Has The ocEan Warmed

Let us begin our discussion of the ocean's role by asking a simple question: Has the ocean interior itself warmed? The ocean itself has in fact warmed over the years, as shown in figure 7.6, which shows an increase in the heat content from 1955. (There are not sufficient data before this time to extend the time series further into the past.) The heat content is defined as the heat capacity of seawater (which is almost a constant) multiplied by the change in temperature, integrated over the entire mass of the world's oceans. Most of the increase is associated with an increase in temperature in the upper several hundred meters of the ocean; measurements in abyssal ocean are much sparser, although what data there are show a similar but smaller trend, with about two-thirds of the total increase in heat content occurring in the upper 700 m (in spite of the fact that the ocean is on average almost 4,000 m deep).

The ordinate of the top panel of figure 7.6 is the total heat content (i.e., the internal energy), which is perhaps not a meaningful number, so let us make a translation.

Figure 7.6. Top: The global heat content for the upper 700 m of the ocean (solidblack line, with shading indicating uncertainty) and upper 100 m of the ocean (dotted line, with thin solid lines indicating uncertainty). Bottom: Increase in sea level as estimated from direct measurements (black line and shading) and by combining the contributing components (dotted line and thin solid lines). The time series are all relative to 1961 and smoothed with a three-year running average. Source: Adapted from Domingues et al., 2008.

Figure 7.6. Top: The global heat content for the upper 700 m of the ocean (solidblack line, with shading indicating uncertainty) and upper 100 m of the ocean (dotted line, with thin solid lines indicating uncertainty). Bottom: Increase in sea level as estimated from direct measurements (black line and shading) and by combining the contributing components (dotted line and thin solid lines). The time series are all relative to 1961 and smoothed with a three-year running average. Source: Adapted from Domingues et al., 2008.

The total heat capacity of the upper 700 m of the entire ocean is about 1024 JK-1 (the area of the ocean is approximately 3.6 X 1014 m2, and the heat capacity of water is about 4 X 103 J kg-1 K-1), so that an increase in heat content over the period 1960 to 2000 of 15 X 1022 J

corresponds to a temperature increase of about 0.15°C over this period. The rate of increase is obviously not smooth (in fact, some recent estimates suggest that heat content went down in the few years following 2003), and these fluctuations are likely caused by some form of natural variability in the climate system. Nonetheless, the overall upward trend is unmistakable.

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