Forested watershed

The largest decreases are expected in Kentucky, Virginia, Tennessee, North Carolina and the northern part of Arkansas. The smallest decreases in cold days are expected along the coastal and southern areas, where these kinds of cold days rarely occur. Similarly, the Southeast region is expected to have more than 20 fewer days with a minimum temperature below 32 degrees by the middle of the century.

The smallest increase of 4 days per year is expected in areas with a currently low number of 95-degree days, including the highest elevation areas along the spine of the Appalachians where historically days above 95-degrees occur fewer than 10 days out of the year. The largest increase in the number of 95-degree days per year (35 days) is expected in south-central Florida, where these kinds of hot days are already common. The western portion of the region is expected to experience the largest number of consecutive 95-degree days, with as many as 16-20 additional days by mid-century.

All climate models agree that temperatures are projected to increase over the 21st century across the Southeast. The spatial variations are projected to be relatively small across the region, with the largest temperature changes occurring in the northwest part of the region (Arkansas, Kentucky, Mississippi, and Tennessee), the smallest variations occurring in southern Florida. Temperature increases will be the greatest in summer. The greatest warming during summer is expected in the northwest portion of the region.

Forest ecosystems throughout the Great Plains are exposed to a range of natural, introduced, and anthropogenic stressors. High-elevation forests and semiarid riparian forests are faced with their own distinct kinds of stressors. Invasive plants, forest pests, diseases, droughts, and floods are expected to become more damaging under climate change, and these factors may interact in unpredictable ways.

Different modeling approaches generally conclude that future climate conditions will increase the risk of wildfire across the Great Plains. Annual fire probability, calculated solely with climate data and physical principles, is projected to increase by 20% to 1200% across the region by the end of the century, with the largest increases occuring in Wyoming and western Montana. Fire probability may decrease in Texas, however.

Water is central to the productivity of the Great Plains. Projected increases in winter and spring precipitation in the Northern Plains may benefit productivity by increasing water availability through soil moisture reserves during the early growing season. The Northern Plains will remain vulnerable to periodic drought because much of the projected increase in precipitation is expected to occur in the cooler months while increasing temperatures will result in additional evapotranspiration.

For an average of seven days per year, maximum temperatures reach more than 100 degrees F in the Southern Plains and about 95 degrees F in the Northern Plains. By the middle of the century, Central Texas north to the Dakotas may experience an increase of 10-20 more hot days per year. East Texas, Wyoming, and western Montana are projected to have 22-30 more hot days by the middle of the century, according to a range of climate scenarios. Similar increases are expected in the number of nights with minimum temperatures higher than 80 degrees F in the south and 60 degrees F in the north.

The freeze-free season is defined as the period of time between the last spring frost (daily minimum temperature below 32 degrees F) and the first fall frost. The length of the annual freeze-free season has been increasing since the 1980s, and all climate models agree that it will continue to increase in the future. The largest increases are projected for the high country of Wyoming and Montana.