finding 9.3 : key-message-9-3

Marine ecosystems and the coastal communities that depend on them are at risk of significant impacts from extreme events with combinations of very high temperatures, very low oxygen levels, or very acidified conditions. These unusual events are projected to become more common and more severe in the future (very likely, very high confidence), and they expose vulnerabilities that can motivate change, including technological innovations to detect, forecast, and mitigate adverse conditions.



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:

Marine heat waves have been described as regions of large-scale and persistent positive sea surface temperature anomalies that can vary in size, distribution, timing, and intensity akin to their terrestrial counterparts.963c8f97-8680-43df-8b28-59a376735f17,b8a1491a-3642-4d34-acec-b2b79bd57b8e Well-documented marine heat waves have recently occurred in the northwest Atlantic in 20121dfd2171-2be3-40b2-a8e2-c0df84ec462a,273a1e86-544e-4a0a-a95b-c0855e9dcc32,8d65a2de-6cac-407f-8906-79fd025c282c and the North Pacific in 2014–2016.e2d6b1b6-6e11-4e40-a7bc-d777f2bbba0f,1d63deea-30f3-4fa1-ba08-3a969b23aa16

Each of these events resulted in documented impacts to ecosystems and, in many cases, to the human communities to which they were connected. The recent major events in the U.S. northwest Atlantic and North Pacific led to economic challenges in the American lobster, Dungeness crab, and Gulf of Alaska Pacific cod fisheries.1dfd2171-2be3-40b2-a8e2-c0df84ec462a,e2d6b1b6-6e11-4e40-a7bc-d777f2bbba0f,e950a4ad-cf0c-4d55-9c68-1c358c2082ec,083694ee-fb95-4c12-bd5c-8edecad138a1

Abrupt warming can induce other ecosystem-level impacts. The North Pacific event featured an extensive bloom of the harmful algae Pseudo-nitzschia5300d778-0b4e-44bb-9449-c6a36ead3636,5509daeb-bffb-4395-8582-1fef669a7a49 that led to mass mortalities of sea lions and whales and the closure of the Dungeness crab fishery. The increase in intensity and occurrence of these toxic algal blooms has been linked to warm events in both the Atlantic and the Pacific.5300d778-0b4e-44bb-9449-c6a36ead3636,5509daeb-bffb-4395-8582-1fef669a7a49,59d0bcfb-805b-472d-b6fe-3b70bacc3d25 Abrupt warming was inferred to trigger the expansion of the North Pacific oxygen minimum zone through reduced oxygen solubility and increased marine productivity.8e65123f-e597-4b51-8321-3f78fdf3b615

Extreme events with corrosive (Ω < 1) and/or low oxygen conditions can occur when deep waters, which are generally corrosive and have low oxygen levels, are brought into the coastal area during upwelling. They can also occur in response to the delivery of corrosive freshwater from the landscape, ice melting, and storms. These conditions now occur more frequently in coastal waters of the Pacific coast of the United States.a080ee24-4ede-4f85-b205-79175e0dc77f,21aa7761-7792-4b6a-b172-7fe3ecd83d13,14ea4d85-e2b5-48d8-b5c8-2d82801e8383,83d5cc5a-349f-406c-a570-1d9a0c0dc75b,7a925d3b-93c1-41ed-8b29-4f283b252c48,9cf6afb1-6606-43f2-a1d2-bfb6a13f96b7,29d05c83-65ba-47ed-b102-d5aeb5e5ef98,32a7c6b7-16ce-49f8-8667-7343d9d40ea9,6b6ee187-783c-447f-943b-25bd1a0a2f9b Such events have led to the elevated mortality of coastal shellfish in hatcheriesbacbf706-64ce-4d4c-95e5-04bc1651fe96 and die-offs of crabs and other animals living on the ocean bottom.b7708c1a-0eb5-47db-a089-ff40be29c884

Heat wave, high-acidity, and low-oxygen events are all produced by variability in the system occurring on timescales ranging from days to years. For example, recent marine heat waves have been linked to natural climate modes such as the North Atlantic Oscillation, Atlantic Multidecadal Oscillation, Pacific Decadal Oscillation, or North Pacific Gyre Oscillation, which change over several years.257c4627-9b78-41ce-a824-ebfe008a4c88,963c8f97-8680-43df-8b28-59a376735f17 Persistent weather patterns lasting several months can further amplify conditions in the ocean, leading to extreme conditions.e2d6b1b6-6e11-4e40-a7bc-d777f2bbba0f,273a1e86-544e-4a0a-a95b-c0855e9dcc32,8d65a2de-6cac-407f-8906-79fd025c282c These climate modes and atmospheric conditions occur on top of the long-term trends caused by global climate change. Thus, as climate change progresses, events with temperatures above a certain level, oxygen below a certain level, or pH below a specified level will occur more frequently and will last longer.430ab97c-0236-4d1e-8291-f3a2c1bce65a,83ad6bee-06cc-4ae3-8691-8a93dbec9145,5ded3542-0f25-487e-901c-0e61408943b4,326a3884-ed4d-4194-b72d-f53a942119ea

The intensity of corrosive events along the upwelling margin of the Pacific coast of the United States is increasing due to more intense winds over the past decade and ocean acidification.5d047224-4e72-46d1-87f5-042c9617472d,41f6e597-2a3d-4a42-886f-54866a25a2e0,b7708c1a-0eb5-47db-a089-ff40be29c884,fbb8d5f6-f170-4f50-aae4-0db84cf891c8 In Alaska waters, these events are associated with freshwater inputs and storm events that may also have a link to climate change.83d5cc5a-349f-406c-a570-1d9a0c0dc75b,7a925d3b-93c1-41ed-8b29-4f283b252c48,9cf6afb1-6606-43f2-a1d2-bfb6a13f96b7,29d05c83-65ba-47ed-b102-d5aeb5e5ef98,32a7c6b7-16ce-49f8-8667-7343d9d40ea9,480b9fbf-ed78-4f41-be7e-c6023a317e6b

There is ample evidence that extreme events motivate adaptive change in human systems. For example, Hurricane Katrina and Superstorm Sandy motivated communities near the affected areas to expand planning against future storms.8f522585-8c4d-4c72-ab01-76c071942cbc,8cbef4be-90a3-4191-b203-4f967eb0e8a4 The 2012 North Atlantic heat wave prompted the development of a forecast system to help Maine’s lobster fishery avoid future supply chain disruptions (Ch. 18: Northeast).16854e43-c4a6-4fe4-bc8a-16d82fdcb38e The impact of corrosive waters on shellfish hatcheries in the Pacific Northwest motivated the development of new technology to monitor and manage water chemistry in shellfish hatcheries.bacbf706-64ce-4d4c-95e5-04bc1651fe96

New information and remaining uncertainties:

The description above assumes that natural modes of climate variability remain the same and can be simply added to baseline conditions set by the global climate. There is evidence that some natural climate modes may change in the future. As mentioned in the narrative, the climate oscillations linked to the 2014–2016 event in the North Pacific increase in amplitude in climate model projections.257c4627-9b78-41ce-a824-ebfe008a4c88,e8aca444-ea23-4765-be6f-4af6dd28af8c,00cf8d09-39c0-419f-b5c3-8c89ea40008f This suggests that extreme events will be more likely in the future, even without accounting for the shift to a warmer temperature baseline. Declines in Arctic sea ice are also hypothesized to impact future climate variability by causing the atmospheric jet stream to get stuck in place for days and weeks (e.g., Overland et al. 2016; Vavrus et al. 2017; but see Cohen 2016).09c51541-b400-4676-80c9-73440de4033f,94300b4b-a766-4038-854c-82949a11f525,2bde765a-e5ff-428f-a2cd-56303c3e5523 This has the potential to create persistent warm (where the jet stream is displaced to the north) and cold (where the jet stream moves south) weather conditions over North America.09c51541-b400-4676-80c9-73440de4033f,94300b4b-a766-4038-854c-82949a11f525 These conditions are similar to the precursors to both the northwestern Atlantic and North Pacific heat waves.e2d6b1b6-6e11-4e40-a7bc-d777f2bbba0f,273a1e86-544e-4a0a-a95b-c0855e9dcc32

For biogeochemistry, other factors may amplify the global changes at the regional level as well, especially in the coastal environment. These factors include local nutrient runoff, freshwater input, glacial runoff, spatial variability in retentive mechanisms, variability in upwelling strength, cloud cover, and stability of sedimentary deposits (for example, methane).5d047224-4e72-46d1-87f5-042c9617472d,fbb8d5f6-f170-4f50-aae4-0db84cf891c8,a7e4205e-2a09-4b92-b90f-0de72be2fb30,8d65a2de-6cac-407f-8906-79fd025c282c,6b6ee187-783c-447f-943b-25bd1a0a2f9b,480b9fbf-ed78-4f41-be7e-c6023a317e6b Most of the factors will amplify the global trends toward lower oxygen and pH, leaving these estimates to be conservative. In addition, temperature, oxygen, and pH have synergistic effects that provide some uncertainties in the projected events.430ab97c-0236-4d1e-8291-f3a2c1bce65a

Assessment of confidence based on evidence:

Because there is very high confidence and very high likelihood that oceans will get warmer, more acidified, and have lower oxygen content in response to elevated atmospheric carbon dioxide levels,5d047224-4e72-46d1-87f5-042c9617472d it is very likely and there is very high confidence that extreme events will occur with increased intensity and frequency in the future.1d63deea-30f3-4fa1-ba08-3a969b23aa16,a1ed7e96-eb33-4566-9bbd-1684f800e9da,83ad6bee-06cc-4ae3-8691-8a93dbec9145,326a3884-ed4d-4194-b72d-f53a942119ea,4a5d60f7-264a-470d-a116-e1c9476e7c9f

References :

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