The effects of climate change are not far off problems for future generations. They are existential problems for everyone alive today.
That’s one big takeaway from the U.S. federal government’s latest roundup of climate science, the National Climate Assessment, now in its fourth iteration.
Released the day after Thanksgiving, the newest report is unequivocal. In heavily footnoted, short declarative sentences it urgently tells readers that climate change is happening, it’s human-caused, and it could make life in the Western U.S. increasingly difficult.
The report is a product of the US Global Change Research Program, an amalgam of 13 federal agencies. Its publication is required by law.
In a chapter dedicated to climate change effects in the southwest, climate scientists say “with very high confidence” that warm temperatures are reducing the water content of mountain snowpack and the flows of rivers and streams that depend on snowmelt. The chapter’s landing page features a photo of low water levels at the nation’s largest reservoir, Lake Mead outside Las Vegas, Nevada, a near perfect symbol of the region’s ongoing water challenges.
The Colorado River feeds the human made lake. Its watershed provides drinking and irrigation water for 40 million people across seven U.S. states and two in Mexico. In addition to sapping the nation’s largest desert reservoirs, the changing climate is also leading to more intense droughts, increasing the risk of severe floods, weakening key infrastructure projects and depleting groundwater, the report says.
Without coming up with new ways to manage water and cut greenhouse gas emissions, the report’s authors say existing gaps between water supplies and demands in the desert southwest will only continue to grow.
Much of the report confirms and reconfirms what scientists already know. Here are some of the biggest takeaways for the southwest, which the report defines as California, Nevada, Arizona, New Mexico, Colorado and Utah:
That’s the sentence that greets readers of the report’s third chapter, dedicated to exploring how climate change will stress U.S. water infrastructure.
In a scenario where greenhouse gas emissions increase, the report says the entire southwest region could see its average annual temperature rise an additional 8.6 degrees by 2100. Southern parts of the region, which include the cities of Phoenix, Tucson and San Diego, could see their summers extend well into the spring and fall, with 45 more days each year where temperatures climb higher than 90 degrees.
That warming means areas of the southwest already at risk for long term drought are seeing the risk increase for more intense, prolonged droughts in the future. Droughts can materialize from both a lack of precipitation and a rise in temperature, and then be exacerbated by human activities. Each drought is the product of particular circumstances, the report notes, and can be intensified by dwindling groundwater, which in some regions acts as a buffer against scarce water supplies above ground.
Other studies have shown that warming temperatures in the Colorado River Basin are causing a decline in river flows, and increasing evaporation in both streams and reservoirs. Between 2000 and 2014, the driest period of record on the Colorado River, climate change tipped the scales toward higher temperatures, resulting in between 17 percent to 50 percent reductions in streamflow.
Drought can occur naturally, the report notes, but the increase in temperature from climate change can amplify the effects causing a more typical drought to last longer and cause more damage. This is true for the recent drought in California and the ongoing Colorado River drought.
Underlining the fact that some climate change effects are already being felt, the Colorado River system hit a new low point earlier this year with its key reservoirs dropping to levels not seen in decades.
2. Shrinking snowpack likely to continue
Southern Rocky Mountain snowpack in Colorado, Wyoming and Utah supplies the vast majority of river flows in the Colorado River watershed. And it’s becoming increasingly scarce.
If greenhouse gas emissions continue unabated climate models project significant reductions in mountain snowpack. The reduced snowpack isn’t just from a decrease in precipitation. Especially at lower elevations, warmer temperatures can cause snow to instead drop as rain and carry less water content. The warming can also shorten snowfall seasons. In the most extreme scenario modeled in the report portions of California’s mountains that currently receive snow, “would begin to receive more precipitation as rain and then only rain by 2050.”
Lower elevations in the southern Rockies that currently receive snow could begin to see their accumulations start to deteriorate, the report notes. Even under a high emissions scenario it’s unlikely snow would be completely eliminated at high elevations where colder temperatures are projected to remain.
To make up for reduced streamflows the region would need a large increase in precipitation, something the report doesn’t project.
Snow droughts, like the dry winter of 2017-2018, can be caused by a lack of precipitation, temperatures that are too warm for snow to form, or a combination of the two, the report notes.
3. We know how to adapt. But can we do it fast enough?
Some parts of the southwest are already showing resilience in the face of a warming climate, the report says.
Many of the region’s biggest cities, like Las Vegas, Denver, Los Angeles and Phoenix, have cut their per capita water use for the last three decades at the same time population has grown. Much of that savings is due to conservation measures like lawn buyback programs, water efficiency upgrades in homes, and investments in water reuse technology. But urban conservation alone can’t solve the region’s water woes.
The near certainty of severe water shortages adds pressure to water managers. The report challenges those leaders to create more flexible methods of managing water supplies than exist now.
Figuring out who should use less water to help the region adapt to a drier future won’t be an easy task. Water use reductions across the entire region are driving the creation of Drought Contingency Plans currently being negotiated among all seven U.S. states that rely on the Colorado River. Even those water managers crafting the plans say they are a temporary stopgap solution to prevent the river’s largest reservoirs from crashing, not a long term response to warming and drying trends.
The report also throws a wet blanket on some of the sexier solutions to southwestern water scarcity. Desalination of ocean water is often trotted out as a possible savior for cities and farms. But desalination technology now, “creates greenhouse gas emissions and its capital costs are high.”
Still, the report’s authors give praise to Colorado River managers for creating some flexibilities in the system. In 2007, amid record dry conditions, Arizona, California and Nevada signed an agreement to allow users to forgo water deliveries and store supplies in Lake Mead. A binational agreement in 2014 between the U.S. and Mexico resulted in an experimental flow of water in the river’s desiccated delta. A 2017 agreement brought Mexican water leaders to the table and resulted in promises to share in future water shortages should reservoirs continue to drop.
The severity of southwestern water shortages is largely up to us, as the report notes in its section on how confident climate scientists are in their findings: “The actual frequency and duration of water supply disruptions will depend on the preparation of water resource managers with drought and flood plans, the flexibility of water resource managers to implement or change those plans in response to altered circumstances, the availability of funding to make infrastructure more resilient, and the magnitude and frequency of climate extremes.”
This story is part of a project covering the Colorado River, produced by KUNC and supported through a Walton Family Foundation grant. KUNC is solely responsible for its editorial content.
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