Sunday, November 28, 2010

Water in the 21st century US

One major reason that it is difficult to achieve consensus about acting to reduce greenhouse gas emissions is the difficulty in grasping the magnitude of the climatic changes they will produce. The significance of a number like 3 degrees Celsius global warming over the next century is lost on most people: for one, many places experience that amount of temperature variation everyday, and much more than that every season. The fact that it is a global average also makes it a nebulous, if not irrelevant figure. Translating these abstract climate estimates to real world experience is a crucial step toward convincing decision-makers and the public about the need to tackle the problem.

Mike and I recently wrote a paper based on that premise: an attempt to help people in the U.S. visualize the local and regional scale environmental changes that are likely to happen over the coming decades. And rather than use temperature &/or precipitation, the two basic variables of climate, we used the availability of water as a means to synthesize their inter-linked significance. This linkage is seen, for instance, in the fact that if precipitation stays about the same over time, but it gets hotter, less water is found in streams or available for crops, because more evaporates. You can find the details and full article here, but the basic idea is to compare the patterns observed for the 20th century with those modeled for the end of the current century (around 2090) - thus changes that many of us are likely to at least begin to witness during our lifetimes.

The patterns of Soil Moisture availability throughout the year summarize the results most vividly. The map series at left shows average levels of water available to plants (i.e., crops and natural vegetation) during the 20th century. Dark blue represents soils that are saturated, while dark brown signifies completely dry conditions. The Southwest and the Great Basin stand out clearly as continuously arid regions, while the coastal strip of the Pacific Northwest and the eastern half of the country as the most humid. Within this general pattern, seasonal variation is also apparent, particularly the drying of humid zones that builds during the summer, followed by the recharge of soil moisture over the winter and spring. A broad gradient characterizes the midsection of the nation: during a trip from Indianapolis to Denver along I-70, an astute traveler would notice gradually drier conditions reflected in the shift from forest to tall grass prairie to short grass steppe.

Climate model projections for the end of this century, which assume major shifts in energy production and significantly reduced consumption by 2050, nonetheless predict a mean annual temperature increase in the US on the order of 4-6 degrees C (7-10 F). The consequences for the geography of water availability are dramatic, taking the 20th century patterns to extremes. The desert conditions of the Southwest become enhanced, and extend throughout much of the Great Plains: the conditions of the Dust Bowl 1930s (and the similar 1950s drought) become the norm for this region. Precipitation levels increase across much of the northern and eastern parts of the US, fully saturating the soils during the spring, but then becoming very dry by the end of the hot summers. Most striking is the sharp boundary that separates the humid east from the arid west- the 100th meridian is no longer the center of a gradual shift from humid to arid, but a stark boundary.

The implications for agriculture and drinking water supplies are huge, with equally significant and varied ripple effects on social and economic conditions; changes that are already being observed in a variety of vulnerable sites, as pointed out in the previous post - When the water ends.

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