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Global Climate Change Digest

A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999



Item #d99feb1

“Ozone Depletion at the Edge of the Arctic Polar Vortex 1996/1997,” Georg Hansen ( and Martyn Chipperfield (,J. Geophys. Res. 104 (D1), 1837-1846 (Jan. 20, 1999).

The northern low-pressure zone of circular winds between 8 and 21 miles above the Earth’s surface over the Arctic is a key factor in producing the conditions necessary for the catalysis of the dissociation of ozone. In 1996-1997, this stratospheric vortex exhibited low temperatures that produced ozone depletion as great as that in the Antarctic during the early 1980s. Ozone depletion was observed from the Arctic Lidar Observatory for Middle Atmosphere Research on the Norwegian island of Andøya. In 1995-1996 and in 1996-1997, the average column ozone concentration was up to 48% below the norm, and at 12 miles in altitude this depletion peaked at 60% below the norm. In 1996-1997, the vortex lasted into early May, which is unusually long, leading to increased ultraviolet radiation exposure in northern Europe and North America. In 1997, chlorine was the principal cause of ozone depletion until the end of March. In April and May of that year, nitrogen oxides contributed to the depletion. This polar vortex is highly variable from year to year, so no trend can yet be determined from these data.

Item #d99feb2

“Drought-Induced Shift of a Forest-Woodland Ecotone: Rapid Landscape Response to Climate Variation,” C. D. Allen and D. D. Breshears,Proc. Natl. Acad. Sci. U.S. 95, 14839-14842 (Dec. 8, 1998).

Detailed aerial photographs taken from the 1930s through the 1970s were used to document and measure a drought-induced boundary (ecotone) shift in the ponderosa and pinon-juniper ecosystems at Bandelier National Monument in northern New Mexico. The shift was caused by a drought that lasted from 1942 to 1956 and was one of the most severe droughts in North America in the past 500 years. It forced a ponderosa pine ecosystem into retreat to be replaced by a pinon-juniper ecosystem that is more drought resistant. The transition zone between the two ecosystems moved 2 km in less than 5 years. The ecotone shift was enhanced by an infestation of pine bark beetles and by the suppression of fires that normally keep the pinion and juniper populations in check. The area lost by the ponderosa pines has not been regained in the ensuing three decades, indicating that they may have been pushed over a threshold from which they may never recover. As a result of the change in vegetation, the study area has experienced increased erosion, apparently because of the loss of ground cover during the drought.

Item #d99feb3

“Atmospheric Moisture Residence Times and Cycling: Implications for Rainfall Rates and Climate Change,” K. E. Trenberth (,Climatic Change 39 (4), 667-694 (1998).

The residence time of water vapor in the atmosphere was found to be a littler more than eight days. Recycling (the production of rain from moisture evaporated locally) varies (logically) with the scale of the domain under consideration. At a scale of 500 km, 9.6% of the moisture is recycled; at 1000 km, just less than 20% is recycled. The percentage of time it precipitates in the United States was found to range from more than 30% in the Northwest during winter to less than 2% in California during summer. Rainfall rates, naturally, are much greater than evaporation rates, and precipitating systems feed mostly on moisture already in the atmosphere. In the United States, that extant moisture has come largely from the Pacific Ocean, the subtropical Atlantic Ocean, or the Gulf of Mexico a day or so earlier.

Increasing temperature increases the water-holding ability of the atmosphere and, along with increased evaporation, this will exacerbate droughts. At the same time, enhanced evaporation must be balanced by precipitation, and the increased water-vapor load of the atmosphere increases the risk of stronger precipitation events and consequent flooding. Indeed, observations indicate that the atmospheric moisture over the United States is increasing at the rate of 5% per year. These noted or expected trends lead to the prediction of increases in the frequency of precipitation events in the southern United States during the winter and decreases in the Northwest from November to January.

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