Ocean Thermal Expansion

The ocean is by far the most important heat reservoir in the climate system, with a heat storage capacity more than 1,000 times larger than that of the atmosphere. In fact, measurements of changes in ocean heat content show that 80 to 90 percent of the heating associated with human greenhouse gas (GHG) emissions over the past 50 years has gone into raising the temperature of the oceans (Levitus et al., 2001 ; Tren-berth and Fasullo, 2010) (see Figure 7.3). One consequence of the large thermal capacity of the oceans is that it takes many years for the climate system to warm in response to GHG emissions; for example, as discussed in Chapter 6, global surface temperatures would continue to warm for many decades even if GHG concentrations and other climate forcings were stabilized at present values). Moreover, as heat is absorbed by the oceans, the volume of the water expands, causing sea levels to rise. Approximately 50 percent of the observed sea level rise since the late 19th century has been attributed to thermal expansion of the warming oceans (Gornitz et al., 1982).

Ocean expansion is neither spatially uniform nor steady in time (Levitus et al., 2009; Lozier et al., 2008). Over the last half century, ocean thermal expansion has varied from approximately one quarter of the total sea level contribution (1961-1993) to a

FIGURE 7.3 Increase in globally averaged ocean heat content (HC) for the topmost 700 m of the ocean. The dashed black line represents estimates from Levitus et al. (2005); the red line shows estimates from Levitus et al. (2009). For both lines, the values are calculated with respect to the 1957 to 1990 periods. The solid black line shows the positive trend in ocean heat content from 1969 to 2008. Units are 1022 Joules. SOURCE: Levitus et al. (2009).

FIGURE 7.3 Increase in globally averaged ocean heat content (HC) for the topmost 700 m of the ocean. The dashed black line represents estimates from Levitus et al. (2005); the red line shows estimates from Levitus et al. (2009). For both lines, the values are calculated with respect to the 1957 to 1990 periods. The solid black line shows the positive trend in ocean heat content from 1969 to 2008. Units are 1022 Joules. SOURCE: Levitus et al. (2009).

little over one half (1993-2003; Bindoff et al., 2007). The absorption of heat energy by the oceans varies from place to place on interannual and decadal time scales, and the warmer waters of the tropics and near the ocean surface expand more in response to a given temperature increase than the cold waters at high latitude and at depth (Fofonoff, 1985). Monitoring spatial and temporal heat content changes of the ocean is thus important for predicting both the global average and spatial patterns of future sea level rise, as is developing a better understanding of mixing processes that distribute heat in the oceans.

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