Executive Summary

In the climate system, the cryosphere (which consists of snow, river and lake ice, sea ice, glaciers and ice caps, ice shelves and ice sheets, and frozen ground) is intricately linked to the surface energy budget, the water cycle, sea level change and the surface gas exchange. The cryosphere integrates climate variations over a wide range of time scales, making it a natural sensor of climate variability and providing a visible expression of climate change. In the past, the cryosphere has undergone large variations on many time scales associated with ice ages and with shorter-term variations like the Younger Dryas or the Little Ice Age (see Chapter 6). Recent decreases in ice mass are correlated with rising surface air temperatures. This is especially true for the region north of 65°N, where temperatures have increased by about twice the global average from 1965 to 2005.

• Snow cover has decreased in most regions, especially in spring and summer. Northern Hemisphere (NH) snow cover observed by satellite over the 1966 to 2005 period decreased in every month except November and December, with a stepwise drop of 5% in the annual mean in the late 1980s. In the Southern Hemisphere, the few long records or proxies mostly show either decreases or no changes in the past 40 years or more. Where snow cover or snowpack decreased, temperature often dominated; where snow increased, precipitation almost always dominated. For example, NH April snow cover extent is strongly correlated with 40°N to 60°N April temperature, reflecting the feedback between snow and temperature, and declines in the mountains of western North America and in the Swiss Alps have been largest at lower elevations.
• Freeze-up and breakup dates for river and lake ice exhibit considerable spatial variability (with some regions showing trends of opposite sign). Averaged over available data for the NH spanning the past 150 years, freeze-up date has occurred later at a rate of 5.8 ± 1.6 days per century, while the breakup date has occurred earlier at a rate of 6.5 ± 1.2 days per century. (The uncertainty range given throughout this chapter denotes the 5 to 95% confidence interval.)
• Satellite data indicate a continuation of the 2.7 ± 0.6% per decade decline in annual mean arctic sea ice extent since 1978. The decline for summer extent is larger than for winter, with the summer minimum declining at a rate of 7.4 ± 2.4% per decade since 1979. Other data indicate that the summer decline began around 1970. Similar observations in the Antarctic reveal larger interannual variability but no consistent trends.
• Submarine-derived data for the central Arctic indicate that the average sea ice thickness in the central Arctic has very likely decreased by up to 1 m from 1987 to 1997. Model-based reconstructions support this, suggesting an arctic-wide reduction of 0.6 to 0.9 m over the same period. Large-scale trends prior to 1987 are ambiguous.
• Mass loss of glaciers and ice caps is estimated to be 0.50 ± 0.18 mm yr^–1 in sea level equivalent (SLE) between 1961 and 2004, and 0.77 ± 0.22 mm yr^–1 SLE between 1991 and 2004. The late 20th-century glacier wastage likely has been a response to post-1970 warming. Strongest mass losses per unit area have been observed in Patagonia, Alaska and northwest USA and southwest Canada. Because of the corresponding large areas, the biggest contributions to sea level rise came from Alaska, the Arctic and the Asian high mountains.
• Taken together, the ice sheets in Greenland and Antarctica have very likely been contributing to sea level rise over 1993 to 2003. Thickening in central regions of Greenland has been more than offset by increased melting near the coast. Flow speed has increased for some Greenland and Antarctic outlet glaciers, which drain ice from the interior. The corresponding increased ice sheet mass loss has often followed thinning, reduction or loss of ice shelves or loss of floating glacier tongues. Assessment of the data and techniques suggests a mass balance of the Greenland Ice Sheet of between +25 and –60 Gt yr^–1 (–0.07 to 0.17 mm yr^–1 SLE) from 1961 to 2003, and –50 to –100 Gt yr^–1 (0.14 to 0.28 mm yr^–1 SLE) from 1993 to 2003, with even larger losses in 2005. Estimates for the overall mass balance of the Antarctic Ice Sheet range from +100 to –200 Gt yr^–1 (–0.28 to 0.55 mm yr^–1 SLE) for 1961 to 2003, and from +50 to –200 Gt yr^–1 (–0.14 to 0.55 mm yr^–1 SLE) for 1993 to 2003. The recent changes in ice flow are likely to be sufficient to explain much or all of the estimated antarctic mass imbalance, with changes in ice flow, snowfall and melt water runoff sufficient to explain the mass imbalance of Greenland.
• Temperature at the top of the permafrost layer has increased by up to 3°C since the 1980s in the Arctic. The permafrost base has been thawing at a rate ranging up to 0.04 m yr^–1 in Alaska since 1992 and 0.02 m yr^–1 on the Tibetan Plateau since the 1960s. Permafrost degradation is leading to changes in land surface characteristics and drainage systems.
• The maximum extent of seasonally frozen ground has decreased by about 7% in the NH from 1901 to 2002, with a decrease in spring of up to 15%. Its maximum depth has decreased about 0.3 m in Eurasia since the mid-20th century. In addition, maximum seasonal thaw depth over permafrost has increased about 0.2 m in the Russian Arctic from 1956 to 1990. Onset dates of thaw in spring and freeze in autumn advanced five to seven days in Eurasia from 1988 to 2002, leading to an earlier growing season but no change in duration.
• Results summarised here indicate that the total cryospheric contribution to sea level change ranged from 0.2 to 1.2 mm yr^–1 between 1961 and 2003, and from 0.8 to 1.6 mm yr^–1 between 1993 and 2003. The rate increased over the 1993 to 2003 period primarily due to increasing losses from mountain glaciers and ice caps, from increasing surface melt on the Greenland Ice Sheet and from faster flow of parts of the Greenland and Antarctic Ice Sheets. Estimates of changes in the ice sheets are highly uncertain, and no best estimates are given for their mass losses or gains. However, strictly for the purpose of considering the possible contributions to the sea level budget, a total cryospheric contribution of 1.2 ± 0.4 mm yr^–1 SLE is estimated for 1993 to 2003 assuming a midpoint mean plus or minus uncertainties and Gaussian error summation.