In my last column I discussed some of the good and bad things that might arise as a result of global warming. Perhaps surprisingly, the good seemed to outweigh the bad. Now, as if we don't have enough to worry about, I want to talk about the dangers of global cooling.

I propose the following questions:

1. Does global cooling form part of a likely future?

2. When and why might the Earth grow cooler?

3. How much confidence should we place in the first two answers?

The evidence that global cooling probably will occur in the future comes from the study of the past. Let's travel backward in time. From about 1640 to 1720, the Earth went through a cool period that is often called the "Little Ice Age." The Thames River froze hard enough to support a coach and horses, the height above sea level at which certain crops (such as grapes) could be grown went down, and some northerly settlements were abandoned. This had been preceded, from roughly 750 to 1250, by a long warm period known as the "Medieval Warming Period," or sometimes the "Little Climatic Optimum." Before that there had been a steady cooling with occasional warm spells, which lasted from around 3,000 BC all the way to about 750 AD. Still moving backward, we had a hot period, the "Holocene Optimum," from 5,000 BC to 3,000 BC. This was preceded by the most recent true Ice Age, a time of widespread glaciers and general cold that ended roughly 12,000 years ago.

In geological terms, all of these events form the recent past. Looking back farther, 50,000 years ago much of North America, Northern Europe and Northern Asia was covered with thick sheets of ice. New England and Scandinavia, for example, lay beneath a continuous layer of ice between one and two miles deep. In fact, for the past million years, the times when everything north of the Great Lakes was covered with ice sheets like this seem to have alternated with periods as warm or warmer than today. It is fair to say that during the past million years, cold and widespread glaciers have been more the rule than the exception. And if we look back even farther, we find evidence of still more extreme events. Between 750 and 550 million years ago, the whole Earth was enveloped in ice to within ten degrees of the equator.

At this point, a skeptic is entitled to ask how we know any of this. Galileo didn't make the first imperfect thermometer until the beginning of the seventeenth century. If anyone was making notes on climate and weather five or ten thousand years before that, those records have not been handed down for us to study.

However, we do have records. They were laid down not by humans but by Nature. Evidence of past climate is provided by a number of independent sources. The width and density of tree rings gives information on both temperatures and rainfall. Trapped gas bubbles in ice cores taken from deep present-day ice sheets in Greenland, Antarctica, and elsewhere offer direct samples of the atmosphere in earlier times. Fossil pollens define shifts in vegetation patterns, while marine sediments and coral growth give evidence of changes in sea temperatures and salinity (and hence of ice cap melting or formation). Finally, the advance and retreat of glaciers is revealed to geologists by the materials left behind. The time of occurrence of all these observations can be estimated using radioactive carbon dating.

Everything adds up to an undeniable conclusion: for the past thousands and millions of years, long before humans were able to influence events, our planet went through prolonged periods of heating and cooling. In the cold periods, the ice sheets covering land were so widespread that sea levels went down by as much as 300 feet. During the hot spells, rises in sea level covered many of today's coastal shores. Through all these periods there were smaller ripples superimposed on the general pattern of climate change.

The historical record tells us what happened. It does not say why. And it is fair to admit that we are still hazy as to the basic natural causes of global warming and cooling. I can list a variety of events, any or all of which have been suggested as a prime cause. Volcanic activity can change the amount of greenhouse gases in the atmosphere. Changes in the orbit eccentricity or inclination of the Earth as it moves around the sun may affect the distribution and amount of solar radiation warming the planet. And then there is the biggest unknown of all: the possible change over time in energy output of the sun itself.

We have no "fossil record" of solar activity to guide us here. We do, however, have a couple of suggestive and relatively recent data points. Britain's Royal Observatory was established in 1675. The first Astronomer Royal, John Flamsteed, made observations of the sun from about 1662 until his death in 1719. Note that this period falls entirely within the "Little Ice Age." In all that time, Flamsteed saw only a handful of sunspots. Admittedly, he could observe only on clear days, and this was England. However, it is tempting to make the correlation between a sun without spots and a colder climate. We certainly know that there is a sunspot cycle with an eleven-year period, and that this affects the ionosphere and electrical activity on Earth. Variations with longer periods are not merely possible, they are likely. It is a measure of our ignorance that we know more about the periodic variation of many stars than of our own sun.

>From a perspective that concerns itself with times of ten thousand or a million years, natural events have usually dwarfed human powers, had any humans been present at the time, to cause or deny change. Given the history of recurring Ice Ages and our current uncertainty as to basic causes, it is probably wisest to prepare for a future that resembles the past.

Of course, our ability to influence Nature may be radically different in that future. A thousand years from now we may have large-scale climate control, inducing or banishing new Ice Ages with a flourish of our technological wand. Until such wizardry is actually in hand, however, I would like to hang on to our fossil fuels. We may need every ounce of them when the glaciers roll back in.

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