The south-western United States is almost certain to experience a “megadrought” lasting decades if global warming continues unchecked, researchers have warned.

The once-mighty Colorado river, which has regularly failed to reach the ocean since the 1960s, is already in the grip of the worst 15-year drought on record with the flow of water in the 21st century nearly a fifth lower than the 20th-century average, a new study found.

And scientists warned the river could be reduced by anything from 35 to 55 per cent by the end of this century if nothing is done to reduce greenhouse gas emissions.

Rising temperatures cause increased evaporation from the river, but also prompt plants to use more water.

A paper about the study in the journal Water Resources Research said: “With continued anthropogenic [human-caused] warming, the risk of multi-decadal megadrought in the Southwest increases to over 90 per cent over this century if there is no increase in mean precipitation.

“Even if modest precipitation increases do occur, the risk will still exceed 70 per cent.”

In the event of “huge and unlikely” increases in rainfall, there would still be a megadrought risk of just under 50 per cent.

Some 40 million people in seven US states and more in Mexico rely on the Colorado river for water.

But the researchers found that the river’s flow between 2000 and 2014 was 19 per cent lower than the average from 1906 to 1999 – equivalent to the amount of water used by two million people over a year.

One of the paper’s lead authors, Bradley Udall, of Colorado State University, said: “The future of Colorado river is far less rosy than other recent assessments have portrayed.

“Current planning understates the challenge that climate change poses to the water supplies in the American Southwest.

“A clear message to water managers is that they need to plan for significantly lower river flows.”

However he added that a prolonged drought – such as has occurred in the past – could make things substantially worse.

“A megadrought in this century will throw all our operating rules out the window,” Mr Udall said.

10 photographs to show to anyone who doesn't believe in climate change Show all 10 1 /10 10 photographs to show to anyone who doesn't believe in climate change 10 photographs to show to anyone who doesn't believe in climate change A group of emperor penguins face a crack in the sea ice, near McMurdo Station, Antarctica Kira Morris 10 photographs to show to anyone who doesn't believe in climate change Floods destroyed eight bridges and ruined crops such as wheat, maize and peas in the Karimabad valley in northern Pakistan, a mountainous region with many glaciers. In many parts of the world, glaciers have been in retreat, creating dangerously large lakes that can cause devastating flooding when the banks break. Climate change can also increase rainfall in some areas, while bringing drought to others. Hira Ali 10 photographs to show to anyone who doesn't believe in climate change Smoke – filled with the carbon that is driving climate change – drifts across a field in Colombia. Sandra Rondon 10 photographs to show to anyone who doesn't believe in climate change Amid a flood in Islampur, Jamalpur, Bangladesh, a woman on a raft searches for somewhere dry to take shelter. Bangladesh is one of the most vulnerable places in the world to sea level rise, which is expected to make tens of millions of people homeless by 2050. Probal Rashid 10 photographs to show to anyone who doesn't believe in climate change Sindh province in Pakistan has experienced a grim mix of two consequences of climate change. “Because of climate change either we have floods or not enough water to irrigate our crop and feed our animals,” says the photographer. “Picture clearly indicates that the extreme drought makes wide cracks in clay. Crops are very difficult to grow.” Rizwan Dharejo 10 photographs to show to anyone who doesn't believe in climate change Hanna Petursdottir examines a cave inside the Svinafellsjokull glacier in Iceland, which she said had been growing rapidly. Since 2000, the size of glaciers on Iceland has reduced by 12 per cent. Tom Schifanella 10 photographs to show to anyone who doesn't believe in climate change A river once flowed along the depression in the dry earth of this part of Bangladesh, but it has disappeared amid rising temperatures. Abrar Hossain 10 photographs to show to anyone who doesn't believe in climate change A shepherd moves his herd as he looks for green pasture near the village of Sirohi in Rajasthan, northern India. The region has been badly affected by heatwaves and drought, making local people nervous about further predicted increases in temperature. Riddhima Singh Bhati 10 photographs to show to anyone who doesn't believe in climate change A factory in China is shrouded by a haze of air pollution. The World Health Organisation has warned such pollution, much of which is from the fossil fuels that cause climate change, is a “public health emergency”. Leung Ka Wa 10 photographs to show to anyone who doesn't believe in climate change Water levels in reservoirs, like this one in Gers, France, have been getting perilously low in areas across the world affected by drought, forcing authorities to introduce water restrictions. Mahtuf Ikhsan

The paper said the current rate of warming was faster than seen during previous megadroughts.

“During the 12th century, flow reductions of approximately 16 per cent occurred during one 25-year period,” it said.

“Evidence indicates that hemispheric and Southwest temperature anomalies were significantly smaller during past megadroughts than the rapid ongoing current warming that could easily exceed 4C to 5C by the end of century under business-as-usual emissions.”

The researchers said there was one way to stop the rapid reduction in the size of the river.

“Temperature-driven threats to the flows of the Colorado are large and real,” the paper said. “The only way to curb substantial risk of long-term mean declines in Colorado river flow is thus to work towards aggressive reductions in the emissions of greenhouse gases into the atmosphere.

“The record warm nature of the ongoing Colorado River drought indicates that this drought is not just a natural drought, and our work demonstrates that flows are unlikely to return to the 20th-century averages if we only wait.