Average temperature for South Carolina has increased about 0.5°F since the early 20th century, less than half that of the United States as a whole. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century, including increases in extreme heat events.
Future changes in precipitation are uncertain, but extreme precipitation is projected to increase. In addition, projected increases in temperature will likely increase the intensity of naturally occurring droughts.
Sea level has risen by 1.3 inches per decade at Charleston since reliable record keeping began in 1921, nearly double the global sea level rise. Global sea level is projected to rise another 1 to 4 feet by 2100, with greater rises for South Carolina. Rising sea levels pose widespread and continuing threats to both natural and built environments in coastal South Carolina.
South Carolina’s geographic position at subtropical latitudes and along the southeastern coast of the North American continent adjacent to the Atlantic Ocean gives it a humid climate with hot summers and mild winters. The Appalachian Mountains to the north and west tend to partially shield the state from cold air masses approaching from the northwest, making for somewhat milder winters than locations to the west of the mountains. However, these mountains are not high enough to fully block these air masses and occasional periods of very cold conditions do occur. Clockwise circulation of air around a semi-permanent high pressure system in the North Atlantic Ocean (the “Bermuda High”) provides a persistent flow of warm, moist from the Atlantic during the warmer half of the year. The annual average temperature varies across the state from the mid-50s (°F) in the mountains to the mid-60s (°F) along the coast. During January, average temperatures range from 40°F in the north to around 47°F in the Lowcountry. Similar northwest to southeast temperature gradients also occur in the summer, with average temperatures in July ranging from 76°F in the north, to 82°F in the Midlands and coastal Lowcountry.
Temperatures in South Carolina have increased about 0.5°F since the beginning of the 20th century (Figure 1). The state warmed during the early part of the 20th century and then cooled substantially during the middle of the 20th century. Since then, warming has occurred but only recently have temperatures reached the levels of the 1930s. The number of extremely hot days (maximum temperature above 100°F) has fluctuated near the statewide average of 3 days per year since the 1990s (Figure 2a), following very high numbers in the 1930s and early 1950s and a period of below average numbers from the late 1950s to the late 1970s. Over the past 30 years (1984–2014), residents in the state capital of Columbia have experienced an average of 3.5 days per year with high temperatures exceeding 100°F, compared to a yearly average of 2.3 days between 1953 and 1983. In the very hot summers of 2015 and 2016, the number of days exceeding 100°F was 10 and 16, respectively, well above average. Very warm nights (minimum temperature above 75°F) have generally been above average since the early 1980s, with the highest 5-year average occurring during the most recent 2010–2014 period (Figure 3). The number of days below freezing (maximum temperature below 32°F) has been below average since the early 1990s (Figure 2b).
Average annual precipitation ranges from 80 inches near Lake Jocassee in the mountains of the far northwest to less than 39 inches at Wateree in the Midlands, the driest part of the state. While isolated mountain areas receive large precipitation amounts, average precipitation for most of the Upstate is 45 to 55 inches. Average precipitation in the rest of the state is mostly 45 to 50 inches in the Midlands and 45 to 55 inches in the coastal Lowcountry. The high amounts of precipitation in the far northwest are due to moist air being forced up the mountains to higher elevations, while the slightly higher amounts along the Coastal Plain are due to sea-breeze front thunderstorms that occur during the summer months.
Annual precipitation has been below average during most years in the 2000s (12 of 16 years during 2000–2015). However, there is no overall trend since the beginning of the 20th century (Figure 2c) and a few years (notably 2003, 2013, and 2015) have been very wet. Since the start of the 21st century, the state has experienced a below normal number of extreme precipitation events (precipitation greater than 3 inches; Figure 2d). Of the last 15 years in South Carolina, 12 have been characterized by warm season drought conditions.
Some of the major storm threats occurring episodically are hurricanes in the summer and fall and severe thunderstorms capable of producing tornadoes in the late winter and spring. The state ranks 23rd in the Nation for annual tornado frequency, with an average of about 24 tornadoes confirmed each year between 2000 and 2014 (compared to the number one state, Texas, with an estimated 127 tornadoes each year). Strong and destructive tornadoes occur two to four times each year. Over the last decade, the state has experienced numerous billion-dollar disaster events involving severe storms, tornado outbreaks, hurricanes, and droughts. One notable event was the 2008 March tornado outbreak consisting of multiple category EF2–EF3 tornadoes occurring in the Columbia area, resulting in more than $45 million in damages.
In early October 2015, torrential rainfall caused catastrophic flooding. The rainfall relieved the moderate to severe drought conditions of summer 2015 and shattered numerous 24-hour and 5-day total rainfall records. The state’s standing 24-hour rainfall record of 14.8 inches set during Hurricane Floyd in Myrtle Beach (September 16, 1999) was broken at White Birch Circle in Columbia, which recorded a 24-hour total of 17.7 inches on October 4th, with 15.1 inches falling in less than 10 hours in the morning hours. The previous record wettest 5-day period of 17.4 inches (Greenville, August 22–26, 1908) was broken at more than 40 locations, with the largest amounts being recorded at Mount Pleasant (Park West) with 27.2 inches (October 2–6); Charleston (Clark Sound) with 23.8 inches (October 1–5); Georgetown County Airport with 23.5 inches (October 1–5); Charleston (James Island) with 22.0 inches (October 2–6); and Folly Beach with 21.5 inches (October 2–6).
Tropical cyclones and hurricanes occur less frequently than severe thunderstorms and tornadoes, but they have more destructive potential. Hurricane Hugo in 1989 was one of the strongest hurricanes (Category 4) in the state’s history, and at that time was one of the costliest Atlantic hurricanes (ranked ninth) ever. Wind speeds up to 120 mph were observed 200 miles inland, and the storm tide reached 20 feet, resulting in extensive damage including severe residential and commercial flooding, loss of life, and power outages, and other essential infrastructure loss. Damages were estimated at $8–$10 billion. Between 1901 and 2009, a total of 27 tropical cyclones made landfall along the coast of South Carolina. Although hurricanes are rare events in coastal South Carolina, Hurricane Hugo demonstrated the state’s vulnerability to hurricanes. Climate models project an increase in the number of the strongest (Category 4 and 5) hurricanes, accompanied by 20% more rainfall, by the end of the 21st century.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual 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.Increases in the number of extremely hot days and decreases in the number of extremely cold days are projected to accompany the overall warming.
Little change in average annual precipitation is projected over the 21st century (Figure 5). However, any increases in temperature will cause more rapid loss of soil moisture during dry spells, increasing the intensity of naturally occurring droughts in the future. The resulting decreases in water availability, exacerbated by population growth, will continue to increase competition for water.
Since 1921, the sea level at Charleston has risen by 1.3 inches per decade, nearly double the global sea level rise of 0.7 inches per decade. Sea level rise is an important concern in South Carolina due to its extensive coastline. The coast includes an abundance of salt marshes and estuaries, as well as natural seaports such as Georgetown and Charleston. Today, more than 800 square miles of coastal land near the Charleston area lie less than 4 feet above the high tide line. Sea level rise has caused an increase in tidal floods associated with nuisance-level impacts. Nuisance floods are events in which water levels exceed the local threshold (set by NOAA’s National Weather Service) for minor impacts. These events can damage infrastructure, cause road closures, and overwhelm storm drains. As sea level has risen along the South Carolina coastline, the number of tidal flood days (all days exceeding the nuisance level threshold) has also increased, with the greatest number occurring in 2015 (Figure 6). Even marginal amounts of sea level rise increase the likelihood of less common flooding events by amplifying tide and storm surge. Global sea level is projected to rise another 1 to 4 feet globally by 2100 as a result of both past and future emissions from human activities (Figure 5) and greater rises are likely for South Carolina following historical trends. Some state-level estimates project a rise of 3.9 feet by 2100 as compared to 2012, after accounting for local effects of land subsidence. Projected sea level rise will likely result in increased coastal flooding, coastal erosion, and disruptions to coastal and estuarine ecosystems.