Drought in the Western U.S. has plunged the largest reservoir in the country into alarming shortage conditions that have rippling impacts for the region. Lake Mead, formed by the construction of the Hoover Dam on the Colorado River, delivers water and hydroelectric power to 25 million residents in the Southwest. But its viability has been pushed to the brink by intensifying drought, exacerbated by climate change, triggering emergency measures to conserve water in the basin.

The region has been in a “megadrought” since 2000, but recently, Lake Mead’s water levels have been breaking ever lower lows, unearthing old shipwrecks and other long-forgotten debris and leaving a “bathtub ring” around the reservoir’s edges. The drought signals a larger trend of dwindling snowfall and longer summers brought on by the growing climate crisis.

New water scarcity measures enacted

Water usage on the Colorado River operates on a tier system. When water levels in a reservoir drop below a certain point, usage by neighboring states is restricted. Lake Mead hit Tier 1 in August 2021 after the elevation of the reservoir dipped below 1,075 feet, leading to a reduction in water supplies that largely impacted agricultural users across counties.

This was the first time a shortage condition has been implemented on Lake Mead. The Tier 2 decision was announced in August of 2022—stating that the water level would fall below 1,050 by the end of the year, triggering a more intense shortage.

This emergency declaration for Lake Mead is part of a plan to increase the water levels in Lake Powell— an upstream reservoir and the second largest in the United States behind Mead. Dealing with shortages in the Colorado River Basin has required officials to weigh the needs of one region over another. The Bureau of Reclamation has indicated that at present, keeping water levels up in Lake Powell supersedes the requirements of Lake Mead. The generators at Powell have a total capacity of 1,320 megawatts and the reservoir is considered a ‘bank account’ for the region to draw on in times of drought—which are anticipated to worsen with climate change.

According to the US Drought Monitor, extreme droughts were rare in the historical climate—a 5.5% likelihood. In 2022 however, nearly all of the watersheds in the Colorado River experienced extreme drought. In a world warmed by 2 degrees C, the likelihood of 12 or more months of extreme drought in the Colorado River Basin becomes as high as 40%.

Meeting water needs in dry times

But Lake Mead also serves a massive population in the lower basin, and filling demand for water even during shortages means some major cities have to turn to reservoirs on other river systems. Arizona, suffering some of the steepest cuts in their allotment of Colorado River water (21%) , will draw from the Salt and Verde rivers. Other strategies include pumping groundwater and implementing more aggressive conservation and re-use strategies, which have so-far helped to spare Las Vegas from the worst effects of the shortage.

The Southern Nevada Water Authority also began using its low lake level intake in 2022, which allows the state to draw water even when the elevation of the lake falls below “dead pool” status— the point at which downstream water releases are no longer possible. But this is only a temporary solution, as the water in the reservoir keeps falling.

The next significant threshold for Lake Mead would be a drop to Tier 3 (1,025 feet) which some experts say could come as soon as 2024. At 950 feet, the reservoir would be considered an “inactive pool”, meaning the dam’s generators can no longer run. Energy shortages could kick off a vicious cycle, requiring backfilling with fossil fuels that would exacerbate the climate crisis and warming-driven drought conditions.

Reversing the drought in the Colorado River Basin will ultimately depend on snowfall in the Rocky Mountains, which will ultimately depend on getting the climate crisis under control. Experts estimate there would have to be several consecutive heavy snow years in the mountains to make back the current deficits further downriver. 2023 is currently experiencing above average snowpack, but if temperatures keep rising, that will be a less likely annual occurrence. Water rights and resource usage will have to adapt rapidly to support residents as reservoir levels continue to drop, but pulling out of emergency scarcity measures for good will require curbing the greater impacts of global climate change.

As the planet warms, drought is an increasing threat in many regions. Research led by Woodwell Research Assistant Isabelle Runde, modeled the frequency of drought across the globe, analyzing drought changes in forest, food, and energy systems as temperatures surpass 2, 3, and 4 degrees Celsius.
Models show that unlike in a stable climate, unreliable water resources and increasing temperatures make drought more likely in many places. For every increase of 0.5 degrees C, an additional 619 million people could become exposed to extreme drought 1 in every 4 years. This is in addition to the 1.7 billion people (nearly a quarter of today’s global population) who are already exposed to these conditions in a world that has warmed by a little more than 1 degree C.

2 Degrees of Warming Risks Damaging our Best Forest Carbon Sinks

Tropical forests are one of the planet’s key natural climate solutions— able to prevent 1 degree of warming through both carbon sequestration and regional cooling effects. Deforestation, fragmentation and degradation from things like fire, and disease threaten to turn these forests from a vital sink to a source of emissions.

In recent years, the Amazon has been a net carbon source due to increased extreme drought and deforestation, leaving the Congo rainforest as the world’s last remaining stable tropical forest carbon sink.

As warming surpasses 2 degrees, the annual likelihood of drought in the Congo rainforest begins increasing faster than in the Amazon. Drought can make a forest more susceptible to further degradation, such as fire or disease, and reduces carbon sink capacity by stressing or killing trees and placing the ecosystem under stress.

Productivity is Threatened in the Breadbaskets of Mediterranean, Mexico, China

Global crop production is highly concentrated in key breadbasket regions— nearly 72% of the world’s maize, wheat, rice, and soy are produced in just 5 countries. Extreme drought can reduce the productivity levels of these staple crops, among others, potentially triggering widespread food insecurity, hunger and economic disruption.

By 2 degrees of warming, the probability of drought in the breadbasket regions of both China and the United States will be greater than 50% — meaning an extreme drought roughly every other year.

Disruption will be much higher in countries where jobs in agriculture comprise a large segment of the economy. In Mexico, one of the world’s top 10 producers of maize, 12% of the workforce is in agriculture and at 1 degree, the country already has among the greatest areas of cropland exposed to drought. 90% probabilities—indicating near-annual drought—begin to emerge in some parts of the country at 2 degrees of warming.This kind of recurrent extreme drought will stress water resources for agriculture.

The Mediterranean also is a drought hotspot. Drought probability in Mediterranean croplands will increase rapidly between 2 and 3 degrees of warming, rising from just 10% to over 50% of cropland affected by drought in 3 out of 4 years.

Drying Rivers Will Plunge Hydro-dependent Countries into Energy-Shortages

Hydroelectricity supplies a sixth of global energy demand, and is a low-cost, low-emission alternative to fossil fuels. The overwhelming majority of new hydropower plants since 1990 have been constructed in fast-growing, developing nations.

High dependence on hydropower makes countries like Brazil and China vulnerable to energy disruption during periods of drought. Brazil draws nearly two thirds of its energy from hydroelectric resources. During a three year drought between 2012 and 2015 in Brazil, hydroelectric generation declined by 20% each year. If warming exceeds 3 degrees C, more than half of Brazil’s hydroelectric capacity will experience a likelihood of annual drought greater than 50%.

Extreme drought can also be counterproductive to reducing carbon emissions. During years of drought, expensive fossil fuel based energy is often brought in to fill demands. In addition, droughts often coincide with extreme heat events, when electricity demand peaks to run air conditioners. Beyond 3 degrees of warming, more than a third of the planet’s hydroelectric capacity will likely be exposed to extreme drought every other year.

Projections of a Dryer World

Current international climate goals aim to limit warming to between 1.5 and 2 degrees C, but without urgent intervention, we are on track to push past that limit to at least 2.5 degrees C. Projections past 2 degrees of warming show a future where extreme drought is common, exposing already-vulnerable people, places, and economies to greater water shortages, while making it even harder to curb emissions. In order to guard water resources and the systems that depend on them, emissions need to be cut rapidly. And places already feeling the impacts of warming will need to brace to adapt to a hotter, dryer version of the world.

A warmer world means snow, rain will be much less predictable

When and where precipitation falls can determine whether or not people have enough drinking water, aquifers can support agriculture, and rivers keep running. Climate change is breaking down the predictability of weather patterns across the globe. Two new releases this week, from the Woodwell Climate Research Center and Probable Futures, flesh out our understanding of how the shifting seasonality of precipitation might impact our future.

Rainy seasons are fluctuating more

A new volume of maps, data, and educational materials launched on the Probable Futures platform today. The volume provides information that helps readers better understand local, regional, and global precipitation trends, showing how they will change with climate change.

The impact of a warmer world on precipitation patterns is not uniform—in some places dry spells will become more common, in others, intense storms, and some places will fluctuate between both. Rainy seasons may start earlier or later in different parts of the world, which will have impacts on growing seasons and agricultural yields.

“Climate change is reshaping both local precipitation patterns and the global water system—and everyone on Earth will be affected,” said Alison Smar, executive director of Probable Futures. “It may seem counterintuitive, but knowing that the future is less predictable is a valuable forecast. Communities need to be more resilient, adaptable, and prepared. It’s within our power today to prepare for the events that are probable, and prevent those with irreversible impacts.”

Snow is melting earlier

Woodwell Associate Scientist, Dr. Anna Liljedahl and Assistant Scientist Dr. Jenny Watts, were co-authors on a paper also released today that documents the impacts of earlier snowmelt in the Arctic. The Arctic is warming more rapidly than anywhere else on earth, which has led to earlier snow melts and longer growing seasons in the tundra.

Conventional hypotheses have predicted that lengthening summers would allow more time for vegetation to grow and sequester carbon, perhaps offsetting emissions elsewhere.

“Our results show that the expected increased CO2 sequestration arising from Arctic warming and the associated increase in growing length may not materialize if tundra ecosystems are not able to continue capturing CO2 later in the season,” said Dr. Donatella Zona, lead author on the paper from the University of Sheffield’s School of Biosciences and the Department of Biology at San Diego State University.

Dr. Liljedahl says that the results highlight the fact that the impacts of climate change will be complex across ecosystems.

“This work shows how important it is to continually assess our assumptions and terminology on how the Arctic system will respond to warming. We often say that warming will lead to a “longer growing season”. We need to be more careful in making that connection,” said Dr. Liljedahl.