Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II

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Key Message 1

San Juan, Puerto RicoFreshwater
Freshwater is critical to life throughout the Caribbean. Increasing global carbon
emissions are projected to reduce average rainfall in this region by the end of thecentury, constraining freshwater availability, while extreme rainfall events, which canincrease freshwater flooding impacts, are expected to increase in intensity. Saltwater
intrusion associated with sea level rise will reduce the quantity and quality of freshwaterin coastal aquifers. Increasing variability in rainfall events and increasing temperatures
will likely alter the distribution of ecological life zones and exacerbate existing problems
in water management, planning, and infrastructure capacity.

Key Message 2
Marine Resources
Marine ecological systems provide key ecosystem services such as commercial andrecreational fisheries and coastal protection. These systems are threatened by changes
in ocean surface temperature, ocean acidification, sea level rise, and changes in thefrequency and intensity of storm events. Degradation of coral and other marine habitatscan result in changes in the distribution of species that use these habitats and the lossof live coral cover, sponges, and other key species. These changes will likely disrupt
valuable ecosystem services, producing subsequent effects on Caribbean island
economies.

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Key Message 3
Coastal Systems
Coasts are a central feature of Caribbean island communities. Coastal zones dominate
island economies and are home to critical infrastructure, public and private property,
cultural heritage, and natural ecological systems. Sea level rise, combined with stronger
wave action and higher storm surges, will worsen coastal flooding and increase coastal
erosion, likely leading to diminished beach area, loss of storm surge barriers, decreased
tourism, and negative effects on livelihoods and well-being. Adaptive planning andnature-based strategies, combined with active community participation and traditional
knowledge, are beginning to be deployed to reduce the risks of a changing climate.

Key Message 4Rising Temperatures
Natural and social systems adapt to the temperatures under which they evolve andoperate. Changes to average and extreme temperatures have direct and indirect effects
on organisms and strong interactions with hydrological cycles, resulting in a varietyof impacts. Continued increases in average temperatures will likely lead to decreases
in agricultural productivity, changes in habitats and wildlife distributions, and risks
to human health, especially in vulnerable populations. As maximum and minimum
temperatures increase, there are likely to be fewer cool nights and more frequent hotdays, which will likely affect the quality of life in the U.S. Caribbean.

Key Message 5
Disaster Risk Response to Extreme EventsExtreme events pose significant risks to life, property, and economy in the Caribbean,
and some extreme events, such as flooding and droughts, are projected to increasein frequency and intensity. Increasing hurricane intensity and associated rainfall rates
will likely affect human health and well-being, economic development, conservation,and agricultural productivity. Increased resilience will depend on collaboration andintegrated planning, preparation, and responses across the region.

Key Message 6Increasing Adaptive Capacity Through Regional Collaboration
Shared knowledge, collaborative research and monitoring, and sustainable institutional
adaptive capacity can help support and speed up disaster recovery, reduce loss oflife, enhance food security, and improve economic opportunity in the U.S. Caribbean.
Increased regional cooperation and stronger partnerships in the Caribbean can expand
the region’s collective ability to achieve effective actions that build climate change
resilience, reduce vulnerability to extreme events, and assist in recovery efforts.

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Executive Summary
Historically, the U.S.
Caribbean region has experienced relatively stable seasonal rainfall patterns,
moderate annual temperature fluctuations, and avariety of extreme weather events, such as tropical storms, hurricanes, and drought. However, theCaribbean climate is changing and is projected to
be increasingly variable as levels of greenhousegases in the atmosphere increase.

plants) is vulnerable to the effects of sea level rise,storm surge, and flooding. In the USVI, infrastructure and historical buildings in the inundation
zone for sea level rise include the power plants on
both St. Thomas and St. Croix; schools; housing
communities; the towns of Charlotte Amalie,Christiansted, and Frederiksted; and pipelines forwater and sewage.
Climate change will likely result in water shortages due to an overall decrease in annual rainfall,a reduction in ecosystem services, and increased
risks for agriculture, human health, wildlife, andsocioeconomic development in the U.S. Caribbean. These shortages would result from somelocations within the Caribbean experiencing
longer dry seasons and shorter, but wetter, wetseasons in the future. 2,3,4,5,6,7,8 Extended dry seasons are projected to increase fire likelihood.9,10
Excessive rainfall, coupled with poor construction
practices, unpaved roads, and steep slopes, canexacerbate erosion rates and have adverse effects
on reservoir capacity, water quality, and nearshore marine habitats.

The high percentage of coastal area relative to thetotal island land area in the U.S. Caribbean meansthat a large proportion of the region’s people, infrastructure, and economic activity are vulnerable tosea level rise, more frequent intense rainfall eventsand associated coastal flooding, and saltwaterintrusion. High levels of exposure and sensitivity torisk in the U.S. Caribbean region are compounded
by a low level of adaptive capacity, due in partto the high costs of mitigation and adaptation
measures relative to the region’s gross domestic
product, particularly when compared to continental U.S. coastal areas.1 The limited geographic andeconomic scale of Caribbean islands means that
disruptions from extreme climate-related events,
such as droughts and hurricanes, can devastatelarge portions of local economies and cause widespread damage to crops, water supplies, infrastructure, and other critical resources and services.1

Ocean warming poses a significant threat to thesurvival of corals and will likely also cause shifts
in associated habitats that compose the coral
reef ecosystem. Severe, repeated, or prolonged
periods of high temperatures leading to extended
coral bleaching can result in colony death. Ocean
acidification also is likely to diminish the structural
integrity of coral habitats. Studies show that major
shifts in fisheries distribution and changes to thestructure and composition of marine habitats
adversely affect food security, shoreline protection,and economies throughout the Caribbean.

The U.S. Caribbean territories of Puerto Ricoand the U.S. Virgin Islands (USVI) have distinct
differences in topography, language, population
size, governance, natural and human resources,and economic capacity. However, both are highly
dependent on natural and built coastal assets;
service-related industries account for more than
60% of the USVI economy. Beaches, affected bysea level rise and erosion, are among the main
tourist attractions. In Puerto Rico, critical infrastructure (for example, drinking water pipelines
and pump stations, sanitary pipelines and pumpstations, wastewater treatment plants, and power

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In Puerto Rico, the annual number of days withtemperatures above 90°F has increased overthe last four and a half decades. During that
period, stroke and cardiovascular disease, which
are influenced by such elevated temperatures,

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became the primary causes of death.11,12 Increases
in average temperature and in extreme heat
events will likely have detrimental effects on
agricultural operations throughout the U.S.
Caribbean region.13,14 Many farmers in the tropics,
including the U.S. Caribbean, are consideredsmall-holding, limited resource farmers and often
lack the resources and/or capital to adapt tochanging conditions. 15

Most Caribbean countries and territories share
the need to assess risks, enable actions acrossscales, and assess changes in ecosystems to
inform decision-making on habitat protection
under a changing climate.16,17 U.S. Caribbean
islands have the potential to improve adaptationand mitigation actions by fostering stronger
collaborations with Caribbean initiatives onclimate change and disaster risk reduction.

Observed and Projected Sea Level Rise(top) Observed sea level rise trends in Puerto Rico and the U.S. Virgin Islands reflect an increase in sea level of about 0.08 inches (2.0mm) per year for the period 1962–2017 for Puerto Rico and for 1975–2017 for the U.S. Virgin Islands. The bottom panels show a closer
look at more recent trends from 2000 to 2017 that measure a rise in sea level of about 0.24 inches (6.0 mm) per year. Projections of sealevel rise are shown under three different scenarios of Intermediate-Low (1–2 feet), Intermediate (3–4 feet), and Extreme (9–11 feet)sea level rise. The scenarios depict the range of future sea level rise based on factors such as global greenhouse gas emissions andthe loss of glaciers and ice sheets. From Figure 20.6. (Sources: NOAA NCEI and CICS-NC).
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Climate Risk Management Organizations

Some of the organizations working on climate risk assessment and management in the Caribbean are shown. Joint regional efforts toaddress climate challenges include the implementation of adaptation measures to reduce natural, social, and economic vulnerabilities,
as well as actions to reduce greenhouse gas emissions. See the online version of this figure at http://nca2018.globalchange.gov/
chapter/20#fig-20-18 for more details. From Figure 20.18 (Sources: NOAA and the USDA Caribbean Climate Hub).

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