Environmental Observation
Several trends continue to positively correlate with the temperature measurements described above. Most conspicuous is the overall reduction in area of surface snow cover, a trend documented in some places since the mid-nineteenth century, and with satellite data since the late 1960's. The recent National Academy of Science assessment, “Climate Change Science” (2001), reviews this evidence succinctly:
The warming trend is spatially widespread and is consistent with the global retreat of mountain glaciers, reduction in snow cover extent, the earlier spring melting of ice on rivers and lakes, the accelerated rate of rise of sea level during the 20th century relative to the past few thousand years, and the increase in upper-air water vapor and rainfall rates over most regions. A lengthening of the growing season also has been documented in many areas, along with an earlier plant flowering season and earlier arrival and breeding of migratory birds. Some species of plants, insects, birds, and fish have shifted towards higher latitudes and higher elevations.
Measurements from submarines and satellite data both suggest that the thickness and extent of Arctic sea ice have diminished since these readings first became available in the 1970’s. In Antarctica, the IPCC Third Assessment documents the retreat of five ice shelves over the course of the 20th century; the National Snow and Ice Data Center put the number at seven since 1974. Less than a year after the Third Assessment appeared, Antarctica experienced the dramatic collapse of the Larsen B ice shelf in the late winter and spring of 2002. Attributed by scientists to “a strong climate warming in the region,” the collapse of Larsen B lasted 31 days, during which a volume of ice larger than the state of Rhode Island—3250 km2—and 220 m thick disintegrated into the sea.
The range of evidence described above is entirely circumstantial, but its cumulative weight is considerable, and has done much to establish beyond question the fact, disputed in the late 1980’s and early 1990’s, that the earth’s atmosphere is indeed warming. “The effect of anthropogenic greenhouse gases is detected,” as the IPCC concludes its Third Assessment. It also suggests the sorts of ecological changes that further warming might accentuate.