Adapted by Melanie Lenart, University of Arizona, and reviewed by Susan E. Moore and Mark A. Megalos, North Carolina State University, from: Carter, L. M., J. W. Jones, L. Berry, V. Burkett, J. F. Murley, J. Obeysekera, P. J. Schramm, and D. Wear, 2014: Ch. 17: Southeast and the Caribbean. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 396-417. Accessible online at: http://nca2014.globalchange.gov/report/regions/southeast.
Sea level rise is one of the biggest climate challenges for the Southeast (Figure 1). The National Climate Assessment reports an 8-inch rise in global sea level during the last century, a change that can affect coastal forests. Higher sea level can convert coastal softwood forests into mangroves, and lead to salt-water intrusion that can make it challenging for any vegetation to survive. Southeastern forests could face more intense wildfires and hurricanes in years to come, as temperatures heat up on both land and sea.
Figure 1. Relative risk that physical changes will occur as sea level rises. The Coastal Vulnerability Index used here is calculated based on tidal range, wave height, coastal slope, shoreline change, landform and processes, and historical rate of relative sea level rise. (Data from Hammar-Klose and Thieler 2001).
The Southeast has not seen as much warming in recent years as most other parts of the country (Figure 2). The 1930s and 1940s registered a warm peak, but a period of cooler weather occurred during the 1960s and 1970s. Since 1970, temperatures have risen about 2 degrees Fahrenheit, putting them back on par with the earlier warm peak.
Figure 2. Average temperature changes by decade for 1901-2012 (relative to the 1901-1960 average) for the Southeast. The far right bar in each graph (2000s decade) includes 2011 and 2012. (Figure source: NOAA NCDC/CICS-NC, at http://nca2014.globalchange.gov/report/our-changing-climate/recent-us-temperature-trends#tab2-images]).
While projections for future temperature rise are lower than other regions, for parts of the Southeast , the region overall is projected to heat up by another 4 to 8 degrees Fahrenheit by the end of the century, with interior states facing larger increases than coastal areas. Buffered by the ocean, Puerto Rico is projected to face a temperature rise of about 2 to 5 degrees Fahrenheit.
The number of nights when temperatures drop below freezing each year is expected to decrease throughout the region, with greater differences in northern parts of the region than southern parts. Kentucky and parts of Arkansas and Virginia are projected to have 20 or more additional freeze-free night by mid-century compared to the 20th Century, with the change becoming progressively less dramatic in southerly sweeps across the region (Figure 3). This can affect trees, especially fruit trees and shrubs that require a certain number of “chill hours” during the cool season to produce adequately.
Figure 3. The projected average number of days per year with freezing temperatures (32 degrees Fahrenheit or less) for 2041-2070 compared to 1971-2000, assuming emissions continue to grow (A2 scenario). Patterns are similar, but less pronounced, assuming a reduced emissions scenario (B1). (Figure source: NOAA NCDC / CICS-NC).
Precipitation, meanwhile, has generally increased in recent decades (Figure 4). In particular, heavy events in both daily and five-day measures have increased. This trend is expected to continue, with precipitation overall is expected to be more extreme in coming decades. Projections also call for hurricanes to be stronger in force overall, although there may be fewer of them. Warmer sea surface temperatures can intensify hurricanes, although other factors are expected will help limit this intensity.
Figure 4. Average precipitation differences by decade for 1901-2012 (relative to the 1901-1960 average) for the Southeast. The far right bar in each graph is for 2001-2012. (Figure Source: NOAA NCDC/CICS-NC, at http://nca2014.globalchange.gov/report/our-changing-climate/precipitation-change#tab2-images)
Southeastern summers have become more variable in recent years, often registering as extremely wet or increasingly dry. The dry years can be particularly problematic when paired with higher temperatures, as hotter air more quickly dries out vegetation and soil, leaving forests susceptible to burning.
Both wildfires and hurricanes can wreak large-scale damage on forests. Many broadleaf trees have the ability to resprout after breaking, a mechanism that allows them to recover more quickly from hurricanes than pine forests. Pines, however, tend to be more adapted to fire than to wind throw.
The Southeast has nearly 40 percent of its land covered in forests, many of them pine plantations. While this indicates a potentially large effect if wildfires increase dramatically, it also offer the potential that the region could benefit economically from agreements to reduce carbon dioxide levels in the air. Roughly half the dry weight of a tree is carbon, so the growth of a carbon market could lead to support for plantations and family forests. The use of wood products could be promoted as part of a carbon market, as steel and concrete take far more energy to produce than wood.
For more insights on how climate change is affecting and is projected to affect the Southeast, see http://nca2014.globalchange.gov/node/5760.