No abstract

A high elevation data set of surface temperatures from the Front Range of the Rocky Mountains in Colorado, USA, is analysed for evidence of long-term change . Sites range from the high plains of Colorado (1509 m) to the alpine tundra (3749 m). Systematic changes in surface-based lapse rates are uncovered, with absolute cooling at the highest elevations, but little temperature change on the high plains. There is lapse-rate steepening at the higher elevations (>3000 m). A synoptic analysis using gridded pressure data shows lapse rate changes to be largely independent of synoptic type. Radiosonde ascents from Denver (1956-98) and Grand Junction (1946-98) are used to derive air equivalent temperatures (AETs) at the same elevations as the surface records. AETs show a contrasting temporal trend, with absolute warming at all levels. Furthermore, free-air lapse rates are weakening at higher elevations, the warming becoming stronger with height. A comparison of the two data sets through derivation of free-air-surface temperature differences shows that the alpine tundra zone of the high Rockies is becoming a progressively stronger heat sink. Possible explanations include increased snow cover, enhanced air movement over the surface and decreased solar radiation input. The heat sink enhancement has led to rapid cooling in the alpine tundra that could not be predicted from the free-air record, casting doubt upon the strong dependence on free-air temperature changes in climate modelling when investigating the potential effects of global warming in mountainous regions. In addition, these local surface trends are of the opposite sign to global and other regional trends identified in many recent observational and modelling studies.

A continuous climate record since 195 1 at Niwot Ridge in the Colorado Front Range shows a decline in mean annual temperature, an increase in annual precipitation amount, and a decrease in mean daily solar radiation for the summer months. The increase in precipitation amount explains about half of the 200% increase in annual wet deposition of NO,-to Niwot Ridge over the last decade. Differences in climate parameters between 1994 and 1995 (increased snow depth and decreased net energy flux to the snowpack) resulted in a 4-5-fold increase in the magnitude of solute release from the snowpack in the form of an ionic pulse. In turn, the high chemical loading of strong acid anions in the seasonal snowpack and release of these solutes from the seasonal snowpack in the form of an ionic pulse is causing episodic acidification (ANC < 0 peq liter-l) in headwater catchments at present deposition levels. Small changes in climate parameters may cause large changes in the hydrochemistry of alpine streams. The changes in climate at Niwot Ridge are not in synchrony with lowland warming in the Great Plains to the east and serve as a reminder that climate in alpine areas is driven by local conditions and may be asynchronous with regional and global climate trends.Many alpine regions are susceptible to environmental damage that will affect both their ecological health and the regional economies. Small changes in the flux of energy, chemicals, and water to high-elevation ecosystems may result in large changes in the climate, ecosystem dynamics, and water quality of these catchments (e.g. Baron 1992). For example, field and laboratory experiments have demonstrated that initial stages of snowmelt often have ionic concentrations many times higher than averages for the whole snowpack-an ionic pulse (e.g. Johannessen and Henriksen 1978; Colbeck 198 1). The magnitude of the ionic pulse may be increased or decreased by small changes in energy flux (Williams and Melack 199 1 b). In turn, the release of solutes from the seasonal snowpack may have a direct and large effect on the solute content of stream waters .

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