Paleoclimatological and historical evidence shows that mountain areas undergo major changes in glacio-hydrological and ecological conditions in response to changes in climate. The question of the relative intensity of these changes, compared with conditions in adjacent lowlands has received little attention, yet this issue is crucial to appropriate interpretations at regional and local scales of global climate model simulations of possible future climates. Global models necessarily represent the orography of mountain areas in a highly simplified manner and model outputs reflect this in the generalized nature of the results. Accordingly, a review has been undertaken of the evidence for past changes in mountain climatic conditions. The physical basis of high-altitude effects is briefly considered in the altitudinal variations of temperature lapse rates, solar radiation, and precipitation amounts, especially snowfall and snow cover. Observational and proxy evidence of climatic change is discussed for three time scales. Long-term evidence of shifts of vegetation and snowline boundaries during the Last Glacial Maximum ca. 18,000 B.P. shows a significant discrepancy between the inferred 5-60C cooling of tropical mountain areas based on glaciological and vege- tational evidence, versus sea-surface temperature reduction of only 20C inferred from planktonic foraminifera. Possible reasons for this paradox are discussed. Historical records of glaciological conditions during the Little Ice Age interval in Europe (ca. A.D. 1550-1850) are summarized and recent instrumental records of meteorological-hydrological conditions are discussed using examples from the Rocky Mountains of Colorado and the European Alps. These comparisons are briefly discussed in the context of model simulations for the climatic effects of increased concentrations of atmospheric greenhouse gases. Modeling studies are urgently required for impact assessment of potential hydro-meteorological changes in mountain areas.
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