Average annual temperature for Puerto Rico and the U.S. Virgin Islands has increased by more than 1.5°F since 1950. 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 total precipitation are uncertain, but extreme precipitation is projected to increase, with associated increases in the intensity and frequency of flooding.
Sea level has risen by 0.6 inches per decade at San Juan, Puerto Rico since 1961, near the global sea level rise rate during the second half of the 20th century. Global sea level rise projections range from 1 to 8 feet by 2100, with similar rises projected for Puerto Rico and the U.S. Virgin Islands. Rising sea levels pose widespread and continuing threats to both natural and built environments in coastal communities.
Hurricanes are a major threat to both Puerto Rico and the U.S. Virgin Islands. Hurricane rainfall rates, storm surge heights due to sea level rise, and the number of the strongest (Category 3, 4, and 5) hurricanes are all projected to increase in a warming climate.
Puerto Rico and the U.S. Virgin Islands are located in the subtropical Caribbean region. Puerto Rico comprises the main island and several smaller nearby islands, including Vieques, Culebra, Isla de Mona, Caja de Muertos, and Isla Desecheo. The major islands of the U.S. Virgin Islands are St. Thomas, St. Croix, and St. John. Surrounded by ocean on all sides, the islands experience warm and humid tropical conditions with minimal temperature variations between seasons. The temperate climate of San Juan, Puerto Rico, illustrates the tropical conditions of these islands. The annual average temperature is 81.0°F. Temperatures are generally cooler in January, with an average minimum temperature of 72.0°F and an average maximum temperature of 83.2°F, and warmer in August, with an average minimum temperature of 78.2°F and an average maximum temperature of 89.2°F.
Precipitation across Puerto Rico and the U.S. Virgin Islands varies seasonally, with wetter summers and relatively drier winters. The predominant synoptic influence on the islands’ climate is the North Atlantic subtropical high, which causes prevailing trade winds predominantly from the east and northeast. The east to west positioning of the Cordillera Central (the highest elevation is 4,390 feet) provide a natural divide that separates Puerto Rico into two climatologically distinct regions. The northern two-thirds of Puerto Rico has a mostly humid climate, and the southern portion has a drier, semiarid climate. The northeast trade winds provide water vapor for precipitation along the northern coast and outlying islands, but passage over this mountain range removes moisture through precipitation, and the resulting downslope flow is drier. The U.S. Virgin Islands are not as high in elevation (the highest point is 1,555 feet on St. Thomas), but topographic effects on precipitation still occur, with the west (upwind) sides being wetter than the east (downwind) sides.
The islands warmed throughout the 20th and early 21st centuries. For example, temperatures in San Juan have increased by more than 1.5°F since 1950 (Figure 1). The number of very hot days (days with a maximum temperature exceeding 95°F) does not exhibit any trend (Figure 2). By contrast, extremely warm nights (days with a minimum temperature exceeding 80°F) have generally been above average since 2000, with the highest number occurring since 2015 (Figure 3). The urban heat island effect has caused temperatures to rise faster in San Juan than across the rest of the islands. The surface temperatures of the surrounding ocean area provide an essential regulator on temperatures in Puerto Rico and the U.S. Virgin Islands and have risen by approximately 3°F since 1910 (Figure 4).
Annual precipitation for Puerto Rico ranges from 172 inches near El Yunque National Forest (located in the Sierra de Luquillo mountains) in the eastern part of the island to less than 30 inches at Ponce in the southwest. Annual rainfall in the U.S. Virgin Islands is less than 50 inches across all the islands. Much of the rainfall in the wet season (May through October) derives from tropical cyclones (hurricanes and tropical storms) and easterly waves, which move from east to west, while high sea surface temperatures can also trigger local thunderstorm activity. In the dry season (November through April), rainfall is caused by cold fronts moving from west to east. Annual precipitation in Puerto Rico varies from year to year. Precipitation totals have been near to above-average since 1955, with only two periods experiencing much below average conditions (Figure 5).
Heavy precipitation across the region has increased since the 1950s. The largest number of extreme precipitation events (days with precipitation greater than 3 inches; Figure 6) has occurred in recent years. In 2011, Puerto Rico and the U.S. Virgin Islands experienced an extended period of heavy rainfall and flooding (not associated with a tropical cyclone) during mid-May through mid-June, in which several locations recorded 2-4 times their normal rainfall during this period. Significant flooding occurred across Puerto Rico, with the exception of the southwest region of the island. Major flooding was also reported in Saint Croix, U.S. Virgin Islands, where weekly rainfall totals were on the order of 6 to 7 inches.
Tropical cyclone events (hurricanes, tropical storms, and tropical depressions) are an important concern for the islands due to their position in the Caribbean hurricane belt. While such weather systems occur near the islands only about once every 2 years (Figure 7), they can have devastating impacts. Tropical cyclones affecting Puerto Rico and the U.S. Virgin Islands in the 21st century include, but are not limited to, Tropical Storm Jeanne in 2004, Hurricane Irene (Category 1) in 2011, and Hurricane Irma (Category 5) in 2017. Hurricane Irene produced very heavy rainfall that resulted in major flooding in northeast Puerto Rico. In the late 20th century, Hurricane Hugo (Category 4) in 1989, Hurricane Marilyn (Category 2) in 1995, and Hurricane Georges (Category 3) in 1998 all caused catastrophic damage to the islands. Earlier hurricanes of note include the San Felipe Segundo storm (Category 5) of 1928 and the San Ciprian storm (Category 4) of 1932. The two most devastating hurricanes occurred in 1899 and 2017. The 1899 San Ciriaco hurricane (Category 4), the most severe natural disaster recorded in the islands’ history, resulted in 3,300 deaths. In addition, a quarter of residents were left homeless, and more than $7 million in coffee production was destroyed (estimated at $225 million in 2012 dollars). Hurricane Maria (Category 4) made landfall in Puerto Rico in September 2017, causing devastating destruction across the islands. Winds that locally reached Category 5 intensity, combined with extremely heavy rainfall and the destructive power of wave action and storm surge, led to extensive damage to buildings and infrastructure. Severe flooding and mudslides affected much of Puerto Rico and the U.S. Virgin Islands, and most residents lost power for months during the largest power outage in American history. Maria is the third costliest hurricane in U.S. history, with total damages across the islands estimated at $90 billion (2017 dollars).
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, annual average 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 warm nights are projected to accompany the overall warming.
A decrease in average annual precipitation is projected over the 21st century (Figure 8). Model projections indicate a decrease in precipitation averaging around 10%, however, there is significant uncertainty in the magnitude of precipitation changes in the Southern Caribbean region. Puerto Rico and the U.S. Virgin Islands may face an increased risk of drought, potentially affecting water supplies, agriculture, and the economy.
Although overall precipitation is projected to decrease, extreme precipitation events are projected to increase due to increased water vapor available in response to climate-change-related warming of ocean temperatures. While there is uncertainty in whether there will be a fewer or greater number of total hurricanes in the future, hurricane rainfall rates, storm surge heights due to sea level rise, and the number of major (Category 3, 4, and 5) hurricanes are all projected to increase.
Since 1962, the sea level at San Juan has risen by 0.6 inches per decade (close to the global sea level rise rate of 0.7 inches per decade). Sea level rise is an important concern in Puerto Rico due to its extensive coastline. Approximately 60% of Puerto Rico’s population lives within the islands’ 44 coastal cities, and these areas are also home to a significant share of the islands’ critical coastal infrastructure. Sea level rise has contributed to significant coastal erosion, resulting in the retreat of the Puerto Rican coastline of up to 3.3 feet per year (1.0 m/year). Even marginal amounts of sea level rise increase the likelihood of less common flooding events by amplifying tidal and storm surge. Most of the U.S. Virgin Islands are well above sea level; however, waterfront property in the capital, Charlotte Amalie, is generally within a few feet of sea level.
Global sea level is projected to rise between 1.0 and 8.2 feet by 2100 as a result of both past and future emissions from human activities (Figure 9), and similar rises are likely for Puerto Rico and the U.S. Virgin Islands. Some island-level estimates for Puerto Rico project a rise of up to 2.1 feet by 2060 and 4.9 feet by 2100 (Figure 10). Rising sea levels will likely result in increased coastal flooding, coastal erosion, and disruptions to coastal ecosystems and critical infrastructures.