Tropical mountain glaciers often are treated as proverbial canaries in the coal mine for the warming of the Earth's climate by human-driven greenhouse gas emissions and land use change. Their rapid disappearance over the last century, however, may not be the result of warmer temperatures but of changes in precipitation that only may be partly attributable to human-driven change. However, their disappearance is not just a way of illustrating climate change but a consequence. Tropical mountain glaciers and their seasonal variability provide humans with water and animals with grazing lands. The timing of their growth or melting can provide guidance to indigenous societies of when to plant or harvest crops. Their loss and the loss of mountain glaciers at higher latitudes can be devastating.

It long has been known that glaciers shape the landscape by carving distinctively shaped valleys with steep, rounded sides as they advance through the landscape. These U-shaped valleys contrast with the more gradually sloping valleys formed by rivers.

This project was part of a larger collaborative project to determine if and why Unaweep Canyon in western Colorado is a glacial valley. Much of the collaborative project involves drilling into the rocks underneath the canyon to determine their age and using seismic techniques to figure out the original shape of the valley. If the valley was a glacial one, its shape would allow us to determine how low in elevation the glacier reached.

The significance of the valley's age is what it would tell us about the climate of the Earth in the past. If the valley were formed by glaciers during the last 2 million years, it would imply that glaciers reached an unusually low elevation compared to nearby areas of the Rocky Mountains but probably would not significantly change our understanding of past climate globally.

What is more likely and more exciting is that the valley has been preserved from the late Paleozoic Era of the Earth's history about 300 million years ago. 300 million years ago, Unaweep Canyon was within 10 or 15 degrees of the Equator and has since moved with the tectonic plates to its current location. Unaweep Canyon also likely would have been at an unusually low elevation for a mountain glacier in the Earth's tropics today. The implication here is that global climate (or at least tropical climate) during the late Paleozoic was much colder than the present day and perhaps much colder that the climate during the peak of the "Ice Age" 20,000 years ago.  

Our main objective was to provide a quantitative relationship between the elevation of tropical mountain glaciers and global mean or tropical mean temperature. To do so, we performed a variety of simulations of snowfall, melting, and sublimation within the part of a climate model that simulates the land surface. These simulations predicted whether glaciers would gain or lose ice at a 1 km resolution close to the scale of a tropical mountain glacier but received precipitation inputs at the 100-200 km scale of a full global climate model. There are a variety of ways scientists can directly or indirectly observe the elevations of past glaciers, which gave us useful estimates with which to compare our model results.    

These simulations and our comparisons with observations had three main results. First, we found that the simulations mostly underestimated the elevations which glaciers reached, by 500?600 meters on average in our most thorough set of simulations. In some cases, the difference seems to stem from the global climate model used, which is known to overestimate rainfall/snowfall over mountain ranges. But in other cases, the difference probably comes from simplifications in bringing information from the 100?200 km scale of the global model to the 1 km scale of the high resolution model. Second, we found, despite this problem with rainfall/snowfall, the model roughly captured the effect in which high snowfall areas have lower elevation glaciers than low snowfall areas. Third, we found that we more accurately modeled the change in glacier elevations as global climate cools than the absolute elevation of glaciers. So if we have an estimate of glacial elevation and local precipitation for the past, we can roughly estimate past global temperature. 

In addition to the modeling work, this project contributed to the larger collaborative project by providing advice and cross-training in geology and climate science to students and postdoctoral researchers at collaborating institutions. These students included student filmmakers at the University of Oklahoma, who are making a documentary about the larger collaborative project.

Project work also contributed to side projects off the larger collaborative project related to the larger context of the hypothesis of late Paleozoic tropical glaciation. One signal achievement of this work was a 20 million year history of dust deposition derived from West Virginia, Ohio, Pennsylvania, and Maryland coals.

Last Modified: 12/28/2022
Modified by: Nicholas G Heavens