What some models say about north america

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Considerable ongoing research is aimed at predicting what the future climate will be like for North America in light of natural changes as well as anthropogenic (human-caused) global warming. Extreme drought is one of the expected consequences of increased global warming, especially in the American Southwest, where it has already been projected to be severe by several models.

The Drought Research Lab at Lamont-Doherty Earth Observatory at Columbia University in New York has identified and studied several

Drifting, blowing soil is burying a farm and its abandoned farm equipment in Kansas during the famous dust bowl in 1935. (NOAA's National Weather Service Collection)

drought cycles that have affected North America in the past. One of the most well-known and publicized droughts was the dust bowl of the 1930s, but it was not the worst. Through analysis of proxy data such as tree rings, scientists at Columbia were able to determine that in the late 1200s North America suffered a severe drought. This coincides with the period in which the Anasazi civilization in the Southwest disappeared (see chapter 7).

They also determined that during the entire Holocene (the past 10,000 years), most of North America has experienced periods of prolonged drought interspersed with wetter periods, called pluvials, and that some of the droughts were extreme. There was also what has been referred to as a megadrought in the late 1500s.

North America has experienced drought in recent times, as well. The mid-1850s to 1860s brought a severe drought to the Plains worse than the dust bowl. During the 1950s, much of the Southwest was dry. Then Arizona, New Mexico, Utah, Nevada, and California suffered a long dry spell from 1998 through 2004, and some of these areas continue to suffer today. Many of the reservoirs in these states are still well below their holding capacities, some of them 50 to 100 feet (15-30 m)

low. Lake Powell, on the border of Utah and Arizona, for instance, is still so low that the original boat docks have been unusable for several years because the water no longer reaches them.

Low reservoir levels not only put a huge strain on drinking water supplies for millions of people who live in the Southwest but also harm farming practices, the grazing industry, and manufacturing industries in the area, all of which rely on a plentiful water supply in order to function at full capacity. Another major problem is the continually growing population. Many people are moving to the Southwest because they enjoy the mild winter climate, but as the population increases, so does the demand for freshwater. Unfortunately, much of the population growth in Arizona occurred from 1977 to 1998, which was an uncharacteristically wet period for the Southwest, meaning that construction and population boomed under atypical natural conditions, giving residents a false sense of security about water. If global warming is not controlled and the drought situation continues to worsen because of human activity, this will make survival in the desert Southwest even more challenging. In fact, the models climate scientists have developed predict that droughts will continue and become even more pronounced because of global warming.

Researchers have come up with two theories about why this area is affected by persistent drought. They believe it relates to subtle changes in sea surface temperatures (SSTs) of the tropical oceans, especially the tropical Pacific, and happens because of the way it causes the atmosphere to move. As hot air is warmed in the Tropics and circulates, it descends in the subtropical higher latitude areas. As this air descends over the American Southwest, it becomes very dry, bringing very little precipitation, which in turn causes drought. They have also noted that when La Niña events occur, they are usually associated with dry winters in the Southwest, which then causes the Plains states to have dry springs. If these dry episodes persist, the entire summer may remain droughtlike.

Another way researchers think the cold equatorial Pacific waters cause drought over North America is by relocating heat sources in the Tropics. If the Intertropical Convergence Zone (ITCZ), a convergence of warm, moist air at the equator, moves north where waters may be warmer, it changes global circulation, which in turn changes wind circulation. The net effect is an increase in the descending motion over western North America, which increases the drying effect and makes drought more pronounced.

One important point researchers have discovered at Columbia is that these shifts in water, atmospheric circulation, or temperature variation do not have to be extreme. It does not take a huge catastrophic event to cause a major change in the climate, such as drought. Instead, it is more likely to happen subtly, in small increments at a time. During the dust bowl and the 1950s droughts, for example, the tropical Pacific Ocean was only a few tenths of a degree Fahrenheit cooler.

According to Richard Seager at the Lamont-Doherty Earth Observatory, the important result of these models is that "persistent droughts are the sum effect of persistent, but small, precipitation anomalies. During the dust bowl precipitation over the Plains was reduced by about 15 percent but, when this happens year after year, the ground moisture gets less and less as evaporation proceeds, resulting in severe drought."

Seager also points out that persistent droughts do not affect only North America. When droughts occur, they occur in a global pattern of "precipitation variability," which means that worldwide some regions will also experience drought while others may experience increased precipitation. When North America experiences significant droughts, it is common for regions such as South America, the Mediterranean-Europe region, North Africa, the Middle East, and Asia also to have droughts, because all these regions lie in the global mid-latitudes (the subtropical regions) and are affected by the descending dry air in the atmospheric circulation.

Climatologists very much want to be able to predict when droughts and pluvial periods will occur. Presently, however, only El Niño and La Niña, which last only a single winter, can be predicted in ocean circulation models with a good degree of accuracy up to a year in advance. Being able to predict long-term droughts is an area researchers are focusing on heavily for the future, especially because of the threat of global warming.

Seager has come up with some theories about the future of the American Southwest and other subtropical regions that will become more arid over time, especially because of global warming. This warming is not something that will occur in the future—it is happening right now and has been going on for several years. It will become an established condition in years to come and create a permanent drought condition. Seager sees this as one of the most critical issues for climate modelers to deal with. He states that 19 modeling groups around the world have looked at the anthropogenic causes of climate change and agree that regions in the subtropics, including the American Southwest, are headed toward much more arid climates in the future. According to the findings in the models, the effects of humans on the land and their overuse of resources and poor land management practices that result in desertification have become very pronounced in the models of the past few decades.

Seager also points out that unlike droughts of the past, models that predict the future do not show the outcome linked so much to natural phenomena as to the overall surface warming driven by increasing greenhouse gases introduced by humans—global warming. The main reason this result is so evident in all the models is that they do not rely on hard-to-model inputs, such as cloud physics, but instead are based on large-scale atmospheric dynamics, a straightforward component that generates nonrefutable output.

These results should be of great concern to many people. Continued drying of already arid lands in the southwestern United States and northern Mexico will have dire consequences for issues such as water resources, land development, economics, industry, political relations, travel, and migration in the years to come.

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