4.5 Changes in Glaciers and Ice Caps
Those glaciers and ice caps not immediately adjacent to the large ice sheets of Greenland and Antarctica cover an area between 512 × 103 and 546 × 103 km2 according to inventories from different authors (Table 4.3); volume estimates differ considerably from 51 × 103 to 133 × 103 km3, representing sea level equivalent (SLE) of between 0.15 and 0.37 m. Including the glaciers and ice caps surrounding the Greenland Ice Sheet and West Antarctica, but excluding those on the Antarctic Peninsula and those surrounding East Antarctica, yields 0.72 ± 0.2 m SLE. These new estimates are about 40% higher than those given in IPCC (2001), but area inventories are still incomplete and volume measurements more so, despite increasing efforts.
Table 4.3. Extents of glaciers and ice caps as given by different authors.
|Reference ||Area (103 km2) ||Volume (103 km3) ||SLEf (m) |
|Raper and Braithwaite, 2005a,c ||522 ± 42 || 87 ± 10 || 0.24 ± 0.03 |
|Ohmura, 2004a,d ||512 ||51 ||0.15 |
|Dyurgerov and Meier, 2005a,e ||546 ± 30 ||133 ± 20 || 0.37 ± 0.06 |
|Dyurgerov and Meier, 2005b,e || 785 ± 100 ||260 ± 65 ||0.72 ± 0.2 |
|IPCC, 2001b ||680 ||180 ± 40 ||0.50 ± 0.1 |
Glaciers and ice caps provide among the most visible indications of the effects of climate change. The mass balance at the surface of a glacier (the gain or loss of snow and ice over a hydrological cycle) is determined by the climate. At high and mid-latitudes, the hydrological cycle is determined by the annual cycle of air temperature, with accumulation dominating in winter and ablation in summer. In wide parts of the Himalaya most accumulation and ablation occur during summer (Fujita and Ageta, 2000), in the tropics ablation occurs year round and the seasonality in precipitation controls accumulation (Kaser and Osmaston, 2002). A climate change will affect the magnitude of the accumulation and ablation terms and the length of the mass balance seasons. The glacier will then change its extent towards a size that makes the total mass balance (the mass gain or loss over the entire glacier) zero. However, climate variability and the time lag of the glacier response mean that static equilibrium is never attained. Changes in glacier extent lag behind climate changes by only a few years on the short, steep and shallow glaciers of the tropical mountains with year-round ablation (Kaser et al., 2003), but by up to several centuries on the largest glaciers and ice caps with small slopes and cold ice (Paterson, 2004). Glaciers also lose mass by iceberg calving: this does not have an immediate and straightforward link to climate, but general relations to climate can often be discerned. Mass loss by basal melting is considered negligible at a global or large regional scale.