finding 20.2 : key-message-20-2

Marine ecological systems provide key ecosystem services such as commercial and recreational fisheries and coastal protection. These systems are threatened by changes in ocean surface temperature, ocean acidification, sea level rise, and changes in the frequency and intensity of storm events. Degradation of coral and other marine habitats can result in changes in the distribution of species that use these habitats and the loss of live coral cover, sponges, and other key species (very likely, high confidence). These changes will likely disrupt valuable ecosystem services, producing subsequent effects on Caribbean island economies (likely, medium confidence).

This finding is from chapter 20 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

The majority of our Key Messages were developed over the course of two separate author meetings. The first occurred March 9–10, 2017, and the second on May 3, 2017. Both meetings were held in San Juan, Puerto Rico; however, people were also able to join remotely from Washington, DC, Raleigh, North Carolina, and the U.S. Virgin Islands (USVI). In addition, the author team held weekly conference calls and organized separate Key Message calls and meetings to review and draft information that was integral to our chapter. To develop the Key Messages, the team also deliberated with outside experts who are acknowledged as our technical contributors.

Description of evidence base:

In 2006, the National Marine Fisheries Service (NMFS) listed elkhorn and staghorn corals as threatened species under the Endangered Species Act, with persistent elevated sea surface temperatures and sea level rise being two of the key factors influencing the listing decision.9d974b61-97ea-4778-977d-26701052c8e9 The Acropora Biological Review Team (2005) found that the number of hurricanes affecting reef ecosystems in the Caribbean has increased over the past two decades (2 hurricanes in the 1970s, 6 in the 1980s, and 12 in the 1990s). Sea surface temperature is expected to continue rising, and this implies an increasing threat to elkhorn and staghorn corals from bleaching-induced mortality and possibly an exacerbation of disease effects. In 2014, NMFS listed an additional 5 species of Atlantic/Caribbean corals (lobed, mountainous star, boulder star, pillar, and rough cactus) as threatened and reevaluated the listing of elkhorn and staghorn corals, confirming them as threatened species; it also listed 15 Indo-Pacific coral species as threatened,8960de56-9b32-4544-9255-046a9ef45e0d with two of the key factors being ocean warming and ocean acidification. Brainard et al.8960de56-9b32-4544-9255-046a9ef45e0d found that ocean warming and related effects of climate change have already created a clear and present threat to many corals that will likely continue into the future and can be assessed with certainty out to 2100. Increases in human population densities and activity levels in the coastal zone are expected to continue, meaning the vulnerability of these populations and infrastructure will likely continue increasing with climate change.895e2c92-d614-4154-8509-7689918c8697 Direct measurements at the Bermuda Atlantic Time-series Study station shows that surface ocean acidity has increased by about 12% and aragonite saturation (Ωarg) has decreased by about 8% over the past three decades.ade3d353-6612-4e45-96d3-9269de56eda0 These values agreed with those reported across the Caribbean0e312df5-ad3d-4709-8b58-8cea86d26415 and Atlantic regionsd51156cc-0034-4afc-b2b7-1ad99efde458,ade3d353-6612-4e45-96d3-9269de56eda0 using regional and global numerical marine carbonate system models.

Many coastal regions already experience low surface seawater pH and Ωarg conditions (localized or coastal ocean acidification) due to processes other than CO2 uptake. As a result, the effect of ocean acidification on coastal zones can be several times higher and faster than typically expected for oceanic waters.dc5e5365-8b0f-4a21-93b4-c6a822be824d

Caribbean coral reefs in the Bahamas, Belize, Bonaire, and Grand Cayman are already experiencing significant reductions in carbonate production rates, with 37% of surveyed sites showing net erosion.0841a7ce-1022-4cf7-be4f-5c20d3b19f2a Friedrich et al. (2012).a03f3916-2084-4353-bce4-adac93801617 concluded that calcification rates may have already dropped by about 15% within the Caribbean with respect to their preindustrial values.

New information and remaining uncertainties:

The link between climate stressors such as increasing sea surface temperatures and bleaching response and increasing prevalence of disease in corals is postulated. There is some scientific evidence indicating a link, but it is hard to make definitive conclusions. Effects of climate change on fisheries in the Caribbean have not been as well studied as the effects on marine habitats, particularly coral reefs.94f95306-0dd9-43cf-8e70-9a379423f31d,272d0750-d233-4495-9c70-22a62cd113a7 Similarly, the social consequences of climate change and associated declines in marine fisheries and the effects on coastal communities reliant on coral reef fishery species have not been as well studied.2c5feeb2-9e41-4529-a0d8-3ba8a36a1e16

Uncertainty with respect to ocean acidification is dominated by uncertainty about how ecosystems and organisms will respond, particularly due to multiple interactions with other stressors.

The value of the loss of ecosystem services to ocean acidification is unknown. Such losses are attributable to the degradation of ecosystems that support important economic marine species such as coral, conch, oysters, fish larvae, urchins, and pelagic fish in the Caribbean. There is strong evidence for decreasing carbonate production, calcification rates, coral cover, and biomass of major reef-building species throughout the Caribbean region. However, there is still not enough evidence to conclude that all these decreased ecosystem processes are due to ocean acidification.

There are only a few studies on ecosystem and organism responses to climate stressors (such as ocean warming) that consider ocean acidification in the Caribbean. For instance, low pH values could affect nursery areas of commercially important species such as tuna, presenting a source of vulnerability for the economy, but studies are scarce. Ocean acidification could also affect the food web dynamics at lower trophic levels and have physiological effects at larval stages that would likely cascade upward, affecting coral and fish recruitment.

The effects of ocean acidification on coral reefs, shellfish, fish, and marine mammals will likely cause an economic effect on fisheries, coastal protection, and tourism in the Caribbean. Ocean acidification can exacerbate the current global warming effects on coral reefs, and it will likely continue deteriorating reef conditions and cause ecological regime shifts from coral to algal reefs.b09adbe5-6a17-4d3c-ab96-b3d9e306af67,e684169c-60a2-4c78-a724-36fb93fb385a The primary effect on reef communities will probably be a reduction in their capacity to recover from acute events such as thermal bleaching.

Sea level rise is currently the most immediate and well-understood climate-related threat to mangroves.c527429c-5661-477a-a6f4-1690355bcbd5 It is not clear how mangroves will respond to elevated CO2, and some studies suggest increases may actually be beneficial to mangroves.c527429c-5661-477a-a6f4-1690355bcbd5 Similarly, in the Caribbean where temperatures are already high, increasing temperatures, as well as declines in rainfall and corresponding increases in soil salinity during periods of drought, will likely increase plant water stress and reduce productivity. There have been limited studies on the effects of climate change on seagrass beds; therefore, these effects remain uncertain.63aac7f9-5e82-4aff-ab62-75c12537612f Sea level rise that results in reduced sunlight due to increased water depths can lead to the loss of seagrass beds from deeper waters. As discussed previously, the loss or degradation of these habitats, which are part of the coral reef ecosystem and serve as nursery habitat for important nursery species, will likely contribute to declines in fishery productivity due to climate change.

Assessment of confidence based on evidence:

There is high confidence that increasing ocean temperatures, changes in ocean acidity, and changes in the frequency and intensity of storms are extremely likely to affect coastal and marine resources. Large storm events within the past decade have resulted in significant effects on marine resources, particularly coral habitats and organisms that rely on them. There is medium confidence in predictions that coral habitats will likely continue to decline throughout the Caribbean, with associated effects on resources dependent on these habitats; although, scientific studies are still needed in terms of climate change effects on fisheries resources, particularly for species that are found in offshore waters or are pelagic. Changes in coral habitats are already occurring as evidenced by massive coral bleaching events (including a three-year global-level bleaching event from 2015–2017) and the increase in these events. Such changes in bleaching events are due to rising sea surface temperatures. There is high confidence that there have been changes in ocean pH and medium confidence on the ecological effects. Due to the lack of studies on the social consequences of climate change and associated losses of resources such as fisheries, there is medium confidence that effects on coastal and marine resources resulting from climate change will affect island economies. These effects can be a result of changes in availability and condition of fishery resources, loss of reefs and other coral communities that serve as coastal barriers, and effects on tourism due to loss of the resources that are primary attractions for visitors.

There is medium confidence in the ecological effects that will result due to changes in ocean pH. The CO2 system of seawater is well understood and established. As such, the understanding of the basic equilibria governing the process of ocean acidification dates back to at least 1960b20e8c2b-cfb8-48b6-85c6-c981c361e267 and represents a foundational understanding of modern chemical oceanography. The ecological consequences of human-induced changes to the system (that is, ocean acidification) is, however, a considerably new field. Both themes were assessed considering recent findings and based on adequate observed local data (for example, atmospheric pCO2 [carbon dioxide partial pressure] values are based on measurements of weekly air samples from St. Croix, the USVI, the United States, and Ragged Point, Barbados), complemented with empirical models. Projected changes in climate for the Caribbean islands were based on the future projections of fossil fuel emissions driven by reasonable models from the Intergovernmental Panel on Climate Change (IPCC).bc140b4c-c2d9-4d99-a684-5c054dc5134f Additional empirical species response data would be useful for increasing the understanding of expected effects of ocean acidification on species and habitats in the Caribbean.

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