The Midwest is characterized by farming, manufacturing, and forestry. The Great Lakes form the world's largest freshwater lake system, providing a major recreation area as well as a regional water transportation system with access to the Atlantic Ocean via the St. Lawrence Seaway. This region encompasses the headwaters and upper basin of the Mississippi River and most of the length of the Ohio River, both critical water sources and means of industrial transportation providing an outlet to the Gulf of Mexico. This area contains some of the richest farmland in the world and produces most of the nation's corn and soybeans. It also has significant metropolitan centers such as Chicago and Detroit. The North Woods are a large source of forestry products and have the advantage of being situated near the Great Lakes, which provide convenient transportation.
During the 20th century, the northern portion of the area warmed by almost 4°F (2°C), while the southern portion, along the Ohio River valley, has cooled by about 1°F (0.5°C). Annual precipitation has increased, in some places up to 20 percent, causing a higher number of rainy days.
Climate models predict that temperatures will increase throughout the Midwest during the 21st century at a greater rate than has been observed in the 20th century. Even over the northern portion of the region, where warming has been the greatest, an accelerated warming trend is projected, with temperatures increasing by 5-10°F (3-6°C). Models also predict that there will be a substantial increase in evaporation, causing a soil moisture deficit, a reduction in lake and river levels, and more droughtlike conditions in much of the region. Unusually heavy and extreme precipitation events are also expected.
Water resources also will be affected. Water levels, supply, quality, and water-based transportation and recreation are all sensitive to climate change. Even though there is a projected increase in precipitation, increased evaporation due to higher summer air temperatures will most likely cause reduced levels in the Great Lakes. The U.S. Global Change Research Program ran 12 different models, and 11 of them projected a one- to five-foot (0.3 -1.5 m) decrease in lake levels for the Midwest. A five-foot (1.5 m) reduction would cause up to a 40 percent reduction in outflow to the St. Lawrence Seaway. Lower lake levels also cause reduced hydropower generation downstream—a clean, renewable energy source—with reductions of up to 15 percent by 2050. As water availability declines, there is also the concern of increased national and international tension related to the increased pressure for water use from the Great Lakes. Decisions will have to be made between the United States and Canada as to how to divide the water-use rights.
Higher temperatures will be a critical factor in the highly populated urban areas because of the urban heat island effect. As an example of this, in 1995 a heat wave hit Chicago, causing more than 700 heat-related deaths. In addition, during heat waves, air pollutants become trapped near the Earth's surface, causing unhealthy air quality. With the likelihood of an increase in extreme precipitation, there also will be an increased risk of water-borne diseases and insect-or tick-borne diseases such as West Nile virus or mosquito-borne St. Louis encephalitis.
Recreational activities will shift as cold-season recreation such as skiing, ice skating, snowmobiling, and ice-fishing are reduced and warm-season recreation such as swimming, hiking, and golf are expanded. Higher temperatures, however, will probably impair summer recreational activities, making people more susceptible to heat stroke.
Another key issue for the Midwest is agriculture. This region is a key producer of crops not only for the United States but also for the world. In the past, it has been able to adapt to changes in climate, and scientists at the U.S. Global Change Research Program think that it will continue to adapt in the future. As temperatures warm, the growing season will lengthen, possibly allowing the opportunity to "double crop," the practice of planting a second crop after the first is harvested. The CO2 fertilization effect will enhance plant growth and most likely contribute to higher crop yields in the region. The northern portions are expected to have the largest increases. Additional productivity could be met if plant-breeding programs create new varieties more tolerant to new growing conditions. Years with droughts and flooding will still cause significant decreases in production, however.
There also will be significant changes in the natural ecosystems. Because of higher temperatures and increased evaporation, forested areas will become more susceptible to pests and diseases. Temperate forests are expected to migrate northward into the present-day boreal forest habitat. There will be additional forest growth on the current forest margins due to the increase in CO2. Most of the climate models predict that higher air temperatures will cause greater evaporation, which will reduce soil moisture, making conditions ripe for wildfires. Drier soils will also put greater stress on both deciduous and coniferous trees, making them susceptible to disease and pest infestation, increasing tree mortality rates.
As the water temperatures in lakes increase, changes in freshwater ecosystems will occur. Current coldwater fish species such as trout will not be able to survive. Warmer water species such as bass and catfish will inhabit and thrive in the ecosystem instead. As heavy precipitation events increase, there will be increased runoff of excess nutrients such as nitrogen and phosphorus from fertilizer into the lakes and rivers. This will cause the growth of algae, depleting the water of oxygen, negatively effecting the fish and other life that inhabits the water. Lower lake levels will also have a negative impact on adjoining wetlands.
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