The authors explain:
At today's rates of water use, the Southwest is projected to use 1,303 million acre feet of groundwater in the 100 years starting in 2010 (the blue plus green segments of the top bar in Figure 1); of this a conservatively estimated 260 million acre feet would be overdraft (shown in green). Taking into consideration only baseline growth of population and income, the Southwest's shortfall of water (today's overdraft plus additional water needed beyond today's annual rates, or green plus yellow in Figure 1) reaches 1,815 million acre feet over the 100-year period. Using the B1 climate assumptions – the least climate change that is still thought to be possible – the Southwest‟s shortfall grows to 2,096 million acre feet (green, yellow, and orange). Under the A2 climate assumptions – the temperature increase expected if the current trend in global greenhouse gas emissions continues – the shortfall reaches 2,253 million acre feet (adding the red segment). This shortfall must be met either from increases to supply (perhaps the most difficult and most expensive options as discussed below), additional groundwater withdrawals, or reductions to use – planned or unplanned.Update, May 13, 2011. From Miller-McCune, via Energy Bulletin:
Between 1920 and 2000, the seven states that share the Colorado River grew from 5.7 million to almost 50 million people. Peter Gleick, co-founder and president of the Pacific Institute for Studies in Development, Environment, and Security, says 23 million more people will be added by 2030 amid mounting evidence that our current practices in water use and management are unsustainable.
As Gleick points out in “Roadmap for Sustainable Water Resources in Southwestern North America,” it’s not land, energy, mining or climate, that is going to be most difficult issue to address in the Western United States — it’s water.
So given the International Panel on Climate Change’s warnings about anthropogenic global warming and a well-documented expanding drought, residents of the West are not just living on borrowed water, they’re also living on borrowed time.