Coastal ecosystems are particularly vulnerable to climate change because many have already been dramatically altered by human stresses; climate change will result in further reduction or loss of the services that these ecosystems provide, including potentially irreversible impacts.

This finding is from chapter 25 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: A central component of the assessment process was a Chapter Lead Authors meeting held in St. Louis, Missouri in April 2012. The key messages were initially developed at this meeting. Key vulnerabilities were operationally defined as those challenges that can fundamentally undermine the functioning of human and natural coastal systems. They arise when these systems are highly exposed and sensitive to climate change and (given present or potential future adaptive capacities) insufficiently prepared or able to respond. The vulnerabilities that the team decided to focus on were informed by ongoing interactions of the author team with coastal managers, planners, and stakeholders, as well as a review of the existing literature. In addition, the author team conducted a thorough review of the technical input reports (TIR) and associated literature, including the coastal zone foundational TIR prepared for the National Climate Assessment (NCA).c9647af9-db7f-4f6a-89bd-2f2293ad26e5 Chapter development was supported by numerous chapter author technical discussions via teleconference from April to June 2012.

Description of evidence base: The key message and supporting text summarize extensive evidence documented in the coastal zone technical input report.c9647af9-db7f-4f6a-89bd-2f2293ad26e5 Technical input reports (68) on a wide range of topics were also received and reviewed as part of the Federal Register Notice solicitation for public input, along with the extant literature. Evidence base is strong for this part of the key message: “Coastal ecosystems are particularly vulnerable to climate change because many have already been dramatically altered by human stresses.” The degradation and depletion of coastal systems due to human stresses (for example, pollution, habitat destruction, and overharvesting) has been widely documented throughout the U.S. and the world.057e69e7-a2a7-48f5-85f5-7214ed7b5bf7 412faea9-5a62-4e2f-95d1-a475b33c39fb 3d9112b9-6aa1-4614-9599-6966c9591ef9 cd8312dc-7405-4bf0-ae48-c5af32814d65 8712f79b-218f-4f08-b4a9-2401d8aa1b14 d9ec3739-08f5-4e71-ada0-194fd3b35d63 d3e0a9e1-9ff9-492c-ba13-9f24976fa65a The degree of degradation varies based on location and level of human impact. However, evidence of degradation is available for all types of U.S. coastal ecosystems, from coral reefs to seagrasses and rocky shores. Human stresses can be direct (for example, habitat destruction due to dredging of bays) or indirect (for example, food web disruption due to overfishing). There is also consistent evidence that ecosystems degraded by human activities are less resilient to changes in climatic factors, such as water temperature, precipitation, and sea level rise (for example, Gedan et al. 2009; Glick et al. 2011; Williams and Grosholz 2008f235a577-2e08-47eb-9ee2-3cc5dbbc234b fdb15eb5-7454-4c42-b331-d9859ea4320e 92da0798-eb2c-44a8-991d-829fdff77dc3). Evidence base is strong: “climate change will result in further reduction or loss of the services that these ecosystems provide.” The impacts of changing coastal conditions (for example, changes associated with altered river inflows, higher temperatures, and the effects of high rates of relative sea level rise) on coastal ecosystems and their associated services have been extensively documented through observational and empirical studies, including recent publications.cfdaea11-95e2-4789-914b-74901b2f26b0 f74345fb-5efc-4b8c-9dcc-44eb3ad7e7a1 24763b7e-38b5-4e8d-b7c2-901431f1d01c 5f01fe1e-d4c6-4974-b238-7dbacf170159 fdb15eb5-7454-4c42-b331-d9859ea4320e febd979a-7e7b-4ae3-8fdc-d9aa62dea49c Many models of coastal ecosystem responses to climatic factors have been well-validated with field data. Given the existing knowledge of ecosystem responses, future climate projections, and the interactions with non-climatic stressors that further exacerbate climatic impacts, evidence is strong of the potential for further reduction and/or loss of ecosystem services. Evidence is suggestive: “including potentially irreversible impacts.” Severe impacts (for example, mass coral bleaching events and rapid species invasions) have been extensively documented for U.S. coastal ecosystems. Many experts have suggested that some of these impacts may be irreversible91134a9b-6dde-4607-bc9f-6301da1e1800 and never before seen conditions have been documented.9ec1ded2-2422-45f4-b84f-c32eeb34b7ff 291c90fe-6766-40c2-9580-aea56178ecaa 497e2536-1556-4905-a4b1-93fa2175e3b5 7ec245d4-187a-493b-9262-d2e89b5c29c4 Recovery may or may not be possible in different instances; this depends on factors that are not well-understood, such as the adaptive capacity of ecosystems, future projections of change that consider interactions among multiple climatic and non-climatic human alterations of systems, the dynamics and persistence of alternative states that are created after a regime shift has occurred, and whether or not the climatic and/or non-climatic stressors that lead to impacts will be ameliorated.b7708c1a-0eb5-47db-a089-ff40be29c884 032cd8fc-fb93-445b-a94b-a18dda336372 4d7318c7-da72-425c-9506-5eba7d21c0f9 5b779eb3-6b85-4781-93ec-30197cfb259c 697b2dc9-5ef5-44da-bc24-5b1ada3e0b45 021db5fd-4164-47ba-ba6a-f7f1895fbd0a

New information and remaining uncertainties: Since the 2009 NCA,e251f590-177e-4ba6-8ed1-6f68b5e54c8a new studies have added weight to previously established conclusions. The major advance lies in the examination of tipping points for species and entire ecosystems (for example, Barnosky et al. 2012; Folke et al. 2004; Foti et al. 2013; Hoegh-Guldberg and Bruno 201091134a9b-6dde-4607-bc9f-6301da1e1800 211334ae-d5e0-48cc-9f29-9e24669b4af3 497e2536-1556-4905-a4b1-93fa2175e3b5 4d7318c7-da72-425c-9506-5eba7d21c0f9). Existing uncertainties and future research needs were identified through reviewing the NCA technical inputs and other peer-reviewed, published literature on these topics, as well as through our own identification and assessment of knowledge gaps. Key uncertainties in our understanding of ecosystem impacts of climate change in coastal areas are associated with: • the interactive effects and relative contributions of multiple climatic and non-climatic stressors on coastal organisms and ecosystems; • how the consequences of multiple stressors for individual species combine to affect community- and ecosystem-level interactions and functions; • the projected magnitude of coastal ecosystem change under different scenarios of temperature change, sea level rise, and land-use change, particularly given the potential for feedbacks and non-linearities in ecosystem responses • the potential adaptive capacity of coastal organisms and ecosystems to climate change; • trajectories, timeframes, and magnitudes of coastal ecosystem recovery; • the dynamics and persistence of alternative states that are created after ecosystem regime shifts have occurred; and • the potential and likelihood for irreversible climate-related coastal ecosystem change. In general, relatively little work to date has been conducted to project future coastal ecosystem change under integrative scenarios of temperature change, sea level rise, and changes in human uses of, and impacts to, coastal ecosystems (for example, through land-use change). Advancing understanding and knowledge associated with this key uncertainty, as well as the others included in the above list, would be fostered by additional research. Assessment of confidence based on evidence We have very high confidence that coastal ecosystems are particularly vulnerable to climate change because they have already been dramatically altered by human stresses, as documented in extensive and conclusive evidence. We have very high confidence that climate change will result in further reduction or loss of the services that these ecosystems provide, as there is extensive and conclusive evidence related to this vulnerability. We have high confidence that climatic change will include “potentially irreversible impacts.” Site-specific evidence of potentially irreversible impacts exists in the literature. This vulnerability is frequently identified by studies of coastal ecosystems. However, methods, research, and models are still being developed for understanding, documenting, and predicting potentially irreversible impacts across all types of coastal ecosystems.

Assessment of confidence based on evidence: We have very high confidence that coastal ecosystems are particularly vulnerable to climate change because they have already been dramatically altered by human stresses, as documented in extensive and conclusive evidence. We have very high confidence that climate change will result in further reduction or loss of the services that these ecosystems provide, as there is extensive and conclusive evidence related to this vulnerability. We have high confidence that climatic change will include “potentially irreversible impacts.” Site-specific evidence of potentially irreversible impacts exists in the literature. This vulnerability is frequently identified by studies of coastal ecosystems. However, methods, research, and models are still being developed for understanding, documenting, and predicting potentially irreversible impacts across all types of coastal ecosystems

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