The Hidden Reason Droughts Are Lasting Longer in the Southwest, According to Climatologists

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Anyone who has watched Lake Mead’s shoreline creep further from the water over the past two decades has probably assumed the culprit is simple: not enough rain. That explanation feels intuitive, and for much of the twentieth century it was mostly true. But climatologists studying the Southwest’s current dry stretch have identified something else working alongside the missing precipitation, something less visible than an empty reservoir but arguably more important for understanding why this drought refuses to end. The answer involves the air itself, and how much water it has started pulling out of the ground, the rivers, and the mountain snowpack simply because it’s warmer than it used to be. It’s a subtle shift, but researchers say it has fundamentally changed the character of Southwestern drought since the year 2000.

A drought unlike the ones before it

The Southwest has been in a dry spell since roughly 2000, and researchers now describe it in terms usually reserved for centuries-old climate events. The drought that has enveloped southwestern North America for the past 22 years is the region’s driest “megadrought” since at least the year 800, according to a UCLA-led study in the journal Nature Climate Change. That statistic alone would be notable, but what makes this particular megadrought different from its medieval predecessors is the mechanism behind it.

Older droughts in the tree-ring record were driven almost entirely by a shortage of rain and snow. This one has a second engine running underneath it. It is also the first megadrought to be driven in large part by human-caused warming, as increased temperatures lead to a thirstier atmosphere in a positive feedback loop that dries out the landscape faster and in turn allows the air to heat up even more.

The real culprit: atmospheric thirst, not just missing rain

For most of the last century, drought in the western United States tracked closely with rainfall totals. When storms skipped the region, soil dried out, rivers shrank, and reservoirs fell. During the later half of the 20th century, drought conditions in the western U.S. were dominated by a lack of precipitation, but a shift in drought dynamics emerged at the turn of the century, fueled mostly by carbon pollution and the resulting warming.

That shift means precipitation deficits alone no longer explain what’s happening on the ground. Heat-driven increases in atmospheric thirst, fueled by human-caused climate change, have been the leading cause of exceptional drought conditions in the western U.S. since 2000, a period that includes an ongoing multi-decade megadrought that is the region’s driest stretch in over 1,200 years. In plain terms, the air is doing more of the drying than the sky is failing to do the wetting.

Vapor pressure deficit, explained simply

Climatologists have a specific term for this atmospheric thirst: vapor pressure deficit, or VPD. It measures the gap between how much moisture the air is actually holding and how much it could hold at a given temperature. High temperatures make the atmosphere thirsty for moisture, which it draws vigorously out of the region’s soil, rivers, lakes, and even the snowpack, and this atmospheric demand reached record highs during the current drought.

The concept matters because it flips the usual drought narrative. Rather than land simply receiving less water from above, the land is losing more water to the air above it. Warmer air acts almost like a bigger sponge, and across the Southwest that sponge has been squeezing harder every decade this century.

How much of this is human-caused, according to research

Scientists have tried to put a number on how much of this atmospheric thirst traces back to human emissions rather than natural variability. One study found that 68% of the increase in vapor pressure deficit is explained by human-caused global warming. That’s a striking figure for a factor most people never think about when picturing drought.

The downstream effect on soil moisture is just as significant. Existing climate models have shown that the current drought would have been dry even without climate change, but not to the same extent, and human-caused climate change is responsible for about 42% of the soil moisture deficit since 2000. In other words, warming isn’t just making a normal dry spell feel worse. It’s actively manufacturing a meaningful share of the deficit itself.

The feedback loop that keeps making things worse

What worries climatologists most isn’t the current numbers but the mechanism that reinforces them. Anthropogenic warming has been shown to substantially raise evaporative demand across the region. Anthropogenic warming increased the annual mean atmospheric vapor-pressure deficit by roughly 9.6 percent, which increased the mean annual total evaporative demand by a wide margin.

Once soil and vegetation dry out, less energy from the sun goes into evaporating remaining moisture and more goes directly into heating the surface air. That extra heat then raises VPD further, pulling out even more moisture. It’s a loop that tends to accelerate rather than settle, which helps explain why droughts that once might have broken after a few wet winters have instead persisted for a quarter century.

The Pacific’s quiet role in the background

Ocean temperature patterns set the stage even before warming amplified things. A shift in ocean and atmospheric patterns over the North Pacific, resembling the drought phase of the Pacific Decadal Oscillation, is considered a key cause of the current megadrought in the Southwest. This pattern, known as the PDO, has stayed locked in its dry phase for an unusually long stretch.

Researchers tracking the PDO have noted just how persistent this negative phase has become. In July 2025, the PDO hit the lowest negative value ever recorded, and the prolonged negative phase generated colder air, which holds less moisture than warmer air, leading to declining precipitation across the US West. The PDO essentially built the stage, and then warming raised the temperature on it. A study by UCLA, NOAA, and CIRES scientists showed that anthropogenic climate warming transformed an ordinary drought into the extraordinary drought that ravaged the Western US in 2020 to 2022, with evaporative demand playing a bigger role than reduced precipitation.

Atmospheric rivers are moving away from the Southwest

Winter storms called atmospheric rivers deliver a huge share of the West’s annual water supply, and their behavior has been shifting for decades. Researchers studying satellite data have found a consistent pattern. Atmospheric rivers have shifted about 6 to 10 degrees toward the poles over the past four decades, and California relies on these systems for up to half of its yearly rainfall.

When storm tracks drift north, the Southwest simply gets fewer chances to catch them. Climatologists connect this pattern directly to the region’s recent winters. Climate change appears to have driven an ongoing 25-year shortfall in winter rains and mountain snows across the U.S. Southwest, and multiple studies now suggest human-caused climate change is boosting an atmospheric pattern in the North Pacific that favors unusually low winter precipitation across the region.

Groundwater is disappearing out of sight

While reservoirs get most of the attention, the water sitting invisibly underground has been vanishing even faster. The American Southwest, including Arizona, New Mexico, and portions of Nevada, Colorado, Utah, and California, is linked to one of four continental-scale mega-drying regions worldwide, with the loss of freshwater resulting from severe droughts and groundwater overuse. Communities have increasingly leaned on aquifers to make up for shrinking surface water.

The scale of that borrowing is hard to overstate. Between 2002 and 2024, the Colorado River Basin lost 27.8 million acre-feet of groundwater, equal to Lake Mead’s full capacity, with Arizona responsible for 74% of that decline. Because these aquifers formed over thousands of years, that water isn’t coming back on any timeline that matters to current residents or farmers.

The Colorado River is the clearest warning sign

The river that supplies roughly forty million people has become the most visible thermometer for this whole phenomenon. Its reservoirs have told a consistent story of decline for years. At the start of 2026, USBR estimates Lake Powell’s elevation will be 3,538 feet, just 48 feet above minimum power pool, while Lake Mead sits at 1,060 feet, only 25 feet above the minimum power pool for its older turbines.

A recent basin assessment ties this decline directly to the atmospheric mechanisms described above, not simply to bad luck with storms. A 2025 assessment of the Colorado River Basin concludes that the Southwest’s long-running drying trend is being driven not only by rising temperatures but also by declining precipitation linked to human-caused climate change. That combination, heat pulling moisture out of the system while storms increasingly miss the basin, is precisely the dynamic climatologists point to when explaining why relief keeps failing to arrive.

What comes next, according to climatologists

The uncomfortable part of this research is the outlook it implies. Scientists studying the underlying mechanisms don’t see an easy reversal on the horizon. The Southwest United States is currently facing its worst megadrought of the past 1,200 years, and according to a recent study from The University of Texas at Austin, the drought could continue at least until the end of the century, if not longer.

Some scientists still hold out hope that natural cycles could eventually swing back toward wetter conditions. Although some scientists anticipate that natural climate variability will bring relief, new research suggests that ongoing warming could be disrupting the natural rhythm of an important climate cycle that brings needed rain to the region. Until that rhythm reasserts itself, or until warming slows enough to loosen the atmosphere’s grip on the region’s water, the Southwest looks set to keep losing more moisture to the sky than it ever loses to a simple lack of rain.

Final thoughts

The story of the Southwest’s long dry spell has quietly shifted over the past twenty five years. It’s no longer just about storms that failed to show up. It’s about air that has learned to pull water out of the ground more aggressively than it used to, a change driven substantially by warming temperatures rather than chance alone.

That distinction matters for how the region plans its future, because a rainfall problem and an atmospheric thirst problem call for different responses. Climatologists studying vapor pressure deficit, shifting storm tracks, and vanishing groundwater are essentially describing the same underlying trend from different angles. The Southwest isn’t just waiting for rain anymore. It’s contending with an atmosphere that has changed the rules.