“It’s a dry heat” is usually considered a positive expression, a relief on high-temperature days, and salve for the reality that the southwestern U.S. has never been what you’d call water-rich. But now human-caused climate change is adding new credence to the region’s bone-dry reputation – and not in a good way.

For starters, that’s because regional temperatures are on the rise, according to a late 2018 federal government assessment report. Between 1901 and 2016, temperatures increased across the Southwest, with the greatest upturns in California and Colorado. This warming trend – together with its diminished snowfall – have intensified recent droughts.

Meanwhile, growing population, aging infrastructure, and groundwater depletion are also compounding long-standing water scarcity issues in the region.

These mounting pressures have a bevvy of potential implications, from human health and ecological function, to food and energy supply. Consider:

• Food production is deeply affected by drought, and recent years have seen crops and livelihoods across the region, ruined.
• Recent droughts have dried out forests, altering habitats and making them more susceptible to fire.
• Indigenous tribes across the region are experiencing adverse effects of drought, such as declines in traditional staple foods like acorns and corn.
• Less snowpack means less water available for hydropower. In states like California, at times, when hydropower supply goes down, carbon emissions go up.

And that’s just the nutshell. To really understand the potential effects of climate change on water supply, it’s useful first to crack into the basic history of water in the American West.

The Southwest’s history with water

Aridity long has been a defining characteristic of the region, from 19th-century maps that labeled it the “Great American Desert,” to those of its states consumed by the Dust Bowl of the 1930s. Cities in the region have grown undeterred by this thirstiness, with 32 percent growth in the last quarter of the 20th century alone. In fact, western states comprise nine of the top 10 slots in population growth over the past century, based on Census records from 1917-2017.

Humans, of course, could not build cities without access to water, so massive efforts have been made all along the way to connect drier areas with wetter ones. Diversions from the Colorado River, as a prime example, have redirected its flow hundreds of miles away, to major cities like Los Angeles and Phoenix, using dams and aqueducts to divvy up the river’s waters (ultimately weakening the river, but that’s a story for another day).

Today, the American West is home to more people than ever – along with their fundamental thirst for water. And the region’s hard-won water supply doesn’t just support the locals, either. It also affects the millions of people across the country and around the globe who eat food grown in agriculture hubs like California, home to an estimated two-thirds of all the country’s fruits and nuts.

The trouble is, climate change is already making water harder to come by in these parts. According to the Fourth National Climate Assessment cited above, human-caused climate change is making the West hotter, with greater temperature increases there than in any other region, and drier; and with “chronic” deficits of precipitation expected in the future, particularly in the Southwest.

What ‘drier, hotter’ means for water across the West

Roughly three-quarters of the country’s water supply comes from surface water, that is, rivers, streams and lakes, which are replenished naturally by rain or snow. The rest comes from groundwater, also replenished by precipitation.

But under the drier and hotter conditions of climate change in the western U.S., there’s less precipitation falling to fully replenish what’s being consumed.

Consider these additional key conditions increasingly affecting water supply in the Southwest:

1. Warmer winters are reducing snowpack across the West.

In the high mountains of the West, winter snowpack is a critical piece of the water supply picture. As these “frozen reservoirs” melt through spring and summer, they keep mountain-fed rivers and streams flowing fast all season long, filling up reservoirs to contribute to regional water supply all year long.

California, for example, receives 75 percent of its rain and snow in the watersheds north of Sacramento. As snow from the Sierra Nevada mountain range melts to fill reservoirs in the spring and summer, it provides roughly 30 percent of the state’s water supply throughout the year.

But the region’s snowpack has been declining dramatically for decades. A new study of long-term snowfall in the western U.S. found declines, one-third of them “significant,” in snowpack at more than 90 percent of monitored sites.

Why? Because the long-term trend toward warmer winters is causing more winter precipitation to fall as rain. And what snow has fallen is often melting earlier, flooding riverbeds and overwhelming reservoirs in spring, then leaving drier conditions behind as the summer months tick on.

Complicating matters further is the fact that warmer air temperatures in summer also mean more water is lost to evaporation.

2. Western states have always endured drought, but it’s getting worse.

Droughts have historically plagued the Southwest, but the region is now considered “one of the more sensitive regions in the world” for heightened risk of drought sparked by climate change. For example, the California drought that stretched from 2011 to 2017 is now considered one of most extreme in the state’s history.

One reason drought is becoming more intense and frequent? Warmer temperatures also leads to higher evaporation rates and plant transpiration, increasing water loss in soil and plants.

It’s important to note here that increasing drought doesn’t mean precipitation is off the table. Climate change is also associated with more extreme weather in the region, including, at times, periods of heavy precipitation followed by possible flash flooding, causing what’s been called “precipitation whiplash.”

In addition to implications for water supply, drought is also a major cost sink. Since tracking began in 1980, drought is the second most economically costly of U.S. weather and climate disasters, costing a net $247 billion as of 2019.

3. Warming weather can lead to harmful algal blooms.

Warmer air temperatures and less snow-fed water can contribute to warmer water, a potential boon to algae growth. Although naturally occurring, algae can sometimes grow out of proportion to its ecosystem and create harmful, even toxic, water conditions.

Algal blooms can also be compounded by excess nutrient pollution that pours in during the severe weather events increasingly associated with climate change. More than just creating murky water, these blooms can produce toxins that can infiltrate a city’s drinking water supply, as recently evidenced in Oregon.

More research is needed to understand the direct links between algal bloom and climate change, but some of the worst blooms on record have occurred within the past decade or so.

So what solutions may lie ahead?

Rising temperatures and more frequent and severe droughts are expected to ramp up competition for water across the western U.S., whether for civic use, agriculture, or hydropower production. So what can be done?

There are a few ways communities are working to protect water supply into the future.

Phoenix, Arizona, for example, has been preparing for increasing drought in several key ways. For starters, it’s been banking water since 1996, to save for a non-rainy day. The fast-growing city also recycles wastewater, uses gray water in agriculture, and is working on “toilet-to-tap” technology. Private citizens are increasingly answering the call: In 2000, roughly 80 percent of city buildings were surrounded by lawns; now it’s estimated at just 14 percent. Plus Arizona, along with other states in the Colorado River basin, recently signed onto an interstate Drought Contingency Plan, which aims to reduce the risk of declining water levels.

Cities can also modernize aging infrastructure to help keep water from being lost to old, leaky pipes and unprotected channels, which are prone to evaporation. In Los Angeles for example, one-fifth of the city’s water pipes were built in 1931 – and are considered the culprits behind nearly half the city’s water loss.

Landscape restoration is another way forward. In the West, restoration  efforts could include increasing natural water storage by restoring prairies and meadows around headlands, which soak up melting snow and hold it for later release.

These and other actions will be needed to sustain water supply for the West’s booming populations. But experts agree that the most powerful, long-term fix for this problem must include addressing the underlying threat: climate change.