Heat stress can exacerbate preexisting health conditions and lead to an increase in human mortality.100,101 Time of year, repetition, duration, time between events, and adaptation of individuals are important determinants of the health outcomes during extreme heat episodes. Vulnerability to heat is a function of exposure and personal sensitivity, which depends on an array of individual factors and may influence the ability to cope with extreme temperatures.102
Urban areas are particularly vulnerable to extreme heat events, given the concentration of built structures, traffic, and other factors that drive the urban heat island (UHI) effect.103,104 Since the middle of the last century, urbanization and population growth have increased the UHI effects in San Juan. Such effects are becoming even more life threatening with a growing and more vulnerable aging population. Heat vulnerability index maps show that the hottest and most vulnerable areas correspond to highly built areas, including within and around the LMM Airport, seaports, parking lots, and high-density residential areas, while cooler areas correspond to vegetated landscapes and urban bodies of water (such as lagoons and wetlands).102
The role of agriculture in Puerto Rico and the USVI is both economic and cultural. The economic role of agriculture has diminished in recent decades compared to the mid-20th century. Currently, less than 1% of Puerto Rico’s gross domestic product (GDP) and approximately 1% of the USVI’s GDP is due to agriculture.13,89 Recent revitalizations in agricultural productivity are vulnerable to climate change. At risk are food security, rural livelihoods, and agroecological services. Increases in average temperature and extreme heat events will likely have detrimental effects on agricultural operations throughout the U.S. Caribbean region.13,14 Climate change affects cattle ranchers and dairy farmers in the U.S. Caribbean by reducing productivity of rangeland, causing a shortage of nutritional feed, increasing heat stress on animals, and increasing energy costs for cooling.105 High temperatures and resultant heat stress reduce animal productivity and increase the proliferation and survival of parasites and disease pathogens. Warming reduces the ability of dairy cattle to produce milk and gain weight and can lower conception rates.105
Tropical cropping systems are often more vulnerable to climatic shifts and anomalies for a number of reasons. Many farmers throughout the tropics, including in the U.S. Caribbean, are considered small-holding, limited resource farmers.1,15 This terminology refers to farmers who own small parcels of land (fewer than 2–5 acres) and often lack the resources and/or capital to adapt to changing conditions.15 Many important tropical crop species, such as coffee, evolved within relatively narrow temperature bands and are more sensitive to variation in rainfall and temperature than are crop species native to temperate regions.24
Finally, rising temperatures will generally increase regional sea surface temperatures, which tends to increase the maximum intensity that hurricanes in the region can achieve.33 This can lead to stronger hurricanes and more active hurricane seasons in general, which the Caribbean region is especially vulnerable to, as evidenced by the 2017 hurricane season (see Box 20.1).
Future Climate Change Relevant to Regional Risks
Cooling degree days (CDDs), used as a proxy for future air conditioning energy demands, are projected to increase over time and to more than double in Puerto Rico by the end of century (Ch. 4: Energy, KM 1).7 The warmer south coast is projected to have the highest increase in CDDs in the first half of the century, while the San Juan metropolitan area is projected to have its highest increases in the second half of the century, suggesting higher energy demands in the island’s largest metropolitan area by the end of the century.7
Warming, along with drying, is projected to affect the terrestrial ecosystems in the region. The ecological life zones of Puerto Rico are projected to shift from rain and wet zones to moist and dry zones based on the projected drying. By the middle of this century, under most scenarios considered, all life zones in Puerto Rico are projected to shift to tropical zones.7 Environmental suitability for species in the region would be altered by life zone shifts, which may lead to biodiversity redistribution in the region. Environmental factors, especially climatic variables, were shown to have higher importance than land-use history on forest species composition in Puerto Rico and the USVI.106 The projected changes in the amount and spatial variability of climatic variables will likely affect the composition and spatial redistribution of species.
Climate change adaptation strategies and national (as well as international) discussions and agreements have focused more on direct socioeconomic implications and less on changes in natural ecosystems; nonetheless, climate-induced species redistribution affects ecosystem functioning, human well-being, and the dynamics of the climate change itself and represents a substantial challenge for human society.107 Species respond to changes in environmental conditions by tolerating the changes, adapting to the new conditions, facing extinction, or moving, which changes their distributions.108 Warming forces species to move toward higher latitudes and altitudes.109 On small islands in the Caribbean with limited latitudinal ranges, species’ adaptive movement is limited to tracking changing temperatures toward higher altitudes.
Challenges, Opportunities, and Success Stories for Reducing Risk
Green and blue infrastructure are, respectively, the natural terrestrial vegetation and water-related components of an urban or other landscape. They provide many beneficial ecosystem services for surrounding microclimates.102,110,111 Urban planning efforts in coastal cities are placing greater emphasis on the use of green infrastructure and water bodies for cooling urban environments. Planners in low-lying cities are also incorporating adaptable spaces that can accommodate occasional flood waters while providing services such as parks or urban open space112 that can also help mitigate the UHI effect. In agriculture, the rapid expansion of electronic and worldwide communications is bringing old and new adaptation practices to a new generation of practitioners as they deal with multigenerational problems of water management and heat stress in crops and livestock.13
Cumulative effects on urban populations, agricultural sectors, and the natural environment add complexity to developing scenarios and prioritizing actions to reduce risks related to climate change. New alliances, collaborations, and governmental structures may be necessary to address these complex challenges.