- Technical Contributor:
- Vicky W. Y. Lam, University of British Columbia
- USGCRP Coordinators:
- Fredric Lipschultz, Senior Scientist and Regional Coordinator
- Apurva Dave, International Coordinator and Senior Analyst
<b>Pershing</b>, A.J., R.B. Griffis, E.B. Jewett, C.T. Armstrong, J.F. Bruno, D.S. Busch, A.C. Haynie, S.A. Siedlecki, and D. Tommasi, 2018: Oceans and Marine Resources. In <i>Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II</i> [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 353–390. doi: 10.7930/NCA4.2018.CH9
Oceans and Marine Resources
From tropical waters in Hawai‘i and Florida, to temperate waters in New England and the Pacific Northwest, to cold Arctic seas off of Alaska, the United States has some of the most diverse and productive ocean ecosystems in the world. Americans rely on ocean ecosystems for food, jobs, recreation, energy, and other vital services, and coastal counties of the United States are home to over 123 million people, or 39% of the U.S. population (Ch. 8: Coastal).8 The fishing sector alone contributes more than $200 billion in economic activity each year and supports 1.6 million jobs.9 Coastal ecosystems like coral and oyster reefs, kelp forests, mangroves, and salt marshes provide habitat for many species and shoreline protection from storms, and they have the capacity to sequester carbon.10,11,12,13
The oceans play a pivotal role in the global climate system by absorbing and redistributing both heat and carbon dioxide.14,15 Since the Third National Climate Assessment (NCA3),16 understanding of the physical, chemical, and biological conditions in the oceans has increased, allowing for improved detection, attribution, and projection of the influence of human-caused carbon emissions on oceans and marine resources.
Human-caused carbon emissions influence ocean ecosystems through three main processes: ocean warming, acidification, and deoxygenation. Warming is the most obvious and well-documented impact of climate change on the ocean. Ocean surface waters have warmed on average 1.3° ± 0.1°F (0.7° ± 0.08°C) per century globally between 1900 and 2016, and more than 90% of the extra heat linked to carbon emissions is contained in the ocean.15 This warming impacts sea levels, ocean circulation, stratification (density contrast between the surface and deeper waters), productivity, and, ultimately, entire ecosystems. Changes in temperature in the ocean and in the atmosphere alter ocean currents and wind patterns, which influence the seasonality, abundance, and diversity of phytoplankton and zooplankton communities that support ocean food webs.17,18
In addition to warming, excess carbon dioxide (CO2) in the atmosphere has a direct and independent effect on the chemistry of the ocean. When CO2 dissolves in seawater, it changes three aspects of ocean chemistry.15,19,20,21 First, it increases dissolved CO2 and bicarbonate ions, which are used by algae and plants as the fuel for photosynthesis, potentially benefiting many of these species. Second, it increases the concentration of hydrogen ions, acidifying the water. Acidity is measured with the pH scale, with lower values indicating more acidic conditions. Third, it reduces the concentration of carbonate ions. Carbonate is a critical component of calcium carbonate, which is used by many marine organisms to form their shells or skeletons. The saturation state of calcium carbonate is expressed as the term Ω. When the concentration of carbonate ions in ocean water is low enough to yield Ω < 1 (referred to as undersaturated conditions), exposed calcium carbonate structures begin to dissolve. For simplicity, the terms ocean acidification and acidifying will refer to the suite of chemical changes discussed above.
Increased CO2 levels in the atmosphere are also causing a decline in ocean oxygen concentrations.15 Deoxygenation is linked to ocean warming through the direct influence of temperature on oxygen solubility (warm water holds less oxygen). Warming of the ocean surface creates an enhanced vertical density contrast, which reduces the transfer of oxygen below the surface. Ecosystem changes related to temperature and stratification further influence oxygen dynamics by altering photosynthesis and respiration.22,23
All three of these processes—warming, acidification, and deoxygenation—interact with one another and with other stressors in the ocean environment. For example, nitrogen fertilizer running off the land and entering the Gulf of Mexico through the Mississippi River stimulates algal blooms that eventually decay, creating a large dead zone of water with very low oxygen24,25 and, simultaneously, low pH.26 Warmer conditions at the surface slow down the rate at which oxygen is replenished, magnifying the impact of the dead zone. Changes in temperature in the ocean and in the atmosphere affect ocean currents and wind patterns that can alter the dynamics of phytoplankton blooms,17 which then drive low-oxygen and low-pH events in coastal waters.
Transformations in ocean ecosystems are already impacting the U.S. economy and the coastal communities, cultures, and businesses that depend on ocean ecosystems (Key Message 1). Fisheries provide the most tangible economic benefit of the ocean. While the impact of warming on fish stocks is becoming more severe, there has also been progress in adapting fisheries management to a changing climate (Key Message 2). Finally, the ability for climate-related changes in ocean conditions to impact the United States was made especially clear by major marine heat wave events that occurred along the Northeast Coast in 2012 and along the entire West Coast in 2014–2016 (Key Message 3). During these events, the regions experienced high ocean temperatures similar to the average conditions expected later this century under future climate scenarios. Ecosystem changes included the appearance of warm-water species, increased mortality of marine mammals, and an unprecedented harmful algal bloom, and these factors combined to produce economic stress in some of the Nation’s most valuable fisheries.
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