and airborne sensors using novel techniques (Bindschadler et al. 1996, Chen et al. 2006a, Kerr 2006, Luthcke et al. 2006, Rignot and Kanagaratnam 2006). These discoveries are possible because of decades of optical and radar images, laser and radar altimeters, and more recently the National Aeronautics and Space Administration’s (NASA) Gravity Recovery and Climate Experiment (GRACE) mission, which measures ice mass directly through its gravitational pull. In addition, airborne laser altimeter data show thinning of ice near the coastline, radar data show faster flow, Landsat data show retreat of the grounding line,2 and the Moderate Resolution Imaging Spectroradiometer (MODIS) data show calving of large icebergs. Warming ocean waters seem to have increased calving of the ice shelves, thereby allowing the ice sheet’s outlet glaciers to flow more quickly (Box 7.1). Glaciers in Greenland have also increased in velocity, perhaps from increased basal lubrication by meltwater penetrating from the surface. These new discoveries indicate that ice stream dynamics (the balance between the forcing, such as ice thickness and surface slope, and the resistance, such as internal stiffness) are the primary drivers of rapid sea-level change instead of the balance between melting and precipitation.
The ability to estimate the overall mass of ice sheets is a remarkable accomplishment of satellite observations. Numerous techniques, including radar images, measurements of surface elevation from laser altimeters, and GRACE’s gravity data, now show that both Greenland and Antarctica have been losing ice over the past 5 to 10 years. From 2003 to 2005, Greenland lost more than 155 gigatons3 per year at lower elevations and gained about 54 gigatons per year at higher elevations, with most of the losses occurring during summer (Chen et al. 2006b, Luthcke et al. 2006, Rignot and Kanagaratnam 2006, Wahr et al. 2006). In Antarctica the gravity data show mass losses of 70-200 km3 per year (60-160 gigatons per year). Most of the loss is from West Antarctica, with East Antarctica in approximate balance (Figure 7.3).
Just as miners once had canaries to warn of rising concentrations of noxious gases, researchers working on climate change rely on arctic sea ice as an early warning system.
—Arctic Climate Impacts Assessment (2004)
For many reasons, observing trends in sea ice reliably has been possible only with the advent of satellite observations. Navigating the remote and frozen seas off Antarctica or in the Arctic to obtain in situ measurements of sea ice extent is treacherous, and sea ice extent is highly variable in time and space due to wind advection and localized melting. Before satellite observations became available, spatial coverage of sea ice was monitored by tracking the location of the ice edge from ships. Because the ice edge is moving with winds and ocean currents, it is not a robust indicator of basin-scale sea ice extent. Thus, accurate and quantitative interannual comparisons of the basin-scale ice coverage became only possible with the availability of the synoptic view from satellites.
Sea ice has been monitored continuously with passive microwave sensors (Electrically Scanning Microwave Radiometer [ESMR], Scanning Multichannel Microwave Radiometer [SMMR], Special Sensor Microwave/Imager [SSM/I], and Advanced Microwave Scanning Radiometer-Earth Observing System [AMSR-E]) since 1979. Not limited by weather conditions or light levels, they are particularly