Timing crop plantings to take advantage of seasonal water availability, although still widely practiced, is no longer sufficient to feed the world. More and more farmers supplement available soil water through irrigation using water collected from different locations or at different times of the year. Irrigated acreage and water withdrawals for agriculture have doubled since 1975. Over the same period, water withdrawals for industrial and domestic purposes have increased four-fold, but they still account for less than half of the amount withdrawn for agricultural purposes.
The majority of the world’s best farmland, in terms of climate and soils, is located in the temperate zones that lie between the tropics and the polar circles. A large percentage of this land is now irrigated. Water for irrigation in several major agricultural regions depends on snow that accumulates in nearby mountains.
Percentage of farmland equipped with irrigation. From Siebert et al. 2005.
Global warming will diminish the quantity of water stored in snowpacks of mountain ranges at mid-latitudes such as the Sierra Nevada and Himalayas. In such ranges, most of the precipitation falls at mid-elevations when moist air, blown in from the oceans, rises and cools as it approaches the mountains. Cooler air has a lower capacity to hold water vapor, and any excess water vapor precipitates. If this precipitation falls as snow, the resulting snowpack provides convenient storage until it melts during warmer, drier seasons. If this precipitation falls as rain, this runoff may engorge rivers, overflow reservoirs, and be discharged directly back to the oceans. Warmer climates will shift snowline to higher altitudes and thereby decrease snowpack and increase rainfall.
In the Sierra Nevada of California, the snowpack at lower elevations (1000 m to 2000 m) currently holds about 30% of the total water in snow and provides about 40% of the water flowing into reservoirs during the spring. Global climate models predict that most of this low-elevation snow will disappear by the end of the century. Moreover, the climate models predict that mid elevations (2000 m to 3000 m), which receive 52% of the total snowfall, will receive only 25% of the snow that they do now. Water flows into reservoirs from snowmelt during the spring will halve, and maximum flows from snowmelt will occur 14 to 32 days sooner in the year. 
In many parts of Asia, water from local precipitation and snowmelt in the Himalayas no longer meet the demands of irrigated agriculture.  To meet this water deficit, farmers are pumping groundwater from wells as deep as 1000 m. Not only are energy costs for pumping from such depths becoming prohibitive, but deep wells tap ancient aquifers that are likely to be depleted in the near future because they replenish only at a very slow pace.
In China, acreage of irrigated farmland has peaked for lack of water, and grain production has leveled off or even declined. China, however, has experienced steady population growth during its rapid economic expansion of the past decade. China receives only 5% of the world’s total precipitation but is home to 21% of the human population. To meet its food needs, China now imports grain and other agricultural commodities.
 Hayhoe, K., D. Cayan, C. B. Field, P. C. Frumhoff, E. P. Maurer, N. L. Miller, S. C. Moser, S. H. Schneider, K. N. Cahill, E. E. Cleland, L. Dale, R. Drapek, R. M. Hanemann, L.S. Kalkstein, J. Lenihan, C. K. Lunch, R. P. Neilson, S. C. Sheridan, and J. H. Verville (2004) Emissions pathways, climate change, and impacts on California. Proceedings of the National Academy of Sciences of the United States of America 101:12422-12427.
 Brown, L. R. (2006) Plan B 2.0: Rescuing a Planet under Stress and a Civilization in Trouble, 1st Edition. W. W. Norton&Co., New York.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents