Some 40 million people in the United States are at risks of losing their water source as the flow of Colorado River continued to decline due to higher temperature since the turn of the century.

Colorado River, according to the U.S. Bureau of Reclamation, supplies water to seven western states and the Mexican states of Sonora and Baja California.

"The future of Colorado River is far less rosy than other recent assessments have portrayed," Bradley Udall, senior water and climate scientist of the Colorado Water Institute at Colorado State University, said.

Reduced River Flows Linked To Warming

The study, published in the journal Water Resources Research, takes into account the future of Colorado River after Udall learned that river flows were less than what the water managers expected due to drought since 2000.

The researchers seek to show the role of rising temperatures in the reduction of the flow of Colorado River.

It has been found out that from 2000 to 2014, the river flows reduced to about 2.9 million acre-feet of water per year.

Researchers have shown that rising temperatures played a major role on the decline of Colorado River flow.

If greenhouse gas emissions remain unchecked, it will "cause Colorado River flow declines of 30 percent by midcentury and over 50 percent by the end of the century," said Jonathan Overpeck, a professor of geosciences and hydrology and atmospheric sciences at the University of Arizona.

"We're the first to make the case that warming alone could cause (the decline of) Colorado River flow," he added.

Old Models, New Realities

Studies have shown that current climate models, which are based largely on the 20th century conditions, are inadequate in providing the information of the megadroughts in the past.

These models cannot even simulate the droughts of the current century. For instance, the American Southwest, according to researchers, is at higher risk of experiencing multidecadal drought as temperatures are rising.

The researchers, meanwhile, warned of the uncertainty in predicting the effects of the amount of rainfall to the basin.

Multidecadal droughts are still expected to occur when the increase of precipitation in the basin will fall below the normal level.

The Message: Plan

Water managers have "to plan for significantly lower river flows."

Udall underscored the inadequacy of the current planning to take into consideration the threat of climate change to the water supplies, especially in the American Southwest.

With the future of the river "far less rosy than other recent assessments have portrayed," the message is clear: plan.