Rising Ocean Temperatures

Cores from corals, ocean sediments, ice records, and other indirect temperature measurements indicate the recent rapid increase of ocean temperature is the greatest that has occurred in at least the past millennium and can only be reproduced by climate models with the inclusion of human-caused sources of heat-trapping gas emissions.2,3,4,20 The ocean is a critical reservoir for heat within Earth’s climate system, and because of seawater’s large heat storing capacity, small changes in ocean temperature reflect large changes in ocean heat storage. Direct measurements of ocean temperatures show warming beginning in about 1970 down to at least 2,300 feet, with stronger warming near the surface leading to increased thermal stratification (or layering) of the water column.5,6 Sea surface temperatures in the North Atlantic and Pacific, including near U.S. coasts, have also increased since 1900.21,15 In conjunction with a warming climate, the extent and thickness of Arctic sea ice has decreased rapidly over the past four decades.7,8,22 Models that best match historical trends project seasonally ice-free northern waters by the 2030s.23,24,25

Figure 24.1: Observed Ocean Warming

Climate-driven warming reduces vertical mixing of ocean water that brings nutrients up from deeper water, leading to potential impacts on biological productivity. Warming and altered ocean circulation are also expected to reduce the supply of oxygen to deeper waters, leading to future expansion of sub-surface low-oxygen zones.26,27 Both reduced nutrients at the surface and reduced oxygen at depth have the potential to change ocean productivity.9 Satellite observations indicate that warming of the upper ocean on year-to-year timescales leads to reductions in the biological productivity of tropical and subtropical (the region just outside the tropics) oceans and expansion of the area of surface waters with very low quantities of phytoplankton (microscopic marine plants) biomass.28,29 Ecosystem models suggest that the same patterns of productivity change will occur over the next century as a consequence of warming during this century, perhaps also with increasing productivity near the poles.30,31 These changes can affect ecosystems at multiple levels of the food web, with consequent changes for fisheries and other important human activities that depend on ocean productivity.19,32

Other changes in the physical and chemical properties of the ocean are also underway due to climate change. These include rising sea level,17 changes in upper ocean salinity (including reduced salinity of Arctic surface waters) resulting from altered inputs of freshwater and losses from evaporation, changes in wave height from changes in wind speed, and changes in oxygen content at various depths – changes that will affect marine ecosystems and human uses of the ocean in the coming years.19

Figure 24.2: Ocean Impacts of Increased Atmospheric Carbon Dioxide

While the long-term global pattern is clear, there is considerable variability in the effects of climate change regionally and locally because oceanographic conditions are not uniform and are strongly influenced by natural climate fluctuations. Trends during short periods of a decade or so can be dominated by natural variability.33 For example, the high incidence of La Niña events in the last 15 years has played a role in the observed temperature trends.34 Analyses35 suggest that more of the increase in heat energy during this period has been transferred to the deep ocean (see also Ch. 2: Our Changing Climate). While this might temporarily slow the rate of increase in surface air temperature, ultimately it will prolong the effects of global warming because the oceans hold heat for longer than the atmosphere does.

Interactions with processes in the atmosphere and on land, such as rainfall patterns and runoff, also vary by region and are strongly influenced by natural climate fluctuations, resulting in additional local variation in the observed effects in the ocean. Marine ecosystems are also affected by other human-caused local and regional disturbances such as overfishing, coastal habitat loss, and pollution, and climate change impacts may exacerbate the effects of these other human factors.