Recent And Future Cryospheric Change

In more recent Earth history, temperature fluctuations of 1°C on interannual to centennial timescales have significant impacts on mountain glaciers, sea ice, river and lake ice, seasonal snow cover, and active layer depths. Fluctuations in precipitation and other meteorological variables are probably secondary to the influence of temperature, overall, although they can be significant on local and regional scales.

Cryospheric sensitivity to temperature changes is apparent through cold and warm phases of the Holocene (the last 10,000 years). Holocene temperature fluctuations of order 1°C, associated with solar and volcanic variability, have driven several periods of glacier advance and retreat in mountain regions. Direct observations and proxy records from the Little Ice Age, which persisted from about 1500 to the late 1800s, offer some of the best evidence for this, with a global expansion of sea ice, snow, and glacier cover through this period. This is nicely documented in literary, artistic, and cultural records from Europe and Iceland. Geological reconstructions from terminal moraines, lake sediments, dendrochronology, and cosmogenic dating evidence indicate that the maximum glacier advances of the Holocene occurred in this period throughout the Northern Hemisphere. Southern Hemisphere glaciers also advanced at this time, but in some locations (e.g., New Zealand) there are records of an even more extended middle Holocene advance.

The glacier advances of the Little Ice Age were the culmination of several millennia of "neoglacial" glacier advances following the early Holocene climate optimum, ca. 6 ka, when global glacier coverage was retracted relative to present day—Northern Hemisphere mountain regions are believed to have been largely bereft of ice during the climate optimum. The Holocene evolution of glaciers is broadly consistent with orbitally driven changes in summer insolation, as discussed in the earlier section "Pleistocene Glacial Cycles," with shorter-term climatic influences superimposed on millennial-scale orbital trends.

Since the late 1800s, other climatic influences—increase in solar output, relatively quiet volcanic activity, and the buildup of atmospheric greenhouse gases— have overridden orbitally driven cooling trends, driving recent warming and a strong cryospheric response. The cryosphere response is particularly marked in the

Northern Hemisphere, where climate warming has been greater. The most dramatic changes are at lower elevations and latitudes, where snow and ice exist close to their 0°C threshold for viability. This has an interesting consequence. The Arctic has warmed more than most other regions on Earth during the past 40 years, but some of the largest changes in the cryosphere are being felt at midlatitudes, where permafrost, river and lake ice, mountain glaciers, and seasonal snow cover are marginal. changes are also dramatic in polar latitudes in the summer months, in particular for the Arctic sea ice and the ablation zones of the Greenland ice sheet and Arctic ice caps. In the northern winter and in most of Antarctica, temperatures are still so low that climate warming is not strongly affecting the cryosphere.

Although temperature changes are less important in cold environments, the highest elevations and latitudes may be experiencing increases in snowfall in both hemispheres, due to increases in atmospheric humidity and more frequent advection of midlatitude air masses into subpolar and polar latitudes, at least in the Northern Hemisphere. This is not enough to prevent a negative mass balance for most of the world's glaciers, but it is a feedback that partially offsets increases in melt. Increases in high-latitude snow also affect the hydrological cycle and the thermodynamics of sea ice, lake ice, river ice, and permafrost.

The sensitivity of the global cryosphere to small changes in temperature has proved invaluable in helping to understand and document climate variability in remote, uninstrumented parts of the planet. It also provides evidence that measured 20th century warming is not an artifact of urban heat island effects or local land-use changes in the populated environments where most direct, long-term observations are derived; significant cryospheric changes are under way in all parts of the world, most of these far removed from urban centers.

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