The State of the Cryosphere - Response to Global Warming

Global mean temperatures have risen over the past 100 years by about 0.6 C. Over half of this increase has occurred in the last 25 years. There is variability in temperatures from year to year, and also from decade to decade superimposed on the longer upward trend. The range of natural variability in global temperature appears to be about plus or minus 0.2 C, so that it is only after the late 1970s that global mean temperatures emerge from the noise of natural variability. In some regions, extreme warming has been detected. Locations in Alaska and northern Eurasia, for example, have warmed by nearly 6.0 C in the winter months over the past 30 years. The cryosphere, or regions where water is found in solid form, are among the most sensitive to temperature change. Average temperatures in snow and ice-covered areas typically remain below 0 C much of the year. Unlike other substances found on Earth, ice and snow exist relatively close to their melting point and frequently change phase from solid to liquid and back again. Consequently, consistent and prolonged warming trends should result in observable changes to the Earth's cryosphere. Water changing from solid to liquid and back often results in dramatic visual changes across the landscape as various snow and ice masses shrink or grow. In this phase of our ongoing study of the state of the cryosphere we investigate, snow cover, glaciers, sea ice, and the related parameter sea level. Various forms of remote sensing allow the monitoring of snow and ice surfaces at varying spatial scales over the most recent 25 to 35 years. Results presented here include long-term monitoring of snow cover and sea ice as well as changes in area and mass of mountain glaciers. In all cases, regardless of parameter or measurement method, the amount of snow and ice has been decreasing over the past several decades. We also introduce the topic of permafrost. However, because reliable data on hemispheric-scale permafrost extent have only recently become available, examples presented provide only a snapshot of current permafrost conditions rather than time series data. Future work will involve similar investigations for ice sheets and ice shelves.