Water stress — the general scarcity of freshwater for people who need it — is considered by many scientists as one of the biggest challenges facing humanity and struggling ecosystems in a world increasingly affected by climate change.
Studies differ on how much the world’s growing population will be affected by the growing difficulty of finding freshwater, but a new report by researchers at the Massachusetts Institute of Technology have found that climate change could actually provide more water to people in some parts of the globe while reducing freshwater for other areas.
Global warming may increase the overall amount of freshwater flowing in rivers worldwide by about 15 percent, easing water scarcity in many places, including the U.S. Midwest, according to MIT’s Energy and Climate Outlook 2014, released Monday.
By the end of the century, during which time greenhouse gas emissions could double globally, the MIT outlook projects that water scarcity could also ease in Mexico, Saudi Arabia, Libya, China and Western Europe. In other places, water stress could worsen, especially in the U.S. Southwest, Pakistan, Turkey, South Africa and parts of North Africa.
“All climate models predict a speedup of the hydrological cycle with warmer temperatures,” said the study’s lead author, John Reilly, co-director of the Joint Program on the Science and Policy of Global Change at MIT’s Center for Environmental Policy Research. “That means faster evaporation, more moisture in the atmosphere and more rainfall.”
MIT researchers project that while more moisture in the atmosphere will increase freshwater flow 15 percent globally by the end of the century, consumption of freshwater for all human uses worldwide is expected to increase 19 percent, including water for industrial, domestic and agricultural uses.
Of those uses, the outlook shows that domestic freshwater consumption could double from 348 billion cubic meters in 2010 to 698 billion cubic meters in 2100, and industrial use of water could increase from 763 billion cubic meters to 1,098 billion cubic meters, or about 45 percent. Irrigation use is projected to decline slightly worldwide.
But more freshwater doesn’t paint the full picture. In a warming world, how and if that water can be made available for people to use gets complicated.
Exploding human populations may overwhelm water supplies, creating new areas of water stress, according to the outlook.
“This water stress arises because of increased water demand, and in some cases reduced runoff,” Reilly said. “As with almost all climate models, we project more precipitation poleward, and generally drier conditions in subtropical regions.”
And, freshwater availability depends on how and when it falls from the sky.
“Water stress, or not, is very much a function of precipitation in the right place at the right time, and in the right form,” Reilly said.
Rain may begin to fall at times when it can’t be used for irrigation or can’t be captured for storage in reservoirs, he said.
A big concern is precipitation falling as rain rather than snow, or snowpack melting earlier in areas that depend on snowmelt, such as much of the western U.S., he said.
“Snowpack is nature’s water storage, slowly releasing water far into summer dry months and therefore providing even timing even when summers are dry,” Reilly said. “With less snowpack storage, we would need to make up for it by building reservoirs where possible.”
Building new reservoirs is a costly proposition, and they’d have to be built to handle the added challenge of capturing water from extreme rainfall.
“There is a general conclusion that more rain is likely to come in heavier downpours, with longer periods in between,” he said. “So that raises the specter of both flooding and drought because in a heavy downpour most of the water runs off, and unless there is man-made storage somewhere, it quickly ends up in the sea, and is no longer fresh water.”
That’s a major concern in Rocky Mountain states such as Colorado, which contains the headwaters of some of the most important rivers in the West, including the Colorado River, which provides water to drought-stricken Phoenix and Los Angeles.
Spring snowmelt in Colorado could come up to 17 days earlier than today, and some rivers the state relies on for fresh water supplies could see streamflows decline by up to 35 percent, according to a 2012 Colorado climate vulnerability study.
“As the climate warms, more water will evaporate and sublimate from mountain snowpacks before it ever reaches reservoirs, and agricultural demand will rise,” meaning that there will be less water to go around as a booming population conflicts with a decreasing and less predictable water supply, Colorado State University atmospheric scientist Scott Denning told Climate Central in January.
The MIT report cautions that any projections of regional precipitation patterns and the processes that control runoff from mountain snowpack in a warming world are extremely uncertain, and rain and snowfall are likely to vary widely from year to year and decade to decade.
Not all studies focusing on water security have shown water stress easing much at all in a warming world.
A 2013 study by researchers at the Potsdam Institute for Climate Impact and Research showed that declining precipitation and increasing evapotranspiration will strain water supplies in many areas, especially the U.S. Southwest, affecting 2 billion people globally.
A Pacific Northwest National Laboratory (PNNL) study published in August showed that without any climate policy curbing global greenhouse gas emissions, half of the world’s population is will be living under “extreme” water scarcity by the end of the century.
These studies reach different conclusions because there are multiple ways to measure water stress: Some studies focus only on water supply, while others, such as the MIT report, focus on both supply and demand. Each study also uses its own assessment of hydrology, the effects of climate change and other factors, said PNNL climate scientist Mohamad Hejazi, lead author of the PNNL water scarcity study.
In some cases, studies may significantly underestimate overall water deficit in some areas in a warming world, he said.
“This (MIT) study constitutes one plausible scenario, but it is not definitive,” he said.
MIT’s report was published Monday following the United Nations Climate Summit in New York the previous week, when the Obama administration committed to greenhouse gas emissions cuts to be included in a treaty expected to be signed in Paris in 2015. The Paris negotiations will be known as COP 21, or the 21st Conference of the Parties to the UN Framework Convention on Climate Change.