Melting Snow and Water Supply

Both lakes and rivers are affected by melting snow, which is an important source of freshwater for people throughout the world. Changes in snowmelt rate could have a serious effect on the availability of water.

"Between 1950 and the 1990s, even though overall precipitation rose slightly, the volume of Rocky Mountain snowpack declined by 16percent."

Although changes in precipitation patterns are uncertain and difficult to predict, scientists are relatively certain that there will be less snow as the temperature warms. In particular, global warming may affect snowpack—naturally formed, compressed snow, especially in mountainous regions, that often melts in springtime. Thus, University of California professor of geography Catherine Gautier, in her book Oil, Water, and Climate, notes that "Perhaps the most reliable prediction of change" in the distribution of water "is the decrease in springtime snowpack over the Northern Hemisphere."10 In 2007, Greg Nickels, the mayor of Seattle, wrote that snowpack in the Cascade Mountains in the Pacific Northwest had declined by half since 1950 "and will be cut in half again in 30 years" without action to address climate change.11

The decrease in water from snowmelt is caused by two factors. First, as temperatures warm, precipitation is more likely to fall as rain than snow. Thus, less snow lands on the ground, and there is less of a buildup in snowpack. Second, snow melts earlier as the world warms. This can result in problems for irrigation systems and reservoirs that depend on regular and predictable snowmelt to replenish water supplies.

In his book Dead Pool, James Lawrence Powell, executive director of the National Physical Sciences Consortium at the University of Southern California, explains how changes in snow patterns affected the Colorado Basin. "Even a slight reduction in snowpack spells trouble. So much of the water in the Colorado River basin evaporates that nearly 90 percent of the water in streams must come from a virtual reservoir: the Rocky Mountain snowfields. Rising temperatures cause more winter precipitation to fall as rain, reducing snowpack levels at the outset. These reductions have already begun. Between 1950 and the 1990s, even though overall precipitation rose slightly, the volume of Rocky Mountain snowpack declined by 16 percent."12 Powell goes on to explain how snow melting early can have an adverse effect on water supplies. "The higher the temperature, the earlier the snowpacks start to melt. . . . Earlier melting sends water downstream in the spring, before cities and fields can use it. If reservoirs have room, they can store the earlier-arriving water; if not, they will overflow."13

The combination of less snowfall and earlier snowmelt may result in crippling droughts in the western United States, according to Tim Barnett of the University of California, San Diego. Barnett used computer models to analyze the water flows in Western rivers over 50 years. He found that changes in western snowmelt "have less than a one percent chance of being due to

PREDICTED CHANGE IN DISCHARGE OF SELECTED RIVER MOUTHS DUE TO CLIMATE AND WATER USE CHANGES

Discharge

Relative

Continent

River Mouth

(km3 yr

-1)

Change

1960s

2050s

(%)

Africa

Kouilou

28.4

20.0

-29.6

Cross

59.9

61.8

3.1

Chari

29.1

34.3

17.9

Senegal

5.7

2.5

-56.0

Congo (Zaire)

1,349.0

1,267.5

-6.0

Volta

32.8

48.1

46.7

Asia

22.3

20.8

-6.7

Chu Salween (Thanlwin)

98.5

135.2

37.2

Nadym

16.0

26.5

65.9

Kura

22.0

13.7

-37.8

Ganges-Brahmaputra

1,186.9

1,388.4

17.0

Indus

121.2

174.6

44.1

Australasia

Merauke

1.5

0.9

-40.6

Fly

135.4

147.5

9.0

Sepik

100.7

133.6

32.7

Murray

11.1

9.5

-14.3

Ramu

32.7

40.1

22.8

Europe

Adour

6.5

6.2

-4.9

Pechora

142.0

174.1

22.6

Mezen

26.5

32.8

23.8

Kuban

13.0

9.7

-25.1

Volga

234.0

246.3

5.2

Severn, Dvina

101.2

123.4

21.9

North and

Patuca

12.3

3.4

-72.0

Central America

Yukon

187.2

246.0

31.4

Kobuk

0.2

0.8

212.2

Grande de Matagalpa

30.1

7.7

-74.3

Mississippi

530.6

540.0

1.8

Colorado

1.3

2.4

81.6

South America

Coppename

10.7

0.7

-93.4

Essequibo

155.1

78.8

-49.2

Santa Cruz

0.9

1.0

17.3

Parnaiba

26.6

5.0

-81.2

Amazonas-Orinoco

6,802.4

5,536.5

-18.6

Doce

24.4

33.4

37.1

Source: Margaret A. Palmer et al., "Climate Change and the World's River Basins: Anticipating Management Options," Frontiers in Ecology and the Environment, vol. 6, no. 2, 2008, pp. 81-89.

I sc natural variability." Barnett suggested that more water rationing and more dam building might be necessary to avert a crisis. "Global warming is an abstraction to most people," Barnett said."Well, the people who live in the West, if they haven't already, are going to very shortly find out what global warming really means to them."14

Changes in snowmelt will affect people far beyond the American West as well. According to Climate Change 2007, part of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, regions that rely on Himalayan ice melt will see a reduction in water supply. The report also notes that "In the Andes, glacial melt water supports river flow and water supply for tens of millions of people during the long dry season." Ultimately, the report says, "With more than one-sixth of the earth's population relying on melt water from glaciers and seasonal snow packs for their water supply, the consequences of projected changes for future water availability, predicted with high confidence and already diagnosed in some regions, will be adverse and severe."15

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