Niwot Ridge LTER

Increasing precipitation produces a climate warming signal in the alpine

Figure 1. The decline in thickness over the period of LTER records corresponds to a thinning at 2.0 cm/yr (r = -0.931; p < 0.01).

The fifty-year climate record from the alpine of Niwot Ridge indicates that the site is becoming wetter but not colder. These climate records are now supported by ice depth, ice duration, and water storage estimates from the Green Lakes watershed. Data collected over the past 20 years by Nel Caine indicates that ice thickness in an alpine lake has declined 25%, in spite of no significant change in temperatures or changes in the duration of ice on this lake. Instead, this change is related to greater thermal inertia of an enhanced volume of water. These findings show that an apparent warming signal can be generated by changes in precipitation rather than temperature.

This trend is not associated with any change in the dates of lake ice inception and decay or in the duration of ice on the lake, which remained constant at about 9 months/year over the last 20 years. Nor is there any evidence of a trend in air temperatures and solar radiation receipts during this period. On the other hand, the last 20 years have experienced an increase in winter precipitation (about 12 mm/yr) in the Niwot Ridge area and this has been accompanied with an increase in late season water storage in Green Lakes Valley. When the effects of increased winter precipitation and increased groundwater storage are combined in a linear model, they explain more than 50% of the annual variability in ice thickness on Green Lake.

Regulating Emissions in the Rockies

Fossil fuel combustion associated with metropolitan areas in the Colorado front range have increased atmospheric deposition of inorganic nitrogen (N) in precipitation in the Colorado Rockies, such that high-elevation areas are switching from N-limited to N-saturated ecosystems. We use both intensive data collected at the Niwot Ridge LTER site and extensive data collected throughout the Colorado Plateau to identify the amount of inorganic N in wetfall that may cause this biological switch. Results from the National Atmospheric Deposition Program (NADP) site located at the Niwot Ridge in the Colorado Front Range shows a significant increase in deposition of inorganic N in wetfall over the 18 years of record. In turn, the increasing amount of inorganic N in wetfall is causing episodic acidification in headwater catchments of the Green Lakes Valley.

Aquatic and terrestrial changes are now occurring in these high elevation areas as a result of the switch from N-limited to N-saturated ecosystems. N deposition was significantly greater at elevations above 2,500 meters because of higher amounts of precipitation at that altitude.

A survey of nitrate concentrations from 91 high-elevation lakes shows that N saturation is occuring throughout the Front Range. These data suggest we have crossed a threshold that puts the region of the Front Range on a slippery slope toward dead fish and dead trees. We make a conservative recommendation that critical loads of inorganic N in wetfall to Class 1 areas in the central Rocky Mountains be set at 4 kg/ha/yr.