A dense dark green forest canopy in the foreground overwhelms the purple and orange sunset peering over the horizon.

Natural Systems

The unique freshwater, marine, and coastal ecosystems throughout the U.S. Caribbean support the region's diverse economy. The effects of climate change on the island are placing stress on these vulnerable ecosystems and putting human communities at risk.

The effects of climate change on Caribbean islands—increases in streambank and coastal erosion, contaminated flood water, and debris transport—coupled with human activities such as agriculture and development are likely to cause shifts in the freshwater, coastal, and marine ecosystems surrounding the islands. Changes in the frequency and intensity of storms can alter physical and chemical properties of the ridge-to-reef connection from island watersheds to ocean waters. As sea level rises, coastal habitat may shift or be lost as plant and animal communities lack unoccupied areas into which they can migrate. These shifts may reduce the ability of coastal and marine ecosystems to provide protection from coastal storms and food from fisheries.

Observed trends and climate model projections show that rapid climate change is underway and expected to continue. The results could have unprecedented impacts on terrestrial and marine communities across the region and lead to negative socioeconomic consequences for human communities. This is of particular concern for small islands with rapidly growing populations that live and work along the coast where they may be vulnerable to sea level rise, intense rainfall events, and outbreaks of marine disease.

Observed and projected temperature changes are shown as compared to the 1951–1980 average. Observed data are for 1950–2017, and the range of model simulations for the historical period is for 1950–2005. The range of projected temperature changes from global climate models is shown for 2006–2100 under a lower (RCP4.5) and a higher (RCP8.5) scenario (see the Scenario Products section of App. 3). Projections from two regional climate models are shown for 2036–2065, and they align with those from global models for the same period. 

Warmer-than-usual conditions, sometimes referred to as heat waves, are now more frequent, widespread, and intense, affecting freshwater, coastal and marine ecosystems, and human populations. In Puerto Rico, coastal and marine development and agricultural practices have negative impacts on coastal and marine ecosystems, and these are likely to be exacerbated by climate change. Similar effects are likely for USVI because of coastal and marine development; however unlike Puerto Rico, agriculture is not common in USVI.

Double trouble: Hurricanes Irma and Maria

Aerial view of Hurricane Irma hovering over the U.S. Caribbean headed northwest toward Florida.

Satellite aerial view of Hurricane Irma hovering above the U.S. Caribbean on September 8, 2017. This storm, along with Hurricane Maria, which hit the Caribbean islands later in the month, brought intense winds, heavy rainfall, storm surge, and riverine flooding, significantly damaging vegetation and infrastructure throughout the islands and permanently changing the landscape.

In 2017, when Hurricanes Irma and Maria, categories 5 and 4, respectively, arrived in the U.S. Caribbean, they significantly changed the landscape. The storms’ intense winds, heavy rainfall, storm surge, and riverine flooding damaged vegetation and infrastructure throughout the islands. Hurricane Maria was the strongest storm to hit Puerto Rico since 1992, bringing maximum sustained winds of 135 knots, 38 inches of rain, flooding up to 5 feet above ground level, and 9 feet of storm surge. The storms also led to high levels of damaged coral around the islands where the strongest waves occurred. Severe damage to nearshore coral and seagrass habitats also occurred due, in part, to the debris and contaminants generated by the storm and transported to nearshore waters.

Hurricane Impacts in 2017

This interactive map contained in the Fourth National Climate Assessment shows the tracks of Hurricane Irma (Category 5) and Maria (Categories 4 and 5) which hit the islands in September 2017. As you follow the storm's path, you can explore data points about the hurricane's and learn about impacts felt throughout the region.
Explore the Hurricane Impacts in 2017 interactive map here

Ocean Chemistry

Human-driven increases in atmospheric carbon dioxide is the causal agent for global warming. Another consequence of having more carbon dioxide in the air is more carbon dioxide in the ocean, which leads to ocean acidification. 

The process of ocean acidification—when the chemistry of ocean waters changes from the basic portion of the pH scale toward the acidic end—decreases the availability of carbonate ions in seawater. Carbonate ions are the principal building block of most marine skeletal material, and the decrease in their availability adversely affects the process of calcification, with potentially detrimental consequences for marine life. 

Blue line graph that shows the monthly mean of CO2 in seawater from 2009 to 2022

Another consequence of having more carbon dioxide in the air is more carbon dioxide in the ocean, which leads to ocean acidification. This graph shows trends in seawater CO2 concentration at La Parguera, PR from 2009-present. 

Reduced availability of carbonate ions leads to slow growth rates and malformed and less dense carbonate shells and skeletons. 1 As a result, species that undergo calcification may become displaced by species that do not. For example, ocean acidification could affect the food web dynamics at lower trophic levels and have physiological effects at larval stages, and these would likely cascade upwards through the food web impacting coral and fish populations. A decline in calcifying species (eg. corals, crustose coralline algae, and calcareous green algae) will also affect the sand supply to island shores, potentially leading to decreases in beach and reef-based tourism and economic activities.

In addition to interfering with calcification in living organisms, ocean acidification is likely to increase erosion and dissolution of carbonate structures, shells, and sediments. 2, 3, 4 Any decline in calcification or increase in net dissolution could compromise coral reef ecosystems. 

Ecosystem Impacts

A large patch of brown sargassum floating in emerald blue water with green hills and a blue sky in the background.

Sargassum spp. is a form of microalgae that negatively impacts coastal and marine ecosystems. The brown tides cause reductions in light, oxygen, and pH, as well as high influxes of nitrogen and phosphorus leading to eutrophication. 

A recent threat to coastal resources is the accumulation of the brown macroalgae Sargassum spp. Sargassum brown tides—massive increases in algal biomass that occur when algae have ample access to essential nutrients and sunlight—are thought to be related to weather fluctuations, nutrient inputs from the Amazon River, changes in ocean currents, and increased iron fertilization from airborne dust that settles in the ocean. 5, 6 The first widespread accumulation or major bloom of Sargassum spp. in the Caribbean was reported in 2011. The next major event occurred in 2015, and the area of brown water was four times larger than in 2011. 7

The effects of Sargassum blooms can cascade through shoreline communities and estuarine wetlands, as well as shallow reef and seagrass areas. The brown tides cause reductions in light, oxygen, and pH, as well as high influxes of nitrogen and phosphorus leading to eutrophication. 8 This led to impacts on the trophic dynamics of the long-spined sea urchin, Diadema antillarum, a keystone herbivore in the coral reef ecosystem; 9 replacement of turtle grass meadows with algal communities; and total or partial mortality of nearshore corals. 8

Climate Change Effects in Freshwater, Coastal, and Marine Systems 

Global climate change predictions suggest a generalized change in precipitation patterns around the world. Projections of future rainfall for the Caribbean suggest a 25-50% reduction in mean annual rainfall. 10 As rainfall patterns change, droughts could be more frequent and severe. Droughts increase the amount of carbon in the soil, which can be drawn into streams and cause a decrease in pH. In Puerto Rico, an experimental addition of CO2 has shown a reduction in stream pH and a negative effect on macroinvertebrate abundance 11 with potential negative consequences for aquatic ecosystems.

This graph indicated the historical drought conditions for Puerto Rico from 2000 to the present day.

The U.S. Drought Monitor synthesizes the best available drought data to indicate the areas of the United States are in drought. This graph indicated the drought conditions in Puerto Rico from the year 2000 to the present (2022). The longest duration of drought (D1–D4) in Puerto Rico lasted 80 weeks beginning on May 5, 2015, and ending on November 8, 2016.

Global temperature is expected to increase by 1° to 3°C in the next 100 years, increasing the amplitude and frequency of summer heat waves. In the Caribbean, sea surface temperature (SST) has been increasing over the past 30 years where the Atlantic Warm Pool (a large body of warm water that comprises the Gulf of Mexico, the Caribbean Sea, and the western tropical North Atlantic) has increased in size, reaching Puerto Rican waters. 12 Ocean temperature is increasing rapidly, both in the sea surface and the deeper layers of the ocean. 13 Likewise, the pH of the ocean is now 0.1 lower (more acidic) due to the increased CO2 concentration in the atmosphere and the subsequent increase of the amount of CO2 absorbed by the ocean. 14 Feedbacks between these physical and chemical factors could have severe consequences for estuarine biodiversity. Intertidal wetland fauna are predominantly vulnerable, as they are simultaneously subjected to increasing temperature and increasing water alkalinity.

Puerto Rico and the USVI are in the path of Atlantic and Caribbean hurricanes. Most models indicate that while storm frequencies may not increase, storm intensity will increase. Increased storm intensity has been associated with climate change, as well as recurrent coral bleaching events and disease outbreaks. Global warming predictions suggest an increase in the average hurricane intensity of approximately 25–30% per °C of warming in all ocean basins. 15 The 2017 hurricane season was particularly active, and between September and October, Puerto Rico and the USVI felt the direct and indirect impacts of storms and hurricanes, particularly Hurricanes Irma, Jose, and Maria. 

Freshwater availability

Overall, climate change may lead to the following consequences related to water: less rain; a reduction in availability of surface water; changes in riparian, estuarine and wetland diversity; greater human demand for water; saltwater contamination of fresh groundwater; increased demands on groundwater; changes in evapotranspiration; changes in relative soil moisture; increases in the probability of urban and coastal floods; and potential increases of sedimentation in rivers, streams, and reservoirs (in Puerto Rico). Due to increases in the magnitude, frequency and intensity of extreme events, more dissolved and particulate material will be transported downstream. Changes in drought patterns will lead to reductions in water availability. This can also lead to changes in the interrelationship between surface and groundwater and reductions in dissolved oxygen, including increases in contaminants in the ocean water column following extreme events. Life cycles and migratory patterns of aquatic species will change as habitat quality and availability change as communities shift in favor of more tolerant species. 

Marine Impacts

Seagrasses are considered important habitats due to the numerous services they provide such as sediment stabilization, marine organisms nursery grounds, and carbon storage, among others. In addition to the impacts of coastal development, sea level rise and increases in hurricane intensity and precipitation generally decrease light penetration into the water column; less light threatens seagrass survival and can result in habitat degradation. Additionally, increases in temperature stress affect seagrass growth and reproduction, altering the photosynthetic process and causing changes on the structure and distribution. The increase in temperature and the combination of factors may result in widespread seagrass damage in Puerto Rico. One potentially bright spot is that high CO2 availability may decrease light requirements for seagrass and help it tolerate temperature stress. Seagrass beds may aid in coral reef survival by ameliorating ocean acidification and decreasing disease levels. Therefore, effective management requires a comprehensive understanding of seagrass life history to protect these ecosystems, as well as an increase of scientific involvement and public awareness.

A bright yellow fish swims through a dense reef of corals.

A bright yellow fish swims through a dense coral reef along the coast of Culebra, Puerto Rico. Ocean acidification is likely to diminish the structural integrity of coral reefs. This could compromise reef resilience in the face of other acute threats, such as thermal stress, diseases, increasing storm intensity, and rising sea level. This will make coastal areas increasingly vulnerable to waves and storm surge with associated effects on the tourism sector, fisheries, and coastal infrastructure.

Along the coast, coral reefs are estimated to reduce the impact of wave energy by an average of 97%.16 In order for coral reefs to continue providing this coastal protection, corals need to grow upward rapidly enough to keep pace with the rising sea level. 17 Ocean acidification is likely to diminish the structural integrity of coral reefs. This could compromise reef resilience in the face of other acute threats, such as thermal stress, diseases, increasing storm intensity, and rising sea level. This will make coastal areas increasingly vulnerable to waves and storm surge with associated effects on the tourism sector, fisheries, and coastal infrastructure.

Anecdotal evidence from commercial fishers around Puerto Rico provides insight to the potential effects climate change is having on fisheries. Fishers have been observing a shift in seasons, specifically with changes in seasonal currents, water temperature, and spawning seasons. A study on the perceptions and socioeconomic effects of climate change on commercial fisheries revealed that pre-Hurricane Maria fishers were more familiar and concerned with the impacts of coastal pollution; although they felt that climate change was responsible for many of the changes observed (i.e., decline in fish stocks, habitat changes, needing to fish further out to sea), fishers have historically contributed these changes to coastal pollution and mangrove destruction. 18 However, the aftermath of Hurricane Maria brought new focus on the threats and impacts of climate change on fisheries. In their survey, Seara et. al (2020) found that fishers have been making modifications to fishing practices over the years: targeting different species, using multiple fishing gears and fishing further out in search of better fishing grounds.

The preceding text is adapted from Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II, Chapter 20: U.S. Caribbean.

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Banner Image Credit
Thomas Shahan, “El Yunque National Forest, Puerto Rico”, CC BY-NC 2.0,https://www.flickr.com/photos/49580580@N02/23086747200, via Flickr
Last modified
18 July 2022 - 9:16am