Louisiana has exhibited little overall warming in surface temperatures over the 20th century. However, under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Hurricanes strike Louisiana an average of once every three years. As the climate continues to warm, hurricane- associated rainfall rates are projected to increase. The resulting flooding caused by these storms is of particular concern for the state.
A large portion of Louisiana’s coastline is extremely vulnerable to sea level rise due to coastal subsidence and low elevation in the southern portion of the state. Projected sea level rise poses widespread and continuing threats to coastal communities.
Louisiana is located on the coast of the Gulf of Mexico and on the southern end of the vast, relatively flat plains of central North America, which extend from the Arctic Circle to the Gulf of Mexico. The state is therefore exposed to the influences of diverse air masses, including the warm, moist air over the Gulf of Mexico as well as dry continental air masses, which are cold in the winter and warm in the summer. Relatively short, mild winters, hot summers, and year-round precipitation characterize Louisiana’s climate. The Gulf of Mexico helps to moderate the climate in the southern portion of the state, while temperatures and precipitation are more variable in the north.
Louisiana has exhibited little overall warming in surface temperatures over the 20th century (Figure 1). Temperatures were hottest in the early 20th century, followed by a substantial cooling of almost 2°F from the 1950s to the 1970s. Temperatures have risen since that cool period by about 2°F, such that the most recent one to two decades have been about as warm as early 20th century levels, with 14 of the 19 years since 2000 being above average. 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 whole as warmed by about 1.8°F. The United States as a whole also cooled from the 1930s into the 1960s, but not by nearly as much as Louisiana. Potential causes for this difference in warming rates have been the subject of research, but this phenomenon has not been fully explained. In Louisiana, the number of very hot days (maximum temperature at or above 95°F) was above the long-term average in the early 20th century. In the 1960s and 1970s, there was a period of a much below average number of occurrences of very hot days. In recent years, the number of very hot days has begun to increase, approaching the long-term average (Figure 2a). By contrast, the number of very warm nights (minimum temperature at or above 75°F) has risen substantially (Figure 3) to record high levels in the most recent 9-year period of 2010-2018. The number of days below freezing (days with maximum temperature at or below 32°F) has also displayed large variability over the past century with no long-term trend (Figure 2b). Historical observed extreme temperatures for the state range from 114°F at Plain Dealing (August 10, 1936), to −16°F at Minden (February 13, 1899).
The state receives abundant precipitation throughout the year. Annual precipitation ranges from around 50 inches in the north to around 70 inches at some locations in the southeast. Statewide annual precipitation has ranged from a low of 36.6 inches in 1924 to a high of 79.5 inches in 1991. The driest multi-year periods are the early 1900’s, late 1930s, and early 1950s, and the wettest multi-year periods were the late 1940s and 1990s and since 2012 (Figure 4). Annual precipitation has ranged from about 50 inches during the driest 5-year period on record (1914–1918) to about 65 inches during the wettest 5-year period on record (1989–1993). Precipitation has generally been above average since 1970. While 2010 and 2011 were very dry, six of the seven years since then have been above average. Springs have generally been wet during that same period, while fall precipitation has been highly variable, with very wet falls in 2015 and 2018 interspersed by very dry 2016 and 2017 falls (Figures 2c and 2d). There is no overall trend in the number of extreme precipitation events, but a record high number occurred in 2016 (Figure 2e). Louisiana averages more than 60 days with thunderstorms annually, the 2nd highest state after Florida. Severe thunderstorms occur most frequently in the late winter and early spring months, and these storms can produce tornadoes. Snowfall is rare near the Gulf of Mexico but can occur occasionally in the north when polar air masses enter the state.
A direct hurricane strike on the coast occurs about once every three years and the Louisiana coast is particularly vulnerable to severe flooding from these storms (Figure 5). In 2005, Hurricane Katrina (a Category 3 storm at landfall) caused massive damage from heavy precipitation and storm surge flooding in the eastern part of the state. New Orleans was particularly hard hit, with more than 80% of the city flooded and some areas under as much as 15 feet of water. Hurricane Katrina caused more than 1,500 fatalities in the state and immense amounts of property damage. A month later, Hurricane Rita (a Category 3 storm at landfall) struck the southwestern portion of Louisiana. Rita produced rains of 5 to 9 inches and caused a 15-foot storm surge along the southwestern coast. In New Orleans, the storm caused an 8-foot storm surge, which breeched provisionally repaired levees and caused additional flooding in the area still recovering from the damage caused by Hurricane Katrina. Storm surges along the central and western coast of Louisiana do not show much variability, 10-foot surges having return periods ranging from 25 to 50 years near Cameron Parish (Figure 6). Along the southeastern coast, variability is much higher due to the orientation of the coast relative to the general path of tropical cyclones through the central Gulf of Mexico. The geography of the coast in the southeast causes a funneling effect of water, leading to higher observed surge heights (Figure 6).
Flooding is also a hazard for regions along the Mississippi River. In 2011, the lower Mississippi Valley experienced one of the worst floods along the Mississippi River in Louisiana history. In early May, to control the massive flooding, the Morganza Spillway was opened for only the second time in history (the first time was in 1973) to protect levees and prevent flooding downstream in Baton Rouge and New Orleans. Interestingly, despite the massive floodwaters, southern Louisiana was experiencing an extreme drought at the time. Both the flood and drought were tied to La Niña conditions in the equatorial Pacific Ocean, which caused storm tracks to shift to the north across the Ohio River Valley. This shift caused the storms to bypass Louisiana, producing drought conditions, but also caused excessive rainfall in the Midwest, producing the flood wave, which moved downstream along the Mississippi River into the drought-stricken area. Most recently, in August of 2016, an historic flooding event affected the state, with 20 to 30 inches of rainfall occurring over several days and exceeding 1 in 500-year amounts in some locations. More than 30,000 people were rescued from floodwaters that damaged or destroyed over 50,000 homes, 100,000 vehicles and 20,000 businesses. With damages estimated at $10 billion dollars, this was one of the most damaging U.S. flood events in recent history.
Despite the lack of a distinct dry season, Louisiana is quite vulnerable to drought. Since the creation of the United States Drought Monitor Map in 2000, Louisiana has only been completely drought-free for approximately 50% of the time (2000–2018) and has had at least 50% or more drought coverage for approximately 18% of the time during that same period.
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. Since the 1970s, Louisiana temperatures have generally been within the range, but on the low end, of model-simulated temperatures. Continuation of the post-1980 warming trend would lead to an approximate additional warming of 1°F by 2050 and 2–3°F by 2100. In this case, the future warming would be on the low end of the model-simulated increases. However, under a high emissions scenario the future rate of warming is projected to increase, potentially leading to considerably larger temperature increases. Any overall warming will lead to increasing heat wave intensity but decreasing cold wave intensity.
Summer precipitation is projected to decrease, although the projected decreases are smaller than natural variations (Figure 7). Even if average precipitation remains the same, higher temperatures will increase the rate of loss of soil moisture during dry periods, which could increase the intensity of naturally-occurring droughts.
Increasing global temperatures raise concerns for sea level rise in coastal areas. Since 1900, global average sea level has risen by about 7–8 inches. It is projected to rise 1 to 8 feet, with a likely range of 1 to 4 feet, by 2100 as a result of both past and future emissions from human activities (Figure 8). 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. Nuisance flooding has increased in all U.S. coastal areas, with more rapid increases along the East and Gulf Coasts. Nuisance flooding events in Louisiana are likely to occur more frequently as global and local sea levels continue to rise. Louisiana is at extreme risk for sea level rise due to its low elevation, which averages only 3 feet above sea level in the southeastern part of the state. Additionally, the coastline is rapidly sinking due to subsidence (settling of the soil over time), which has already caused the state to lose almost 2,000 square miles of land since the 1930s. Due to subsidence, sea level rise at some locations is more than 4 times the global rate. The New Orleans metropolitan area, the most populous in the state, is at particular risk for sea level rise impacts. Sea level rise will present major challenges to Louisiana’s existing coastal water management system and could cause extensive economic damage through ecosystem damage and losses in property, tourism, and agriculture.