Climate scientists have a pretty good idea what is going to happen to much of the Earth’s snow as the planet warms over the next century: It’s going to melt. But the melting will occur at different rates in different places, which has major implications for the 2 billion people who rely on snowmelt for water.

What’s more, over the next few decades, some areas are likely to see increased snow and rainfall as climate changes in complicated ways.

“Such confounding factors complicate how water managers will be able to respond to climate change,” says Justin Mankin, a climate scientist at Columbia University’s Earth Institute.

In order to help water providers better forecast their supply, Mankin led a team of scientists in a new study published Thursday in the journal Environmental Research Letters. Climate models were used to predict changes in rainfall and snowpack across basins in the northern hemisphere that supply water to large numbers of people. These areas include much of the American West, the Middle East, Central Asia, and southern Europe.

The scientists concluded that overall there is a 67 percent risk of less water available from snowpack by 2060. But over the next few decades, some regions face more risk than others.

In some areas precipitation could actually increase. That’s because a warmer atmosphere can hold more moisture. Yet “the extent that people have the capacity to capture and use that water is a different matter,” says Mankin.

Snow’s Holding Power

Traditionally, when precipitation falls as rain only some of it can be captured in aquifers, lakes, and manmade reservoirs. When the moisture falls as snow, it often sticks to mountains for long periods of time, where it melts slowly, trickling down as a nearly steady water supply. (Read more about this process.)

“In the future, water managers are going to have to adjust to a decrease in the amount of water available from snowpack,” says Mankin. Strategies could include building more reservoirs, either above or below ground, tougher water conservation measures, desalination, deeper wells, or other plans. (Learn how scientists measure snowpack.)

The team did not attempt to consider rising human population or migrations, which may increase water needs. They assumed that demand would remain constant, which is unlikely given recent trends. The study also did not take into account the ecological needs of the basins to support fish and other wildlife.

Dwindling Reserves Certain regions of the Northern Hemisphere are expected to face major challenges due to loss of snowpack; click on the dots to move through the four areas. Regional snowmelt risk* < 50% 50-74% 75-94% 95% + Major cities by population 1,000,000+ 500,000+ SWIPE FOR MORE NG MAPS; ANDREW UMENTUMSOURCE: JUSTIN MANKIN *LIKELIHOOD THAT FUTURE SNOWPACK WILL NOT BE ABLE TO FULFILL HUMAN WATER DEMAND UNMET BY RAINFALL. .

1. The Central Valley, California

“The American West is iconic in its reliance on a limited water supply, but even so, I was surprised by how strong the signal was,” says Mankin.

Perhaps nowhere is this more apparent than California’s Sacramento River Basin, which is home to 4.9 million people. In 95 percent of the trials run through the team’s climate models, the snowmelt fell short of demand by mid-century. The same result was seen to the south, in the San Joaquin Valley, home to 6.3 million people.

California is already suffering its worst drought in recorded history, and the future doesn’t look much better under the new modeling. The result will likely be an even stronger push on people and farms to conserve, tighter restrictions on use, and an intensified search for alternate sources.

Watch: How do you determine how much water is in snow?

2. The Colorado River Basin

Home to 11 million people, the Colorado River system fared only somewhat better in the analysis, with a decline in snowpack in 74 percent of the tests.

“Our water supply is not going to look the same in the future,” says Mankin. “We're going to have to get innovative about what management practices really make sense.” (Read more about the embattled Colorado River.)

3. Rio Grande Basin

The Rio Grande Basin that straddles Mexico and the U.S. is home to 16 million people. Like the Central Valley, in 95 percent of the trials run through the climate models, the snowmelt runoff fell short of demand by mid-century.

4. The Indus and Ganges River Basins

View Images The Ganges River, seen in Rishikesh, India, is sacred to millions. A new study shows that the basin faces a 37 percent risk of a decreased snowpack, far less risk than other areas dependent on snow for water supplies. Photograph by Pete McBride, Nat Geo Image Collection

At the other end of the spectrum are the highly populous Indus and Ganges systems in southern Asia, home to a combined 966 million people. They do face some melting snowpack, but climate models suggest the region will also experience higher rainfall in the coming decades. As a result, the Indus saw a snowpack decline in 37 percent of the trials, and for the Ganges, 63.

5. Southern Europe and North Africa

These populous basins in North Africa and Southern Europe are among the handful that are “particularly sensitive” to a changing climate, according to the study, with greater than 95 percent risk of declining snow resource potential.

6. Eastern Mediterranean and the Middle East

A broad swath of this region shows risk of 95 percent or greater, from Greece to Iran. Some research has suggested that the lingering drought is partially to blame for recent migration and conflicts in Syria.

WATCH: A first-of-its-kind river restoration campaign, Change the Course brings the public, sponsors, and conservation groups together to restore water to depleted ecosystems throughout the Colorado River Basin.