finding 9.1 : key-message-9-1

The Nation’s valuable ocean ecosystems are being disrupted by increasing global temperatures through the loss of iconic and highly valued habitats and changes in species composition and food web structure (very high confidence). Ecosystem disruption will intensify as ocean warming, acidification, deoxygenation, and other aspects of climate change increase (very likely, very high confidence). In the absence of significant reductions in carbon emissions, transformative impacts on ocean ecosystems cannot be avoided (very high confidence).

This finding is from chapter 9 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

The goal when building the writing team for the Oceans and Marine Resources chapter was to assemble a group of scientists who have experience across the range of marine ecosystems (such as coral reefs and temperate fisheries) that are important to the United States and with expertise on the main drivers of ocean ecosystem change (temperature, deoxygenation, and acidification). We also sought geographic balance and wanted a team that included early-career and senior scientists. 

We provided two main opportunities for stakeholders to provide guidance for our chapter. This included a town hall meeting at the annual meeting of the Association for the Sciences of Limnology and Oceanography and a broadly advertised webinar hosted by the National Oceanic and Atmospheric Administration. Participants included academic and government scientists, as well as members of the fisheries and coastal resource management communities. We also set up a website to collect feedback from people who were not able to participate in the town hall or the webinar.

An important consideration in our chapter was what topics we would cover and at what depth. We also worked closely with the authors of Chapter 8: Coastal Effects to decide which processes and ecosystems to include in which chapter. This led to their decision to focus on the climate-related physical changes coming from the ocean, especially sea level rise, while our chapter focused on marine resources, including intertidal ecosystems such as salt marshes. We also decided that an important goal of our chapter was to make the case that changing ocean conditions have a broad impact on the people of the United States. This led to an emphasis on ecosystem services, notably fisheries and tourism, which are easier to quantify in terms of economic impacts.

Description of evidence base:

Ocean warming has already impacted biogenically built habitats. Declines in mussel beds, kelp forests, mangroves, and seagrass beds, which provide habitat for many other species, have been linked to ocean warming and interactions of warming with changes in oxygen levels or other stressors (see Ch. 27: Hawaiʻi & Pacific Islands, Key Message 4 for impacts on mangrove systems in the Pacific Islands).ca6665ed-2cb2-4a8c-959b-47a6a7dbbc29,76e7cb83-a820-43f3-97e2-01b1260709e9,73811ef4-cada-439e-8118-cfb52e69b7a1,a40d3ab9-ef26-4894-af9e-ab6ff67bcee5 Sea level rise will continue to reduce the extent of many estuarine and coastal habitats (for example, salt marshes, seagrass beds, and shallow coral reefs) in locations where they fail to accrete quickly enough to outpace rising seas.a84974cf-4526-41a9-8073-4d73f1f9846b,cefeec26-f91a-4855-8a84-1a90562a0524 The composition and timing of phytoplankton blooms are shifting, and dominant algal species are changing, which can cause bottom-up changes in food web structure.6f235775-bc4f-4dc8-ab04-5bb4e45761d7,b0d0f60e-b8b2-447c-9bdd-2b2fdba0e958,4fe22675-e201-4922-a491-9c382d98530c

Some of the most apparent ecosystem changes are occurring in the warmest and coldest ocean environments, in coral reef and sea ice ecosystems. Live coral cover in coral reef ecosystems around the world has declined from a baseline of about 50%–75% to only 15%–20% (the current average for most regions; see Bruno & Valdivia 2016; Eddy et al. 2018),907c63d3-2172-4d33-a185-620258ec628d,a834e38d-a794-4f3a-a81d-87250bcf93a1 primarily due to ocean warming.b09adbe5-6a17-4d3c-ab96-b3d9e306af67,732d35cc-654b-450e-8c8a-a61f65d3afc4 Exposure to water temperatures just a few degrees warmer than normal for a given reef can cause corals to bleach; bleached corals have expelled their colorful symbiotic dinoflagellate algae, and the lack of algae can partially or wholly kill coral colonies.ae028434-6dfc-48a7-acca-eb5e76857801 Over the past four decades, warming has caused annual average Arctic sea ice extent to decrease between 3.5% and 4.1% per decade; sea ice melting now begins at least 15 days earlier than it did historically (Ch. 26: Alaska, KM 1).61d6757d-3f7a-4e90-add7-b03de796c6c4,a19b5228-b54b-4f6b-9476-c60decd90b95,6d2a6ea3-2248-43a2-ad47-064c9fe3315a Several studies have shown that sea ice loss has changed food web dynamics, caused diet shifts, and contributed to a continued decline of some Arctic seabird and mammal populations.0077ea5b-e28a-4ecb-83e8-1250e7f8837c,5e94e812-1e3f-492c-aff2-30cd17439952,0b9f55e4-a97c-48c9-9b51-861ce9e3d5a6,7152e039-263f-42da-81d6-6a9be12566e6,5ac759a6-dd6e-42f7-bb61-facddf418a38 For instance, polar bear litter sizes have already declined and are projected to decline further; models suggest that sea ice breaking up two months earlier than the historical normal will decrease polar bear pregnancy success in Huntington Bay by 55%–100%.bce0fc11-cac8-4295-b49b-210fe6221d09,77fa1096-3c8e-4d78-b891-4463ba85e499

Species differ in their response to warming, acidification, and deoxygenation. This imbalance in sensitivity will lead to ecosystem reorganization, as confirmed by a number of recent ecosystem models focused on phytoplankton6f235775-bc4f-4dc8-ab04-5bb4e45761d7,d77e86de-f50f-4b78-88b0-fa43c357eff3,3c8ef20a-1560-403a-b556-b1a6278b5f40 and on entire food webs.79fffe59-14cc-48e9-b6e2-0e70906f6d28,4290e202-dc09-4a6d-9c0e-380d43afc208,2716b10a-54e8-4ba6-affc-90fd49f9e006,32fd2a21-42cb-4db0-897f-61a3b9653730,f12c40d3-4265-4e90-b852-026deec041fd,bbae7008-bc11-4aa6-b0a5-40fe1c37b197 Local extinction and range shifts of marine species due to changes in environmental conditions have already been well documented, as have the corresponding effects on community structure.e4313895-fb80-4d31-906c-2fadb9da71de,71910705-6b0b-4f61-8249-5a2d444ad5fb

Global-scale coral bleaching events in 1987, 1998, 2005, and 2015–2016 have caused a rapid and dramatic reduction of living coral cover; as the regularity of these events increases, their effects on ecosystem integrity may also increase.1f10cc44-9e2f-491c-a0ac-5ab533130318,732d35cc-654b-450e-8c8a-a61f65d3afc4,2dcec73a-c9eb-4f23-9182-ac6bd27e716c,f5c42e1b-b79b-4fe4-938e-9d50a88b21da Warming increases the likelihood of coral disease outbreaks and reduces coral calcification, reproductive output, and a number of other biological processes related to fitness.07709e73-c331-4953-a578-909aa80ae86e,f379739a-5554-49b2-954a-d008e604d801 Under the higher scenario (RCP 8.5), all shallow tropical coral reefs will be surrounded by water with Ω < 3 by the end of this century.5d518479-27a0-47b4-b30b-4b84f25fe4d2 Laboratory research finds that many coral species are negatively impacted by exposure to high CO2 conditions,4c63a501-3601-49a3-bf29-422edb72c754,6327a193-36ee-4405-a209-49b40dc289cf,062ef102-c2ac-4462-a7c4-5d927ad946d3 and field research conducted near geologic CO2 vents have found that exposure to high CO2 conditions changes some, but not all, coral communities.bed1473e-4ee0-48b5-8d08-d8c13b7d6e8f,12b7b31d-d070-4dc7-9cef-493a0d720b0a,4ef045c2-b5db-4d2d-b229-b178b62ef463,7a42ee69-7fac-47d7-9b1f-494e5e4768d4 Sea ice loss in the Arctic is expected to continue through this century, very likely resulting in nearly sea ice-free late summers by the middle of the century (Ch. 26: Alaska, KM 1).61d6757d-3f7a-4e90-add7-b03de796c6c4 Ice-free summers will result in the loss of habitats in, on, and under the ice and the emergence of a novel ecosystem in the Arctic.bf21b6fb-c6f8-431a-82a5-09c4e12fe5f5 Arctic waters are also acidifying faster than expected, in part due to sea ice loss.3edd9d0c-4aed-49d8-b248-483dcb0dfff0

Conservation measures, such as ecosystem-based fisheries management (Key Message 2) and marine-protected areas that reduce or respond to these other stressors, can increase resilience;3febdda6-8289-4929-a0f6-2184ff560ff8,4e96682c-abfa-4a04-a3ea-3a9e2e5820e1 however, these approaches have limits and can only slow the impact of climate change and ocean acidification.4290e202-dc09-4a6d-9c0e-380d43afc208 Ocean warming, acidification, and deoxygenation, among other indirect stressors, will lead to alterations in species distribution, the decline of some species’ calcification, and mismatched timing of prey–predator abundance that cannot be fully avoided with management strategies.f331e549-0949-43d9-9ec0-898c2cedb116,6c64179b-4b32-4538-b908-e24a8dd7bd53 Coral bleaching occurs on remote reefs, suggesting that even pristine reefs will be impacted in a warmer, more acidified ocean.907c63d3-2172-4d33-a185-620258ec628d,994e6b3d-5b20-4f51-9cc2-a3a523349078 Without substantial reductions in CO2 emissions, massive and sometimes irreversible impacts are very likely to occur in marine ecosystems, including those vital to coastal communities.dffeda67-312f-49ab-bbd5-a41c08bec353

New information and remaining uncertainties:

Further research is necessary to fully understand how multiple stressors, such as temperature, ocean acidification, and deoxygenation, will concurrently alter marine ecosystems in U.S. waters. More research on the interaction of multiple stressors and in scaling results from individual to population or community levels is needed.d3f2fc9b-6acf-48b0-b5d1-0f8c620e7f35,db3aaad3-b368-4b7c-99bd-e190a5c89c82,b8a1533d-6a8e-4d70-a318-a6f98bbf7dd0,8c2b8977-79a8-4ba0-9211-a94fb2864042

Most species have some capacity to acclimate to changes in thermal and chemical conditions, depending on the rate and magnitude at which conditions change, and there may be enough genetic variation in some populations to allow for evolution.b003220c-7fb3-44b3-b329-d6581d727dd4,33aaaa4b-92cf-4bf7-9a3d-a38098c0025d,eef4a3d2-e0a2-4a83-860e-2b363e488636,04a02114-e2b5-4c87-8c34-5658bc4f3c05 Some research suggests that only microbes have the ability to acclimate to the expected anthropogenic temperature and pH changes, suggesting a reduction in the diversity and abundance of key species and a change in trophic energy transfer, which underpin ecosystem function of the modern ocean.f331e549-0949-43d9-9ec0-898c2cedb116

Assessment of confidence based on evidence:

The amount of research and agreement among laboratory results, field observations, and model projections demonstrate very high confidence that ecosystem disruption has occurred due to climate change, particularly in tropical coral reef and sea ice-associated ecosystems due to the global increase of ocean temperatures. It is very likely that ecosystem disruption will intensify later this century under continued carbon emissions, as there is very high confidence that warming, acidification, deoxygenation, and other aspects of climate change will accelerate. While conservation and management practices can build resilience in some ecosystems, there is very high confidence that only reductions in carbon emissions can avoid significant ecosystem disruption, especially in coral reef and sea ice ecosystems.

This finding was derived from scenario rcp_4_5
This finding was derived from scenario rcp_8_5

References :

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