Rock Glaciers and Permafrost in Rocky Mountain National Park: What Warming Would Do

The question

Rock glaciers are masses of angular boulders of all sizes held together by ice. They form only where the ground can stay frozen, so active and relict rock glaciers in Rocky Mountain National Park record where permafrost lies today and where it lay in the past.

Permafrost is ground that stays at or below 0°C for at least two years in a row. It responds directly to temperature, so it will change as the climate does. And when the ice that binds it melts, slopes become unstable — which is why predicting that change matters.

The project

Jason R. Janke of Metropolitan State College of Denver mapped the rock glaciers of the park and the adjoining Indian Peaks Wilderness, using aerial photographs, earlier motion studies and fieldwork — including ground-penetrating radar on Taylor rock glacier to measure how thick the permafrost is. He sorted them into three kinds:

He then tied present-day permafrost to the terrain each class occupies (aspect, slope and so on) and calibrated a permafrost model with the standard mountain lapse rate — air cools 0.5°C for every 100 m of elevation gained. With that, he asked how the permafrost would change if the climate warmed or cooled by up to 4°C.

The results

The past. Relict rock glaciers show that in the late Pleistocene, over ten thousand years ago, mean annual air temperature was at least 3.0–4.0°C cooler, and permafrost reached 600–700 m lower than it does today.

The present and the possible futures:

The response is nonlinear: the first half degree of warming does a great deal of damage.

The lag. Air and ground temperatures interact in complex ways, and the ground can take a long time — possibly a few hundred years — to catch up. But if global temperatures keep rising, permafrost will keep shrinking.

What it means for the park

Scientists project a rise of 2–4°C in the 21st century — enough for a significant loss of permafrost. As melting ice lubricates slopes, the park could see more:

• debris flows
• rockfalls
• slumps under Trail Ridge Road
• other slope failures

Monitoring the frozen ground will help predict them.

Sources

Based on "Rock Glacier Response to Climate Change," Rocky Mountain National Park, National Park Service, a research summary written by Diane Escobedo, Geological Society of America intern (August 2007, updated January 2008), from Jason R. Janke's research; the NPS summary is a work of the United States government in the public domain. It is not a statement of park policy or a definitive account of the results. The summary's photograph and permafrost map, credited to the researcher, are not reproduced here.