Snow is not just for family skiing trips or the winter Olympics every four years. Mountain snow provides water for billions around the world and is a key part of annual water cycles. About 75% of water supplies in the western U.S comes from snowmelt, according to the U.S. Geological Survey. But scientists say global warming is causing snow to finish melting earlier in the springtime than historically has been the case – and in smaller quantities – putting western water resources at still more risk.

Why is snowmelt important?

When snow melts high in the mountains, the runoff flows into rivers, streams, and lakes, replenishing ground and surface water reserves for agriculture and consumer uses. In western U.S. water supply systems, mountains act as natural water reservoirs, filling up large bodies of surface water to be distributed for a variety of uses – growing crops, drinking, bathing – essentially everything humans do. Eastern parts of the U.S generally receive sufficient precipitation throughout the year in the form of rainfall, but still benefit from water from snowmelt through recharging of groundwater, rather than surface water, supplies.

The amount of snowfall varies based on current climate conditions each year, whether it is influenced by the natural environment or humans. Some years are high snow years, others low snow years. Water managers use April 1 as a standard date to begin estimating how much water will be available once it melts based on the level of snowfall. Mountain landscapes across the world vary on when their snow melts. At higher elevations, snow doesn’t start melting until much later in the year, while at lower elevations it melts far sooner. In the U.S., snowmelt period typically begins in the springtime and extends throughout the duration of the summer.

How does climate change affect snowpack?

But the warming of the atmosphere is causing snow to melt earlier in the spring than usual. As a result, there isn’t enough water to extend throughout the summer in some cases. But what is causing such a large-scale change? The answer lies in the human activities that produce carbon emissions – in particular the burning of coal and other fossil fuels – causing global temperatures to rise. As long as carbon dioxide and other greenhouse gases continue to be emitted and atmospheric concentrations continue to increase, snow will melt earlier and earlier in the year, and that trajectory has troubling consequences for water resources.

Mountains are often referred to as “water towers of the world.” They provide major water resources for humans thousands of miles away, not just for nearby communities. Rainfall is another key source of water for regions that have no mountains, but most global regions are dependent on mountains as their primary source of water. In the U.S, cities nowhere near mountain regions, like Los Angeles, benefit from snowmelt in the distant Colorado Rockies. More than 40 million people between the seven states the Colorado River passes through – Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming – rely on snowmelt that runs off into that river.

Snowpack in Colorado and elsewhere in the West melts earlier

In Colorado, snow is melting as much as a month earlier than the historical norm, and the change is affecting when water will be available in reservoirs for down-stream urban communities. The acceleration of snowmelt timing resulting from higher global temperatures is causing a myriad of issues for water managers, increasingly challenged, given the uncertainties, to predict how much water will be available with snowmelt.

“What climate change is doing is it’s making it warmer and more humid,” said Paul Brooks, professor of hydrology at the University of Utah. “With climate change, a larger fraction of the snow and snowmelt sublimates and evaporates, so there is just less snow overall.”

Farmers and ranchers are affected by earlier snowmelt, mainly because agricultural production relies heavily on an influx of water to arrive on time for growing season. With snowmelt happening a month earlier than expected, irrigation systems receive too much water too early, and human-designed irrigation systems are ill-suited for holding large amounts of water for long periods of time. Farmers accordingly are facing major water deficits, leading to water supply shortages in parts of the growing season.

Disadvantaged communities living in poorer regions of the U.S. are at the highest risk of being adversely affected. Several small towns in the western U.S. states, including Arizona, California, and New Mexico, are facing significant strains on their water supplies that will only intensify as global temperatures continue to rise. California, one of the driest states in the country, has seen major cut-backs to the state’s water availability in recent years.

Western water managers stress the importance of saving water. With snowmelt becoming so unpredictable, water is being allocated to the last drop.

“[In California] the impacts are very significant,” said Ryan Jacobsen, executive director and president of the Fresno Irrigation District. “The way that the snowpack comes off is literally what we live and die by on an annual basis.”

Erin Chessin is a journalism master’s candidate at the University of California, Berkeley.