Record average annual temperatures in the 1920s and 1930s were followed by large cooling in the middle of the 20th century. Temperatures have warmed about 1.5°F since the 1970s to levels slightly below the 1920s and 1930s. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
There are no robust trends in total annual precipitation and the number of extreme events. Future changes in mean precipitation are uncertain while increases in the frequency and intensity of extreme rainfall are projected.
Sea level along the Alabama coast has risen at the rate of 1.5 inches per decade, faster than the global rate. Projected sea level rise poses widespread and continuing threats to both natural and built environments in coastal Alabama.
Alabama is located at subtropical latitudes on the coast of the Gulf of Mexico and on the southern end of the vast, relatively flat plains of central North America extending from the Arctic Circle to the Gulf of Mexico. The state is therefore exposed to the influences of diverse air masses, including warm, moist air from the Gulf of Mexico and dry continental air masses, which are cold in the winter and warm in the summer. Clockwise circulation of air around a semi-permanent high pressure system in the North Atlantic (the “Bermuda High”) causes a quite persistent southerly flow of air off the Gulf during the warmer half of the year. Thus, relatively mild winters, hot summers, and year-round precipitation characterize Alabama’s climate. In addition to serving as a predominant source of moisture, the Gulf helps moderate temperatures along the coast. This mild climate is an important economic driver for agricultural production and tourism.
Temperatures in Alabama were hottest in the 1920s and 1930s, followed by a substantial cooling of almost 2°F into the 1960s and 1970s (Figure 1). Temperatures have risen since that cool period by about 1.5°F, such that the last two decades have been above the long- term average, slightly cooler than the 1920s and 1930s. Because of the large cooling that occurred in the middle of the 20th century, the southeastern United States is one of the few regions globally that has not experienced overall warming since 1900, while the United States as a whole has warmed by about 1.8°F.
The United States also cooled from the 1930s into the 1960s, but not by nearly as much as Alabama. Potential causes for this difference in warming rates have been the subject of research, but this phenomenon has not been fully explained. Hypothesized explanations include increased cloud cover and precipitation, increased small particles from coal burning, natural factors related to forest re-growth, decreased heat flux due to irrigation, and multi-decade variability in North Atlantic and tropical Pacific sea surface temperatures.
In the summer, daytime high temperatures are typically in the range of 85°F to 95°F, with temperatures regularly exceeding 95°F across the state. In recent decades, the number of very hot days (daytime high temperatures at or above 95°F) has been well below the numbers experienced during the 1930s and early 1950s (Figure 2a). The state has also experienced a below average number of very warm nights (nighttime low temperatures at or above 75°F), with the 2010–2014 period being an exception (Figure 2b). In the winter, average nighttime low temperatures range from 30°F in the northern portion of the state to more than 45°F along the coast. The average annual number of nights below freezing is 51 and 61 for Birmingham and Huntsville, respectively, compared to only 15 for Mobile.
The statewide average annual precipitation is 55.5 inches, distributed rather uniformly through the year except for a relatively dry period occurring from August to October. Variable precipitation patterns in the summer months combined with widespread soils with poor water-holding capacity frequently give rise to short-term drought conditions. Mean annual precipitation is highly variable from year to year (Figure 2c). While there is no net trend throughout the 1895–2018 period, annual precipitation for the last four- year period (2015–2018) was above average. The 2nd driest year on record (2007) and 2nd driest consecutive 3-year period (2006–2008) was followed by the 3rd wettest year (2009). The driest multi-year periods were in the late 1890s and the 1950s, and the wettest in the late 1940s and the 1970s (Figure 2c). The driest 5-year period was 1895–1899 and the wettest was 1971–1975. The annual number of heavy rain events (number of days with precipitation of 3 or more inches) was near the long-term average for 2015-2018 (Figure 3), with no statistically-significant long-term trend.
Tornadoes and hurricanes are two of the deadliest weather hazards in the state. Tornadoes typically occur in the spring and fall, with an estimated 22 tornadoes touching down each year. In 2011, a deadly tornado outbreak swept across the southern, midwestern, and northeastern United States. Alabama was one of the hardest-hit states, suffering millions in property and infrastructure damages and large loss of life, with an estimated 238 tornado-related deaths. Hurricanes and tropical storms can also result in massive property damage. On average (1900–2018), Alabama is directly impacted by a hurricane about once every 6 years; however, there has been no overall long-term trend over the past century (Figure 2d). In 2005, Hurricane Katrina brought hurricane force winds along the Alabama coastline. It caused widespread damage from winds, flooding following a storm tide of 14 to 18 feet, and tornadoes. In 2012, Hurricane Isaac resulted in storm surge (the abnormal rise of water generated by a storm over and above the predicted astronomical tide) of 4.63 feet above normal tide levels in the Mobile Bay area and 3 to 5 feet inundation (the total water level that occurs on normally dry ground as a result of storm tide) levels along the coast of Alabama.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a lower pathway of greenhouse gas concentrations, temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records. Warming is projected despite the lack of a long-term temperature trend in Alabama because the increased warming influence of greenhouse gases will become greater than the natural variations that have dominated Alabama’s temperature climate.
Future changes in average annual precipitation are uncertain (Figure 4). However, any increase in temperature will cause a more rapid rate of loss of soil moisture during dry periods. This will likely increase the intensity of naturally-occurring droughts in the future. Increases in extreme precipitation are projected for Alabama, because it is virtually certain that atmospheric water vapor will increase in a warmer world.
Increasing temperatures raise concerns for sea level rise in coastal areas. Since 1900, global average sea level has risen by about 7–8 inches. Based on observed data from 1966 to 2006, the local sea level at Dauphin Island, Alabama, has increased 1.2 inches per decade, faster than the global rate. Global sea level is projected to rise 1 to 8 feet by 2100, with a likely range of 1 to 4 feet, as a result of both past and future greenhouse gas emissions from human activities (Figure 5). Sea level rise has caused an increase in nuisance flooding—a rise in water level above the minor flooding threshold set locally by NOAA’s National Weather Service. Nuisance flooding has increased in all U.S. coastal areas, with more rapid increases along the East and Gulf Coasts. These floods can damage infrastructure, cause road closures, and overwhelm storm drains. Nuisance flooding events in Alabama are likely to occur more frequently as global and local sea levels continue to rise.
Naturally occurring land subsidence is a major contributor to higher levels of sea level rise in Alabama, with land in the Dauphin Island area projected to subside an additional 6.6 inches by 2100. A recent U.S. Department of Transportation study found that highways and port and marine waterway systems along the low- lying coast of Mobile, as well as coastal wetlands, are particularly vulnerable to storm surge and sea level rise. The percentage of critical ports exposed to sea level rise ranges from 46% under the study’s lowest scenario (1 feet of sea level rise by 2050) to 92% under the highest scenario (6.6 feet of sea level rise by 2100).