Communities in the Rocky Mountain region of North America rely on snowmelt to provide water for drinking, sanitation, irrigation, and industry. Snow, which falls in the mountains during the winter, acts like a massive frozen water tower, providing a steady supply of water throughout the drier summer months. Water usage in many cities is growing rapidly, and some are already encountering the limits of water availability. The threat of climate change looms large—warming temperatures would push the snowline to higher elevations, decreasing the capacity of that frozen water tower.

Two recent papers shed some light on the long-term history of water availability in the region to provide insights into the current situation, as well as a future outlook.

The first paper, published in Geophysical Research Letters, provides a unique, 5,200-year record of water levels at Lake Athabasca in Canada, which is fed by rivers coming down from mountain catchments. Water usage in that region has increased 88 percent since 2000, in large part due to the booming oil sands industry, which now accounts for about 65 percent of water use. At the same time, streamflow feeding Lake Athabasca has decreased over the last few decades. Effective planning requires accurate predictions of future water availability.

In a sediment core from the bottom of a pond connected to Lake Athabasca, the researchers used the ratio of carbon to nitrogen as a proxy to infer lake level in the past. The ratio tracks changes in vegetation, which can shift from peat to open water plants. The advantage of this technique is that it can probe deeper into the past than records generated from tree rings (which go back about 1,000 years). A comparison with existing tree ring data verifies that this carbon/nitrogen technique does, in fact, accurately track lake level.

The deeper history of the record turns out to be pretty important. The researchers found that "modern society in western Canada developed during a rare interval of relatively abundant freshwater supply." We happen to be living in an especially wet period, a result of the extra accumulation at mountain glaciers during the 1700s (a period known as the Little Ice Age). At only one other point in the record (around 0-500 A.D.) was water this plentiful—the 2,500 years previous to that were much drier.

In addition, the team notes that transitions from wet conditions to drier conditions happened quickly—within a human lifetime. They conclude that, as warming continues, the region will have to prepare, as they put it, for "water-supply reductions well beyond the magnitude and duration of societal memory."

The second paper, which appears in Science, uses a compilation of 66 tree ring records to look at how snowpack in the US Rockies has changed over the last 1,000 years or so. Like the Canadian work, they show that snowpack peaked during the Little Ice Age, but their network of records also shows some pretty interesting geographic trends. Snowpack accumulation tends to oscillate between the northern and southern halves of the Rockies. In times where snowpack is increasing in the northern half, it's decreasing in the southern half, and vice versa. The researchers attribute this behavior to changes in the latitudinal position of winter storm tracks, controlled by the El Niño/La Niña cycle and the Pacific Decadal Oscillation (another alternating Pacific Ocean pattern that shifts every 20-30 years).

The effects of warm periods look different. Instead of an alternating see-saw between north and south, snowpack decreases throughout the Rockies. That behavior only pops up three times in their record—twice for short periods between 1350 and 1450, and then from 1980 to present. This tells us rising temperatures likely have a lot to do with the observed decline of snowpack over the last few decades.

Coming from two different angles, both papers end up reinforcing the same point—in the context of historical records, modern trends in water availability and usage in western North America represent real challenges for the future.

Geophysical Research Letters, 2011. DOI: 10.1029/2011GL047599 Science, 2011. DOI: 10.1126/science.1201570 (About DOIs).

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