Natural Variability

Perhaps, a more detailed discussion of what is meant by natural variability is called for. Both historical data and the results of GCMs show that both the globally averaged temperatures and regional and local temperatures will have their ups and downs. Historical changes that have occurred both recently and during the past thousand years bear out the fact that sizeable changes in the climate might occur naturally in the climate system. This is what climatologists call natural variability. For example, there was a relatively cool period that occurred between about 4500 and 2500 years ago, ending at about the time of the dawn of the Roman Empire.4 The decline and fall of the empire and the beginning of the Dark Ages about A.D. 500 to 1000 saw a return to colder climates. After this period was a relatively warm period called the Medieval Optimum, which lasted from about A.D. 1100 to 1300 in Europe but apparently not in the northern hemisphere as a whole as can easily be seen in Figure 3.7, which shows a rather persistent cooling of the northern hemisphere throughout the period. It most likely occurred as a result of warming of the North Atlantic Ocean. During this period, wine grapes flourished in England, and the Arctic ice pack retreated, to the extent that Iceland and Greenland were settled by Europeans for the first time. About 1450, there was a return to a cooler climate. This period, which lasted through most of the 19th century, is known as the Little Ice Age. One can easily see why some people believe that the recent warming that has occurred is simply the result of natural climate variability as the Earth emerges from the most recent cold period. One of my graduate students recently decided to study past climate variability to see whether he could learn anything about future natural climate trends based on what has happened in the last 1000 years.7 Based on statistical analysis, we concluded that natural variability should have caused warming of a few tenths of a degree until about 1940, but since that time, the Earth should be cooling. Extrapolating statistical data into the future is risky business at best, but the result provides food for thought because it might refute one of the arguments against the supposition that the recent warming only reflects natural variability. The argument goes like this. During the early 20th century when global emissions of CO2 were small, the global temperature was increasing rapidly, but between 1940 and 1970, when CO2 releases had grown substantially, the increase in global temperature was nil. If this preliminary hypothesis is correct, the increase in temperature early in the century was caused mostly by natural variations in the climate. Later on, the natural tendency for the climate to cool was opposed to the greenhouse effect, and the climate neither warmed nor cooled. After about 1970, the climate began to warm again, as the greenhouse warming became stronger than the natural cooling. But why did the cooling episode occur? I believe that it is variability of the thermohaline circulation. It happens that the ocean temperatures at depths of 500 to 2000 m in both the North Atlantic and North Pacific warmed while the surface temperatures were cooling during this period.8 The deep ocean was taking all the heat! I will say more when in the next chapter, I respond to the skeptics' Challenge 12. In any case, detecting a greenhouse warming signal within the historical temperature records is clear. It is clear that the globally averaged temperature of Earth's surface has risen during the present century, and most importantly, it has done so over a shorter time period than at any time since the end of the last ice age about 15,000 years ago.

The warming of about eight tenths of a degree centigrade that has occurred during the last hundred years is faithfully predicted by the GCMs. It is certain that the climate of the Earth will warm as the atmospheric concentrations of greenhouse gases increase. It is very likely that it will warm, on average, at about the rate predicted by the GCMs that exhibit medium sensitivity. These models predict that the global temperature will increase by 2°C to 5°C by 2100, depending on how committed we are to reducing greenhouse concentrations. (I will be more specific about this later when I discuss various energy use scenarios.) Moreover, it is important to remember that high latitudes will warm as much as two to three times as much as this globally averaged value. This we know from both models and historical data.

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