This story is part of a special National Geographic News series on global water issues.

Much of the U.S. West's water supply is under threat as rapidly warming temperatures melt more snowpack annually than is created by precipitation, a new government study suggests.

Each spring, melted snow and ice from the Rocky Mountains recharge up to 80 percent of the Columbia, Missouri, and Colorado River Basins.

Together, these basins form the primary water source for nearly 70 million people in an area plagued by droughts—including Denver, Las Vegas, Los Angeles, and Salt Lake City. (See an interactive map of the Colorado River Basin to learn more about continued drought in the West.)

The new study, by the U.S. Geological Survey (USGS), appears to confirm a key source of the droughts: Snowpack has been disappearing over the past several decades.

What's more, 60 percent of the decline is due to greenhouse gas-induced warming, the researchers conclude.

Reading the Tree Rings

USGS scientists studied tree rings dating as far back as a thousand or more years. Narrow rings indicate years of slow growth and meager water supply.

Based on the predominance of narrow rings more recently, "the last 20 to 30 years were more intense than last 800," in terms of snowpack loss, said lead researcher Gregory Pederson, an ecologist at the USGS Northern Rocky Mountain Science Center in Bozeman, Montana.

"The study confirms previous research asserting that human-induced warming has had a strong impact since the 1980s on the snowpack."

The diminished snowpack is not simply the result of natural temperature and precipitation cycles, Pederson said.

Similar patterns in tree rings were seen in another study that identified the mid-1300s and early 1400s as periods of higher than usual temperatures in the West. The climate returned to cooler temperatures afterward. But now, Pederson said, “we have no expectation that things will cool off again.”

"Across the West this is an issue," Pederson added.

"The post-1980s period may well be that inflection point for this new era of temperature-driven [snowpack] decline. Meaning, by the time we reach the warm, dry summers, there is less water in the bank account, so to speak."

Runoff Running Out?

Climate-change and drought predictions are nothing new to western water managers.

For example, Denver Water, the public agency that supplies water to 1.3 million Coloradans, bases its planning in part on U.S. Bureau of Reclamation climate-change analyses. In one scenario, in which the Denver area's annual average temperature increases by 5 degrees Fahrenheit, the region would lose 20 percent of its water supply by 2050.

Marc Waage, Denver Water's manager of water-resources planning, said his agency is trying to prepare for a range of outcomes.

"We can't afford to develop for the worst-case scenario. The trick is preserving options to act on when we see a shift in that direction."

While Waage and his colleagues have not identified a declining water supply in their own research, they're already dealing with the undeniable fact that snowpack is melting earlier.

But compared to a trend of snowpack shrinking, "melting earlier is not as large a concern, because we have storage to capture the snowmelt," he added.

Silver Lining?

The USGS data could be "an impetus to invest in more ways to adapt to a drier climate," Waage said.

Those larger-scale and more costly investments include:

building recycling facilities for so-called gray water, such as from sinks and baths;

getting more aggressive with "xeriscaping"—landscaping that minimizes water use;

retrofitting homes with efficient fixtures;

and purchasing more water rights from farmers (agriculture accounts for more than 70 percent of the region's water use).

"Size is more important," said Waage, underscoring how much direr a reduction in total water supply would be, versus simply earlier spring melts.

"It is easier to adapt to changing runoff patterns than a loss of supply."