Cryospherebiosphere Interactions

There are numerous interesting ecological influences of the cryosphere, such as marine mammals' penchant for the sea-ice edge, the hibernation rhythms of bears, the deadly nature of rain-on-snow events for Peary caribou, and the importance of deep snowpacks for winter feeding of woodland caribou, which, lacking giraffe necks, rely on the extra elevation to forage higher into spruce, fir, and cedar trees. Photosynthetic activity and ecological rhythms of high-l atitude rivers and lakes are also closely connected with the ice cover.

Snow, ice, and permafrost play an interesting but poorly understood part in the global carbon cycle. Po-lynya tend to be nutrient-rich, ecological hot spots, associated with C02 drawdown into the ocean, so it is suspected that open water (versus sea-ice cover) would be conducive to a stronger sink for atmospheric carbon in the polar regions. There are algae that thrive in sea ice, however, so carbon-ice-ocean exchanges are not fully predictable.

Glacier and permafrost cover alter carbon storage in the landscape. Glaciers and ice sheets override vegetation and soil, effectively removing this carbon from the system. Some of this organic carbon can be stored sub-glacially, as evidenced by retreating glaciers, but most of it decomposes and is washed out via subglacial meltwa-ter, leaving a relatively barren, inorganic subglacial and periglacial environment. Soil is still trying to establish itself in the till deposits of most glacier forefields, which have been exposed by glacier retreat since the sun set on the Little Ice Age in the late 19th century.

Permafrost in much of northern Canada, Russia, Alaska, and Scandinavia is found in organic-rich muskeg and peatlands, where there is high soil moisture and carbon content. When frozen, this carbon is removed from the atmospheric cycle, and it can be locked up in the ground for long periods. As introduced in chapter 7, thawing of permafrost and deepening of the active layer in recent decades are causing a reverse effect: the release of soil carbon to the atmosphere, as both methane (CH4) and carbon dioxide (C02). Carbon fluxes from thawing permafrost may be offset by increased carbon uptake as vegetation and biomass expands at high elevations and in high northern latitudes. Similarly, biological carbon uptake may increase in high-latitude ocean waters and as the snow-free growing season is extended in midlati-tudes. The net impact of cryospheric change on the carbon cycle is therefore unclear.

Variations of C02 and CH4 during glacial-interglacial cycles pose an ongoing puzzle to our understanding of the carbon cycle. Levels of both greenhouse gases dropped systematically during the Pleistocene glaciations, as documented by the air bubbles trapped in cryosphere-climate processes glacial ice in Greenland and Antarctica. Ice cores show a remarkably close correlation between the concentrations of CO2 and CH4, air temperature, and the volume of ice on the planet, so it is clear that greenhouse gas reductions acted as an important feedback in driving the world in and out of glaciations. However, the nature of the glacial carbon sink remains uncertain. The quantity of carbon in the ice sheets is negligible, so the carbon sink lies elsewhere.

The direct effect of the Pleistocene ice sheets should be to release carbon from overridden vegetation and soil to the atmosphere, decreasing terrestrial cO2 and increasing atmospheric levels. This means that even more CO2 must have been taken in by the oceans or by tropical vegetation during the glaciations. One aspect of the cryosphere may be implicated in the glacial carbon sink: carbon sequestration in frozen ground. Midlatitude permafrost may have acted as a major carbon sink during the glaciation, both under the ice sheets and in the proglacial regions. If so, the coupling between ice sheet advance/retreat and uptake/release of this carbon was extremely tight, implying a century-scale response time for permafrost carbon storage.

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