Climate Change in the Southeast USA: Executive Summary

This book is based on a technical report the National Climate Assessment (NCA) document that was prepared for submission to the President of the United States and the United States Congress. That document summarized the scientific literature with respect to climate impacts on the Southeast (SE) USA, in particular the literature that has been published since 2004. A national assessment was produced in 2009; however, no technical report was developed in support of that document.

For the Third US National Climate Assessment, the Southeast region includes 11 southern states (Figure 1.1), Puerto Rico, and the US Virgin Islands. This region differs slightly from the previous National Climate Assessment in that it follows state borders and does not include the Gulf Coast of Texas.

1.1 Diversity and Vulnerabilities

The SE USA is characterized by great diversity in terms of climate, natural and managed ecosystems, social and political attitudes, and vulnerabilities. While most of the SE is classified as humid, temperatures vary widely across the regions, with a transition from tropical rainforests in Puerto Rico and the US Virgin Islands to temperate forests in the southern Appalachian Mountains. This climatic diversity, which is described in detail in Chapter 2, results from a range of weather patterns that affect the region, including frontal systems that dominate during fall and winter, convective systems that dominate during the spring and summer, tropical systems that are important during the summer and fall, and sea breeze systems that are important for the coastal regions. In addition, the region is prone to other extreme weather phenomena, including droughts, floods, winter storms, and tornadoes.

The region also is subject to related risks that interact with climate variability and change. For example, sea level change and salt water intrusion already threaten many coastal communities (Chapter 5) and ecosystems (Chapters 9 and 11). Sea level change, which includes both sea level rise and land subsidence in parts of Louisiana, Mississippi, and Alabama, makes the region more vulnerable to storm surges produced by tropical storms or winter storms in the Gulf of Mexico (Mitchum 2011). Increasing atmospheric carbon dioxide concentrations might benefit agricultural (Chapter 7) and forest systems (Chapter 8) of the region through increasing photosynthesis, but benefits are likely to be offset by losses of productivity that would result from increased temperatures. Increasing atmospheric carbon dioxide concentrations are also projected to acidify surface waters, which would likely inhibit the growth of corals, shellfish, and crustaceans (Chapter 9). Finally, increasing atmospheric carbon dioxide increases pollen production by many plant species, which has been linked with increased levels of asthma and respiratory illnesses (Chapters 3and 7).

Climate also interacts with social conditions in the Southeast, which has experienced unprecedented population growth during recent decades. All states in the region had positive growth from 2000 through 2010, with overall population growing by 8.9 million people, or about 13% (Table 1.1). Population grew the most in North Carolina (18.5%), Georgia (18.3%), Florida (17.6%), and South Carolina (15.3%), and most of that population growth has been in urban and peri-urban areas (Mackun and Wilson 2011). In the region, only Puerto Rico and the Virgin Islands had negative growth (- 2.2 and -2 percent respectively) (Mackun and Wilson 2011). States with the fastest growing populations on a percentage basis were mostly states that already had relatively large populations. This trend indicates widening differences in population density among states in the Southeast. Population growth likely will compound climate related risks for most sectors. Increasing competition for water resources (Chapter 10) will likely affect the energy (Chapter 4), agriculture (Chapter 7), fisheries and aquaculture (Chapter 9), natural ecosystems (Chapter 11), and built environment (Chapter 5) sectors.

The diversity of people, natural and managed ecosystems, and resources of the Southeast provide the region with great richness. With coastlines along the Gulf of Mexico and South Atlantic seaboard, the SE has a wealth of estuaries (Chapter 12) with associated fishing industry (Chapter 9), ports with associated transportation hubs (Chapter 6), and beaches with associated tourism (Chapter 13). Inland forests constitute an important carbon sink (Chapter 8), which mitigate greenhouse gas effects on climate (Chapter 12). Its relatively humid, high rainfall environment provides the SE sufficient water resources (Chapter 10) to be a major exporter of energy (Chapter 4) to other regions at present, though future increases in competition for water resources might diminish the region's energy production capacity. Climate change threatens all of these natural resources and the industries that depend on them. Thus, it is not surprising that there are numerous efforts already underway in the SE to mitigate and adapt to climate change (Chapters 12 and 13). In addition there are ongoing programs to educate people about climate variability, climate change, and ways society can manage climate related risks (Chapter 14).

1.2 Time-scales of Interest to Southeast Decision Makers

Decision makers in the SE have the greatest interest in seasonal and decadal timescales (Bartels et al. 2011). Typically, the time-scale of interest matches the time-scale of investments and expenditures, most of which are 20 years or less. In order to engage decision makers in the use of climate information, it is important to provide information at time-scales that are relevant to the decisions for which they need information. If the science community can provide useful information at these shorter timescales, as decision makers use that information to manage seasonal and near term climate risks, they also begin to adapt to and mitigate climate change (Fraisse et al. 2009).

An advantage to providing climate information at shorter time-scales in the SE is that seasonal climates of the Florida peninsula and coastal plains from Louisiana to North Carolina are affected by sea surface temperatures in the equatorial Pacific, or El Niño-Southern Oscillation phenomenon (Chapter 2). For these areas, El Niño conditions typically result in cool, wet fall and winter conditions whereas La Niña conditions typically result in dry, warm fall and winter conditions. To the north of the coastal plains, seasonal climate does not typically exhibit an El Niño-Southern Oscillation signal.

1.3 Future Scenarios

In this report, we use the term "projection" to describe how future climate is expected to respond to various scenarios of population growth, greenhouse gas emissions, land development patterns, and other factors that might affect climate change. The report uses two of the Intergovernmental Panel on Climate Change (IPCC) scenarios for climate projections, A2 and B1. It is important to recognize that these scenarios describe potential situations for both greenhouse gas (GHG) emissions and societal development alternatives.

The A2 scenario is the most pessimistic in that it assumes that nations will have little or no response to anticipated adverse effects of climate change (EPA 2012). The A2 storyline and scenario describes a heterogeneous world with an underlying theme of self-reliance and preservation of local identities. Birth rates across regions converge slowly, which results in continuously increasing population. This scenario is often called "business as usual."

The B1 storyline and scenario describes a convergent world with the same global population as in the A1 storyline, but with rapid change in economic structures toward a service and information economy (EPA 2012). This scenario includes a reduction in material intensity and the introduction of clean and resource-efficient technologies. It emphasizes global solutions to economic, social, and environmental sustainability, including improved equity, but without additional climate initiatives. The B1 scenario is the most optimistic in that it generally reflects a concerted, global effort to mitigate human impacts that would further warm the planet.

An important feature of these scenarios is the difference in population growth for the region (Figure 1.2). In the A2 scenario the population of the SE nearly triples from 2010 through 2100 whereas in the B1 scenario, population increases about one-third over the same period. If population growth follows trends similar to those simulated in the A2 scenario, the SE will experience far more cross-sectoral competition for land and water resources, which will compound climate change impacts. On the other hand, the B1 scenario with its more modest population growth provides more opportunities for adaptation and mitigation.

Another important factor for the SE will be changes in land use and land cover. As population grows, development is inevitable. How that development proceeds, however, will have great impact on regional climate (Shin and Baigorria 2012).

1.4 Process for Developing this Book

This document has been produced through collaboration among three Regional Integrated Sciences and Assessments Centers (RISAs): the Southeast Climate Consortium; the Carolinas Regional Sciences and Assessments; and the Southern Climate Impacts Planning Program; and with contributions from numerous local, state, federal, and non-governmental individuals and agencies. We established a leadership committee that was charged with the design of the overall report and the organization of a two-day workshop with about 90 participants that was held in Atlanta, GA, in September 2011. From September 2011 through February 2012, more than 100 contributors provided information to lead authors, who drafted the chapters, which were reviewed and revised to the extent possible given the time constraints, and submitted as a draft to the NCA on March 1, 2012. Following this submission the report was fully reviewed, revised, and resubmitted to the NCA on July 23, 2012 (Ingram et al. 2012). Following submission of the final report to the NCA, the authors agreed to publish the report as a book through Island Press, which entailed another several rounds of edits and revisions that led to this book. Despite our best efforts to produce a comprehensive, fully documented assessment for the SE USA, we have likely missed some important documents, for which we apologize. If you are aware of such documents, please contact the NCA (engagement@usgcrp.gov) to learn how to submit documents for future reports.

1.5 Report Organization

Three sections of the report follow this introduction: (1) Climate of the SE, which has one chapter that reviews the historic climate, current climate, and projected future climate of the region; (2) Climate interactions with important sectors of the Southeast, which includes nine chapters loosely organized from most to least anthropocentric; and (3) Cross-sectoral issues, namely climate change mitigation, adaptation, and education and outreach.

Climate of the Southeast USA: Past, Present, and Future

This chapter describes the climate of the Southeast USA, including past, present, and future conditions. In addition, it provides a historical perspective on extreme events and context for assessing future impacts of climate change.

Key Findings

* The Southeast USA experiences a wide range of extreme weather and climate events, including floods, droughts, heat waves, cold outbreaks, winter storms, severe thunderstorms, tornadoes, and tropical cyclones. These events have contributed to more billion-dollar weather disasters in the Southeast (SE) than in any other region of the USA during the past three decades.

* Historical records of precipitation and temperature reveal much interannual and interdecadal variability across the SE, with no long-term trends since the end of the 19th century. However, since the 1970s, temperatures have steadily increased, with the most recent decade (2001 to 2010) noted as the warmest on record. A portion of this warming may be due to increased nighttime temperatures resulting from human development of the earth's surface. Interannual precipitation variability has also increased across the SE region, with more exceptionally wet and dry summers compared to the middle of the 20th century.

* Mean annual precipitation is expected to decrease across the southern tier of the SE, including the Caribbean, and increase across the northern tier through the first half of the 21st century. The greatest changes are expected during the summer months. However, there is much uncertainty in projected precipitation by the end of the 21st century due to the overall lack of model agreement on the sign and magnitude of these changes (except across the Caribbean). There is better model agreement and overall confidence in temperature projections, which indicate an overall increase across the SE through the end of the 21st century. Increases in the length of the growing season, the number of cooling degree days, the number of consecutive hot days, and interannual temperature variability are projected through the end of the 21st century.

* There is much uncertainty regarding future projections of drought, severe thunderstorms tornadoes, and air quality. The frequency of major hurricanes is projected to increase in the Atlantic Basin, while the overall number of tropical cyclones is projected to decrease through the end of the 21st century. Mean relative sea level rise across the SE coast is generally consistent with the global trend and is expected to increase between 20 and 200 cm by the end of the 21st century. The exact rate will depend largely on the rate of ice sheet loss as well as local land motion (e.g., subsidence).

2.1 General Description

The climate of the Southeast (SE) is quite variable and influenced by a number of factors including latitude, topography, and proximity to large bodies of water. The topography of the region is diverse. In the southern and eastern portions of the region, extensive coastal plains stretch from Louisiana eastward to southeastern Virginia, while rolling low plateaus, known as the Piedmont, are present from eastern Alabama to central Virginia. North and west of these areas, mountain ridges are found, including the Ozarks in Arkansas (1,500 to 3,000 ft) and the Appalachians, which stretch from Alabama to Virginia (2,000 to 6,600 ft). Finally, elevated, dissected plateaus lie from northern Alabama to Kentucky. Temperatures generally decrease with increasing latitude and elevation, while precipitation decreases away from the Gulf-Atlantic coasts, although rain is locally greater over portions of the Appalachian Mountains. Overall, the climate of the Southeast is generally mild and pleasant, which makes it a popular region for relocation and tourism.

A semi-permanent high pressure system, known as the Bermuda High, is typically situated off the Atlantic Coast. Depending on its position, the Bermuda High commonly draws moisture northward or westward from the Atlantic and Gulf of Mexico, especially during the warm season. As a result, summers across the SE are characteristically warm and moist with frequent thundershower activity in the afternoon and early evening hours. Day-to-day and week-to-week variations in the positioning of the Bermuda High can have a big influence on precipitation patterns. When the Bermuda High builds to the west over the region, hot and dry weather occurs, although humidity often remains relatively high. This pattern can cause heat waves and poor air quality, both of which negatively affect human health. When the Bermuda High persists over or immediately south of the area for extended periods, drought conditions typically develop. This places stress on water supplies and agricultural crops and can reduce hydroelectric energy production. Variations in the positioning of the Bermuda High also affect how hurricanes move across the region.

During cooler months of the year, the Bermuda High shifts southeastward as the jet stream expands southward. Accompanying the jet stream are extratropical cyclones and fronts that cause much day-to-day variability in the weather. As the jet stream dives southward, continental air can overspread the SE behind extratropical cyclones, leading to cold-air outbreaks. Sometimes subfreezing air reaches as far south as central Florida, causing major damage to citrus crops. Extratropical cyclones also draw warm and humid air from the Atlantic Ocean and Gulf of Mexico northward over frontal boundaries, and this can lead to potentially dangerous snowstorms or ice storms. These winter storms are generally confined to the northern tier of the region (35°N latitude and greater) where temperatures are cold enough to support frozen precipitation. In the spring, the sharp contrast in temperature and humidity in the vicinity of the jet stream can promote the development of severe thunderstorms that produce damaging winds, large hail, and tornadoes.