What New Evidence Is There That The Climate Is Changing In Response To Societal Activities?

Observed Changes In The Timing Of The Seasons May Signal Greenhouse Effect

A 340-year record (1651-1991) of monthly average temperatures from central England has provided the first evidence that the timing of the annual temperature cycle had been advancing by a little more than a day each century up until 1940; this result was confirmed in other records and is expected as a result of the very slow shifts in the variations of the Earth's orbit about the Sun. However, since 1940 the timing of the seasons as measured in the central England and many other Northern Hemisphere records has shifted much more rapidly, suggesting some other effect is now becoming dominant. The timing of the seasonal cycle of temperature at any given location results from the interplay between changes in the solar and infrared heating of the Earth and transport of heat from elsewhere on the globe. The greatly accelerated rate of change in the timing of the seasonal cycle seems to be occurring because local climates are becoming more dominated by the radiative mode, as might be expected from the enhanced concentrations of greenhouse gases in the atmosphere reinforcing winter-time solar heating. The timing of the seasons in the Southern Hemisphere has been less affected, as expected, because the radiative influence of the greenhouse gases and solar radiation and the transport modes are currently more nearly in phase.
Reference: The Seasons, Global Temperature, and Precession, Thomson, D. J., Science, 268, 59-68, 1995.

Local Warming Leads To Ice Shelf Disintegration

On February 27th, 1995, the British Antarctic Survey (BAS) announced that an ice shelf that formerly blocked the Prince Gustav channel between James Ross Island and the Antarctic Peninsula had disintegrated, and an enormous iceberg had broken off the Larsen Ice Shelf further south. BAS scientists have ascribed the cause of the events to a 2.5°C warming of the regional climate since the 1940's. Climate models have predicted enhanced warming at the poles, associated with the buildup of greenhouse gases.

The ice shelf in the Prince Gustav channel spanned approximately 270 square miles prior to its disintegration. The iceberg broken from the Larsen ice shelf is roughly 1,150 square miles in size (the size of Rhode Island), and approximately 656 feet thick. BAS scientists also stated that the Wordie ice shelf on the west coast of the Antarctic peninsula also recently disintegrated. BAS scientists also maintain that there is little doubt that the retreat of these ice shelves is, in the short term, irreversible.

References: (1) British Antarctic Survey Press Release, February 27, 1995; (2) Rapid Disintegration of Wordie Ice Shelf in Response to Atmospheric Warming, Doake, C.S.M., and D. G. Vaughan, Nature, Vol. 350, pp. 328-330, 1991.

Reduction In Northern Hemispheric Snow Cover Over The Last 20 Years And Enhanced Surface Warming In Northern Latitudes

The extent of annual snow cover over all three continental-scale regions of the Northern Hemisphere has declined by about 10% over the past 20 years. This reduction in snow cover, primarily in the spring months, has altered the amount of outgoing long-wave radiation and the surface albedo, creating a positive feedback effect that is further warming surface air temperatures.
Reference: Observed Impact of Snow Cover on the Heat Balance and the Rise of Continental Spring Temperatures, Groisman, P., T. R. Karl, and R. W. Knight, Science, Vol. 263, pp. 198-200, 1994.

Increase In Water Vapor May Be Linked To Climate Warming

Water vapor in the lower stratosphere above Boulder, Colorado has increased significantly, a tendency consistent with model predictions. The measurements taken at Boulder should be representative of the stratosphere over the highly populated northern mid-latitudes. An increase of 0.34 to 0.84% per year from 1981 to 1994 has been observed for altitudes from 9 to 27 kilometers. The water vapor increase was greatest in the region from 20-25 kilometers, with an average annual increase of 1%. The increase in water vapor below 20 kilometers is larger than might be expected from the stratospheric oxidation of increasing concentrations of atmospheric methane (methane is broken down chemically to produce water vapor). This suggests that the increase in water vapor may be the result of the rise in the global temperature that has been observed over the past few decades. Recent work also indicates that the water vapor concentration determines the effectiveness of the chemical reactions that destroy ozone in the lower stratosphere. Thus, an increase in water vapor could be contributing to ozone losses. This fifteen-year study also demonstrates that there is now excellent agreement between satellite-derived data (UARS and SAGE II satellites) and balloon-borne data regarding water vapor.
Reference: Increase in Lower-Stratospheric Water Vapor at a Mid-Latitude Northern Hemisphere Site from 1981 to 1994, Oltmans, S. J., and D. J. Hofmann, Nature, Vol. 374, p. 146-149, 1995.

Changes In Atlantic Ocean Temperatures

In measurements made along a transect across the central North Atlantic, oceanographers funded by the USGCRP found a distinct warming in the upper 2500 meters, compared to the original measurements made in 1957 and 1981. This transect is also the approximate route of Columbus' first voyage to the New World. This warming could represent some of the first evidence of a large-scale change in the ocean, but could also be indicative of a shifting of masses of warmer water in the Atlantic. It should be noted that another set of repeat ocean sections in the northeast Atlantic showed marked cooling between 1962 and 1991. An extensive series of cruises in the Atlantic is being planned for 1996-97 as part of the World Ocean Circulation Experiment (WOCE) to examine these possibilities.
References: (1) Rising Temperatures in the Subtropical North Atlantic over the Past 35 Years, Parrillo, G., A. Lavin, H. Bryden, M. Garcia and R. Millard, Nature, Vol. 369, pp. 48-51, 1994; (2) Cooling and Freshening of the Subpolar North Atlantic Ocean Since the 1960's, Read, J. and J. Gould, Nature Vol. 360, pp. 55-57, 1992.

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Last updated 04/10/96