Climate change and Elevational Dependence at a Mid-Latitude Mountain System, Niwot Ridge, Colorado Rocky Mountains

Mid-latitude mountain systems are critically sensitive to recent and projected climate change under an elevated greenhouse gas world. It is often taken that climatic change at high elevation sites will reflect those at lower sites - regional warming is assumed to be consistently played out in mountains, or even amplified by the snow-albedo feedback. The anticipated outcome is that the alpine will eventually be "pushed off the top of mountains." There are several reasons why this might not be the case, or at least considerably delayed - one is whether high elevation climates reasonably reflect regional lowland trends or if they are decoupled from them as a result of mountain climatic processes. We evaluated standard climatological variables (minimum & maximum temperature, precipitation) and derived variables [diurnal temperature range, growing season length (using both 0° & -3°C thresholds), and growing degree days (0°C base)] from subalpine (C1, 3048m) and high alpine (D1, 3749m) sites from 1953 to 2006 at Niwot Ridge in Colorado, the longest high- elevation climate record in the US. Over the last 54 years, mean maximum temperature (Tmax) increased through much of the year in the subalpine (trend in annual Tmax=+0.4°C/decade), but in the alpine decreased in early winter (-0.4 to -0.6°C/decade). These patterns resulted in altered seasonal cycles for the two sites, but in different ways: a positive offset in the subalpine (C1) and amplification in the alpine. Precipitation increased at the alpine site from October through April (trend in annual ppt=+100mm/decade), but not during any season in the subalpine. At both sites, summer onset is later and termination earlier, so that the "growing season" has shortened - this reflects long-term tendencies in minimum temperatures. An apparent contradiction is that growing degree-days have gone up at the subalpine site; this due to the positive trend in maximum temperatures. The alpine showed no corresponding trend. An integrated view of these trends infer synoptic dynamics and surface energy processes that act differently in the high alpine near the Continental Divide vs. in the subalpine dominated by closed conifer forest. At the same time, these climates are affected by multidecadal hemispheric circulation changes. Nearly all temperature-related timeseries for both sites show a period of cooling until around 1980, followed by warming. Precipitation series show corresponding periods of increasing then decreasing precipitation. On the face of it, this pattern resembles that of the Pacific Decadal Oscillation (PDO). This suggests that alpine and subalpine climate signals are not as decoupled as they appear, but rather that across a relatively short elevational gradient (Δ700m) synoptic and landscape-scale processes react differently to and differentially modify a prevailing hemispheric signal.