Two recent events have been particularly well documented: the 2012 marine heat wave in the northwestern Atlantic Ocean (Ch. 18: Northeast, Box 18.1) and an event occurring between 2014 and 2016 in the northeastern Pacific Ocean, nicknamed the Blob (Figure 9.3) (Ch. 24: Northwest, KM 1; Ch. 25: Southwest, KM 3; Ch. 26: Alaska, KM 1). Ecosystems within these regions experienced very warm conditions (greater than 3.6°F [2°C] above the normal range) that persisted for several months or more.1,2,3 Additionally, the very warm temperatures during the 2015–2016 El Niño led to widespread coral bleaching, including reefs off of American Sāmoa, the Marianas, Guam, Hawaiʻi, Florida, and Puerto Rico (Ch. 20: U.S. Caribbean, KM 2; Ch. 27: Hawaiʻi & Pacific Islands, KM 4).7
Coastal communities are especially susceptible to changes in the marine environment,110,111 and the interaction between people and the ecosystem can amplify the impacts and increase the potential for surprises (Ch. 17: Complex Systems, KM 1). In the Gulf of Maine in 2012, warm temperatures caused lobster catches to peak 3–4 weeks earlier than usual. The supply chain was not prepared for the early influx of lobsters, leading to a severe drop in price.1 The North Pacific event, centered in 2015, featured an extensive bloom of the toxic algae Pseudo-nitzschia4,120 that led to mass mortalities of sea lions and whales and the closure of the Dungeness crab fishery.121,122 The crab fishery then reopened in the spring of 2016, normally a time when fishing effort is low. The shift in timing led to increased fishing activity during the spring migration of humpback and gray whales and thus an elevated incidence of whales becoming entangled in crab fishing gear.122 Continued warm temperatures in the Gulf of Alaska during 20165 led to reduced catch of Pacific cod.78
Extreme events other than those related to temperature can also occur in the oceans. Short-term periods of low-oxygen, low-pH (acidified) waters have occurred more frequently along the Pacific coast during intense upwelling events.15,123,124,125,126 The acidified waters were corrosive (Ω < 1) and reduced the survival of larval Pacific oysters (Crassostrea gigas) in commercial hatcheries that support oyster aquaculture127,128 and increased dissolution of the shells of pteropods, a type of planktonic snail important in many ocean ecosystems.129,130,131,132
The extreme temperatures experienced during both recent heat waves exposed ecosystems to conditions not expected for 50 or more years into the future, providing a window into how future warming may impact these ecosystems. In both regions, southerly species moved northward, and warmer conditions in the spring shifted the timing of biological events earlier in the year.1,133
In the future, the same natural patterns of climate variability associated with the heat waves in both ocean basins3,134,135,136,137 will continue to occur on top of changing trends in average conditions, leading to more extreme events relative to current averages.138
Human-caused climate change likely already contributed to the events observed in 2012 and 2015, helping drive temperatures to record levels.139,140 Ocean acidification events such as those described along the Pacific coast are already increasing and are projected to become more intense, longer, and increasingly common.53,141 The increase in intensity and frequency of toxic algal blooms has been linked to warm events and increasing temperatures in both the Atlantic and Pacific Oceans.4,120,142 Changes resulting from human activities, especially increased nutrient loads, accelerate the development of hypoxic events in many areas of the world’s coastal ocean.15,143
Opportunities for Reducing Risk
Extreme events in the oceans can lead to significant disruptions to ecosystems and people, but they can also drive technological adaptation. Several corrosive events along the Pacific Northwest coast prompted the Pacific Coast Shellfish Growers Association to work with scientists to test new observing instruments and develop management procedures.128 The hatcheries now monitor pH and pCO2 (partial pressure of carbon dioxide) in real time and adjust seawater intake to reduce acidity. Similar practices are being employed on the East Coast to adapt shellfish hatcheries to the increasing frequency of low-pH events associated with increased precipitation and runoff.144
Similarly, the need to forecast El Niño events led to the development of seasonal climate forecast systems.145 Current modeling systems make it possible to forecast temperature, pH, and oxygen conditions several months into the future.101,102,146,147,148 Operational forecasts are also being developed for harmful algal blooms149 and for the timing of Maine’s lobster fishery.150 Further engagement with users would improve the utility of these emerging forecasts.101,148
Emerging Issues and Research Gaps
The recent extreme events in U.S. ocean waters were the result of the interaction between natural cycles and long-term climate trends. As carbon emissions drive average temperatures higher and increase ocean acidification, natural climate cycles will occur on top of ocean conditions that are warmer, acidified, and have generally lower oxygen levels. A major uncertainty is whether these natural cycles will function in the same way in an altered climate. For example, the natural patterns of climate variability that contributed to the formation of the Blob show increasing variability in climate model projections.3 This suggests that similar temperature events in the North Pacific may be more likely. Unusually persistent periods of warm weather led to the formation of both the North Atlantic and North Pacific heat waves.2,134,151 Observational and modeling studies suggest that the loss of Arctic sea ice may disrupt mid-latitude atmospheric circulation patterns, making extreme weather conditions more likely (e.g., Overland et al. 2016, Vavrus et al. 2017, but see Cohen 2016152,153,154). This mechanism suggests that extremes in the ocean may be more extreme in the future, even after accounting for climate trends.
Ocean ecosystems provide economic, recreational, and cultural opportunities for all Americans. Increasing temperatures, ocean acidification, and deoxygenation are likely to alter marine ecosystems and the important benefits and services they provide. There has been progress in developing management strategies and technological improvements that can improve resilience in the face of long-term changes and abrupt events. However, many impacts, including losses of unique coral reef and sea ice ecosystems, can only be avoided by reducing carbon dioxide emissions.