Climate change has had large impacts on the ecosystems and landscapes of the US territory Puerto Rico. According to a 2019 report by Germanwatch, Puerto Rico is the most affected by climate change. The territory's energy consumption is mainly derived from imported fossil fuels.

The Puerto Rico Climate Change Council (PRCCC) noted severe changes in seven categories: air temperature, precipitation, extreme weather events, tropical storms and hurricanes, ocean acidification, sea surface temperatures, and sea level rise.

Climate change also affects Puerto Rico's population, the economy, human health, and the number of people forced to migrate.

Surveys have shown climate change is a matter of concern for most Puerto Ricans. The territory has enacted laws and policies concerning climate change mitigation and adaptation, including the use of renewable energy. Local initiatives are working toward mitigation and adaptation goals, and international aid programs support reconstruction after extreme weather events and encourage disaster planning.

Greenhouse gas emissions

Puerto Rico is the 19th-biggest emitter of carbon dioxide among the 33 Latin American and Caribbean countries; its industrial emissions, energy supplies, and transportation are among the main sources of the island's net greenhouse gas emissions. The territory's gross carbon dioxide emissions rose to 80% between 1990 and 2005. Since 2005, emissions decreased by 42% until 2018, due to Puerto Ricans migrating to the United States. Between 2010 and 2020, the population declined 12% from 3.8 to 3.3 million inhabitants as a result of the country's vulnerability to natural disasters and its economic insecurities.

Energy consumption and fossil fuels Puerto Rico's energy consumption is nearly 70 times higher than its energy production. Petroleum products supply 63% of Puerto Rico's electricity generating capacity; 23% comes from natural gas, 8% from coal, and 6% from renewable energy sources, specifically from solar, wind, hydropower, and biomass.

Puerto Rico has no reserves of fossil fuels; the territory's energy consumption comes mostly from imported fossil fuels.

The U.S. Department of Energy published the PR-100 Study in February 2024, which provided empirical evidence of how Puerto Rico could achieve its goal of 100% renewable energy (see Policies and legislation).

+Per capita CO2 emissions of Puerto Rico in 2021 by fossil fuel compared to the U.S. (in metric tons)Puerto RicoUnited StatesTotal Fossil Fuels5.5214.78Petroleum3.376.76Natural Gas1.234.99Coal0.923.02Population of Puerto Rico (2021): 3,263,584; Population of the U.S. (2021): 331,893,745

Impacts on the environment

The El Niño–Southern Oscillation and other periodic events, such as Volcanic eruptions, cause natural climate variability. These natural factors are summarized as "internal climate variability" and are always present in the climate system. The climate system is also affected by anthropogenic emissions of greenhouse gases and changes in atmospheric concentrations (e.g. carbon dioxide (CO2), methane) and land surface changes. Climate change signals can be observed as changes in the magnitude of variability and systematic trends over time. Variability is a natural feature of the climate system and its understanding is fundamental for identifying signals of anthropogenic climate change.

Changes in climate parameters Puerto Rico has a yearly tropical climate with moderately high temperatures and high humidity. According to a 2019 report by Germanwatch, Puerto Rico has been the territory most-affected by climate change, with a climate risk index score of 6.67. The Puerto Rico Climate Change Council (PRCCC) has identified changes in seven climate parameters: air temperature, precipitation, extreme weather events, tropical storms and hurricanes, ocean acidification, sea-surface temperatures, and sea level rise.

Air temperature The mean annual temperature in Puerto Rico has increased from about in 1921 to in 2021. The central regions of Puerto Rico, which are at higher elevations, are warmer than any other regions of the island. Urban areas such as San Juan have become urban heat islands (UHIs), meaning they are warming more quickly than rural or vegetated regions. Studies in San Juan have shown the UHI effect is permanent due to urban expansion and it leads to a temperature increase of over 4C (7.2 F) compared to surrounding rural areas. This trend of greater urban warming is expected to continue with San Juan reaching an average temperature of by 2050, an increase of 1.5 C (2.7 F).

Extreme weather events The number of days with high temperatures is increasing while the number of days with very low temperatures is decreasing. During the early 20th century, there were approximately 100 days per decade that reached temperatures of over . In 2010, the same number of days above that temperature was reached within a year.

Rainfall Puerto Rico receives large amounts of rainfall, especially in the north and central regions, which receive 3–4 m of rainfall every year. Within the territory, rainfall varies between regions due to topography and geography in relation to trade winds and oceanic circulation. Climate change is predicted to exacerbate these regional differences. For example, in some areas such as Old San Juan, there is a long-term trend towards a decrease in precipitation. This decline is expected to continue in future projections. The southern regions of Puerto Rico, which have been drier in the past, show a positive trend in precipitation. The seasonal distribution of precipitation is also expected to become more pronounced, with wetter winters and drier summers. Future projections indicate a further decrease in precipitation.

Heavy rainfall has increased by 20% on average in North America, the largest increase being in the wettest regions. Puerto Rico has seen a 37% increase in heavy precipitation. There is only limited data available for future projections, which leads to conflicting forecasts for extreme precipitation in the territory. General projections for the Caribbean region suggest a higher incidence of extreme precipitation events despite a decrease in total rainfall. Rainfall will be less frequent but more intense.

Water resources Potential impacts on water resources in rain-fed areas of the Caribbean include higher precipitation extremes, greater seasonal variability in water runoff, a higher likelihood of prolonged dry spells, and an increased risk of droughts and floods. In this context, prolonged dry periods and increased evaporation can lead to a decline in lake levels. This is a problem because they are an essential source of freshwater. Groundwater use is expected to increase with demand for water, especially in dry months when surface sources decline. Prolonged dry periods could also lead to reduced soil moisture, resulting in an increased need for irrigation in agriculture.

Sea surface temperature Since 1920, the surface temperatures of the Caribbean Sea have warmed by . The warming of the sea surface on Caribbean coasts of Puerto Rico is faster than that on Atlantic coasts. In addition, temperatures below the water surface are rising more quickly than surface temperatures. In 2018, researchers estimated there will be an increase of more than within 50 years. This would mean temperatures would exceed the threshold for coral bleaching for about one-third of the year and the threshold for the formation of deep convection storms will be exceeded throughout the year.

Sea level rise The rate of sea level rise (SLR) in Puerto Rico has been measured at 1.7 mm a year based on historical records from tidal gauges since 1900, which is consistent with global trends. However, recent satellite data from 1992 onwards shows that this rate has almost doubled to 3.2 mm a year. Projections for the future SLR in Puerto Rico are similar to global estimates, with the National Oceanographic and Atmospheric Administration (NOAA) recommending updated bounds of 0.3 m to 2.5 m for end-of-century projections. The Caribbean region, including Puerto Rico, is particularly vulnerable to coastal flooding due to SLR.

Ocean acidification

As more carbon dioxide (CO2) is released into the atmosphere, oceans absorbs more CO2, leading to a decrease in pH and carbonate saturation of the ocean. This process, which is known as ocean acidification, negatively impacts marine life and geological processes by reducing calcification rates and mineral precipitation. Puerto Rico experiences similar trends to the rest of the world, with decreasing average pH and carbonate saturation state. , the rate of decline for aragonite saturation states in Puerto Rico is 3% per decade and is expected to continue with ongoing emissions.

Tropical storms and hurricanes

In Puerto Rico, hurricanes Irma and Maria in 2017 are an example of predicted climate change impacts on tropical cyclones that had devastating effects in the Caribbean, including damage to the coral reefs that dissipate wave energy on Puerto Rico's coasts. The lack of protection from the reefs leads to an increased risk of damage by winter swells, resulting in coastal erosion and sediment displacement. The southern part of the island is particularly vulnerable to Atlantic hurricanes. In the future, rising sea-surface temperatures are likely to lead to more intense rainfall, winds, and storm surges.

Ecosystems

Despite the relatively small size of territory there are several ecosystems in Puerto Rico; coastal and marine ecosystems, dry forests and rainforests, the Puerto Rican karst, and mountainous areas. Climate change is expected to have synergistic effects on ecosystems and species in Puerto Rico, meaning systems that are already under stress will be exposed to additional stressors, exceeding their adaptive capacity. This may result in loss of habitat, adverse changes in structure and function, and reduced benefits to society. While certain ecosystems and species may be able to adapt to the changing environment, others may have difficulty coping with new conditions.

Dry forests and rainforest

, about 40% of Puerto Rico's land area is covered by forests, mainly young mixed forests, that provide important habitat for numerous plant and animal species. They are also an important source of water for municipalities, agriculture, and industry, which is vital for nearly four million people. Forests also provide recreational areas. Using the changes in Puerto Rico's forests, models for understanding similar changes in other tropical islands as a result of human expansion can be derived. With the introduction of economically important crops and deforestation for pasture and charcoal production, Puerto Rico's forests have dramatically changed over the past two centuries. Economic development and increasing urbanization are the main factors in the disappearance of forests and old agricultural lands. The diversity of Puerto Rican forests in terms of location and type is so great the number of forest types has yet to be fully documented.

Climate change is having a strong effect on forests through higher air temperatures, changes in precipitation, more intense wind and water events, and sea-level rise. All of these factors are altering species composition and forest structure due to changes in habitat.

Karst landscapes