Implications for global warming

Earlier we noted that one possible cause of the warming over the past century is natural variability, with one candidate mechanism being a variation in the ocean that releases heat to the atmosphere, thus causing atmospheric warming. This is a very plausible conjecture because the ocean certainly has the potential to vary on the timescale of decades to centuries; however, the empirical evidence is against it. If the ocean were to have given up its heat to the atmosphere, then it would have become cooler over the past few decades. In fact, the heat content of the ocean has increased over that period, as we saw in figure 7.6. This increase implies that global warming is not coming from the ocean, for this would cause the ocean to cool; rather, the ocean is warming because of global warming.

A temperature increase of 0.15°C is fairly small, much less than the increase in surface temperature of about 0.6°C that occurred over the second half of the twentieth century. The temperature increase of the abyssal ocean is likely to be even smaller, although observations are sparse. The small increase is important because it tells us that global warming still has a long way to go. Eventually, except for possible effects caused by changes in the ocean circulation, we expect that the increase in temperature of the deep ocean will be similar to that at the surface. Even if we were to stop adding greenhouse gases to the atmosphere today, it would take a very long time for the deep ocean temperature to rise to that level, and until that time, the surface temperature will keep rising, as we discuss later in this chapter.

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