Of course the models might be wrong and the temperature increase might come from natural causes—natural variability in the climate system, such as we discussed in chapter 6. Thus, the observed warming might not be anthropogenic but rather might be related to our emergence from the Little Ice Age over the past century. This idea is not so much an argument as a speculation because no viable mechanism has been posited that could cause the warming seen over the course of the past century, except possibly for the influence of the ocean, a topic we deal with in the next section. We might imagine that for some reason the ice sheets have retreated, decreasing the albedo and warming the planet, or we might imagine that the clouds have changed configuration in such a way as to cause warming, but there is no evidence for either possibility. If the climate system were so sensitive that such changes were possibilities, we might expect to see evidence of similar changes in past climates. The climate has varied, of course, but looking back at figure 7.3, we see that over the past 1,000 years it has never varied in the same manner as it has in the past 100—the rate of increase of temperature is, so far as we can tell, unprecedented.
One oft-mentioned possibility is that global warming arises because of variations in the sun's output. On the decadal and centennial timescales, variations in the sun's output occur mainly through the solar cycle, a cycle of solar magnetic activity that affects sunspots. The cycle has main periods of about 11 and 22 years, and the former period modulates, albeit slightly, the total solar flux coming into Earth's atmosphere, mainly at ultraviolet wavelengths. The cycle itself cannot cause global warming, but it is possible that there may be longer term variations in the sun's output that modulate the solar cycle, and it is sometimes hypothesized that the Little Ice Age might have been caused by such variations. Indeed, there was a period from about 1645 to 1715, known as the Maunder Minimum, when sunspots seem to have been exceedingly rare, and this period coincided with low temperatures in the middle of the Little Ice Age. Although there is some uncertainty, solar irradiance during the Maunder Minimum is believed to have been about 0.2% less than that of today, or about 0.7 W m-2 less.8 (The change in the ultraviolet component is larger, about 0.7%, but is a small fraction of the total.) The increase in solar irradiance since 1750 is estimated to be at most 0.3 W m-2, and the change since 1900, much less than that. The total solar radiative change is thus considerably less than the changes in radiative forcing that have arisen through changes in the levels of greenhouse gases—a 50% increase of C02, for example, is roughly equivalent to a forcing of 2 W m-2. Unless there are mechanisms in the atmosphere of which we are totally unaware, perhaps somehow amplifying the changes in the ultraviolet component, variations in the solar output are almost certainly insufficient to have caused the bulk of the global warming that we have seen over the past century. If the climate system were so sensitive to variations in the solar cycle, we would expect to have seen a more pronounced variation across the Maunder Minimum.
Finally, we might plausibly ask, could causality of the greenhouse gas-temperature relationship be reversed— that is, might the recent warming be causing the carbon dioxide increase? First, for this to be the case our comprehensive climate models would have to be completely wrong, but that should certainly not be wholly discounted. Second, and perhaps more compelling, no known mechanism could give rise to this reversal of causality on the timescales of decades on which we have seen the warming take place. There are some suggestions in the paleo-record that changes in temperature have preceded changes in carbon dioxide in past ice ages. However, these changes occur on much longer timescales and are likely the result of a positive feedback in which temperature changes cause changes in the ocean circulation, which affect the absorption of carbon dioxide, which changes the temperature, and so on (although the precise mechanism of this sequence of events remains a mystery). By far the simplest and most compelling explanation of the current warming is that it is caused by the increase in greenhouse gas concentration.
So what does the future hold, and how certain can we be about it? Before we can answer that, we need to understand the role of the ocean in global warming, so let's turn to that.
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