Marine species are sensitive to the physical and chemical conditions of the ocean; thus, warming, acidification, deoxygenation, and other climate-related changes can directly affect their physiology and performance.27,28,29 Differences in how species respond to physical conditions lead to changes in their relative abundance within an ecosystem as species decline or increase in abundance, colonize new locations, or leave places where conditions are no longer favorable.30,31,32,33 Such reorganization of species in marine communities can result in some species losing resources they depend on for their survival (such as prey or shelter). Other species may be exposed to predators, competitors, and diseases they have rarely encountered before and to which they have not evolved behavioral responses or other defenses.34,35,36 Climate change is creating communities that are ecologically different from those that currently exist in ocean ecosystems. Reorganization of these communities would change the ecosystem services provided by marine ecosystems in ways that influence regional economies, fisheries harvest, aquaculture, cultural heritage, and shoreline protection (Figure 9.1) (see also Ch. 7: Ecosystems, KM 1; Ch. 8: Coastal, KM 2).37,38,39,40
While climate-driven ecosystem changes are pervasive, the most apparent impacts are occurring in tropical and polar ecosystems, where ocean warming is causing the loss of two vulnerable habitats: coral reef and sea ice ecosystems.41,42 Warming is leading to an increase in coral bleaching events around the globe,7 and mass bleaching and/or outbreaks of coral diseases have occurred off the coastlines of Puerto Rico, the U.S. Virgin Islands, Florida, Hawai‘i, and the U.S.-Affiliated Pacific Islands.43,44 Loss of reef-building corals alters the entire reef ecosystem, leading to changes in the communities of fish and invertebrates that inhabit reefs.45,46 These changes directly impact coastal communities that depend on reefs for food, income, storm protection, and other services (Figure 9.1) (see also Ch. 27: Hawaiʻi & Pacific Islands, KM 4).
The extent of sea ice in the Arctic is decreasing, further exacerbating temperature changes and increasing corrosiveness in the Arctic Ocean (Ch. 26: Alaska, KM 1).15 The decline in sea ice represents a direct loss of important habitat for animals like polar bears and ringed seals that use ice for hunting, shelter, migration, and reproduction, causing their abundances to decline.47,48,49 The Arctic Ocean food web is fueled by intense blooms of algae that occur at the ice edge. Loss of sea ice is also shifting the location and timing of these blooms, impacting the food web up to fisheries and top predators like killer whales (Ch. 26: Alaska, Figure 26.4).50,51,52 Surface waters around Alaska have or will soon become permanently undersaturated with respect to calcium carbonate, further stressing these ecosystems (Ch. 26: Alaska, Figure 26.3).
The majority of marine ecosystems in the United States and around the world now experience acidified conditions that are entirely different from conditions prior to the industrial revolution (Ch. 7: Ecosystems).14,53,54 Models estimate that by 2050 under the higher emissions scenario (RCP8.5) (see the Scenario Products section of Appendix 3 for more on scenarios) most ecosystems (86%) will experience combinations of temperature and pH that have never before been experienced by modern species.54 Regions of the ocean with low oxygen concentrations are expected to expand and to increasingly impinge on coastal ecosystems.15,55,56 Warming and ocean acidification pose very high risks for many marine organisms, including seagrasses, warm water corals, pteropods, bivalves, and krill over the next 85 years.57 Ocean acidification and hypoxia (low oxygen levels) that co-occur in coastal zones will likely pose a greater risk than if species were experiencing either independently.58 Furthermore, under the higher scenario (RCP8.5), by the end of this century, nearly all coral reefs are projected to be surrounded by acidified seawater that will challenge coral growth.59
Changes in biodiversity in the ocean are underway, and over the next few decades will likely transform marine ecosystems.33 The species diversity of temperate ecosystems is expected to increase as traditional collections of species are replaced by more diverse communities similar to those found in warmer water.60 Diversity is expected to decline in the warmest ecosystems; for example, one study projects that nearly all existing species will be excluded from tropical reef communities by 2115 under the higher scenario (RCP8.5).61
Climate-induced disruption to ocean ecosystems is projected to lead to reductions in important ecosystem services, such as aquaculture and fishery productivity (Key Message 2) and recreational opportunities (Figure 9.1) (Ch. 7: Ecosystems, KM 1). Eelgrass, saltmarsh, and coral reef ecosystems also help protect coastlines from coastal erosion by dissipating the energy in ocean waves (Ch. 8: Coastal, KM 2). The loss of the recreational benefits alone from coral reefs in the United States is expected to reach $140 billion by 2100 (discounted at 3% in 2015 dollars).62 Reducing greenhouse gas emissions (for example, under RCP4.5) could reduce these cumulative losses by as much as $5.4 billion but will not avoid many ecological and economic impacts.62
Opportunities for Reducing Risk
Warming, acidification, and reduced oxygen conditions will interact with other non-climate-related stressors such as pollution or overfishing (Key Message 2). Conservation measures such as efforts to protect older individuals within species,63,64 maintain healthy fish stocks (Key Message 2),65 and establish marine protected areas can increase resilience to climate impacts.66,67,68 However, these approaches are inherently limited, as they do not address the root cause of warming, acidification, or deoxygenation. There is growing evidence that many ecosystem changes can be avoided only with substantial reductions in the global average atmospheric CO2 concentration.57,69,70
Emerging Issues and Research Gaps
Species can adapt or acclimatize to changing physical and chemical conditions, but little is known about species’ adaptive capacity and whether the rate of adaptation is fast enough to keep up with the unprecedented rate of change to the environment.71,72,73 Furthermore, ocean ecosystems are becoming increasingly novel, meaning that knowledge of current ecosystems will be a less reliable guide for future decision-making (Ch. 28: Adaptation, KM 2). Continued monitoring to measure the effects of warming, acidification, and deoxygenation on marine ecosystems, combined with laboratory and field experiments to understand the mechanisms of change, will enable improved projections of future change and identification of effective conservation strategies for changing ocean ecosystems.