Figure

Change in carbon dioxide levels since beginning of the Industrial Revolution

That the amount of carbon dioxide in our atmosphere, as evidenced at the carbon dioxide laboratory, has increased is undisputed. What is disputed, however, is what has caused this rise, and what (if anything) it will mean to global climate.

Few, except those who for political or economic reasons (such as representatives of the big oil companies and the politicians that they have bought off ), dispute that humanity is rapidly changing the composition of the atmosphere (although there is still great debate about whether those changes are causing a rise in mean global temperature, also known as global warming). The carbon dioxide is largely coming from automobiles and human industry. These anthropogenic, or human-induced, sources of gas go beyond carbon dioxide: There are also methane, chlorofluorocarbons, sulfur dioxide, and nitrogen oxides, the levels of which have been rising dramatically since the Indus trial Revolution. All of these gases have the ability to absorb infrared radiation and reradiate it back to Earth, producing the well-known greenhouse effect. Predictions about the possibility of future global warming over the next decades and centuries to come from a class of models known as global circulation models (GCMs). A starting point of these models is the prediction that the amount of carbon dioxide in the atmosphere will double over the next century. Most climate scientists agree that this doubling is sure to have profound ecological effects, including greater temperature increases in midlatitude, temperate, and continental interior regions relative to the rest of the globe; decreases in precipitation in these same midlatitude regions; and an increase in severe storm patterns.

The debate about global warming can be categorized as follows: first, that it is not happening; second, that carbon dioxide and other greenhouse gases are rising but that this rise is caused by volcanic sources, not people; third, even though greenhouse gases are rising, they will have no effect on current and future climate. Thus the naysayers. A 2004 novel written by the inimitable (and curiously science-hating) mega-author Michael Crichton, State of Fear, uses each of the points above to argue against any sort of human-caused global warming. But what do the climate scientists, not the authors and politicians, say?

A new computer model developed by scientists from the University of East Anglia in England has factored in the role of human-made global warming. This model suggests that the human input of greenhouse gases will indeed delay the next ice advance by perhaps as much as 50,000 years—but that when it does arrive, it will be an even more extreme and longer period of ice than otherwise might have occurred. The amount of hydrocarbons that can be burned is finite, and sooner or later they will run out.

Although many experts do think that human-produced global warming could postpone the next ice advances by many millennia, there is another school of thought suggesting that the rapid global warming that is now underway may actually trigger the next glacial advance. According to this model, seemingly paradoxically, the advent of global warming now could kick us back into the time of ice. Let us look in more detail at the carbon dioxide curve, and another curve as well—the methane curve—and let us take a less obvious perspective— that anthropogenic global warming is not just a two-century phenomenon but one that began soon after humans discovered agriculture.

TEN THOUSAND YEARS AGO, HUMANS BEGAN TO SHIFT FROM HUNTER

gathering to farming, from isolated bands to living in cities by the new crops, from small population numbers to larger and better-fed populations. Doing so brought with it a new kind of atmospheric input: human-created methane.

This proposition is the major thesis of veteran climatologist William Ruddiman in his 2005 book Plows, Plagues, and Petroleum. Ruddi-man readily conceded the obvious: that the last two centuries have witnessed an unusual rise in carbon dioxide and methane levels. But Ruddiman takes a longer view that the more recent rises are sitting on the shoulders of more ancient human events, noting, "Slower but steadily accumulating changes had been under way for thousands of years, and the total effect of these earlier changes nearly matched the explosive industrial era increases of the last century or two." This can be analogized to the great volcano in Hawaii where so much of our information about carbon dioxide rise has come from: Mauna Loa rises 14,000 feet above sea level, and while it is an imposing mountain, at this maximum elevation it is about half the elevation of the Everestlike peaks. But the subaerial, visible portions of Mauna Loa are but part of the mountain. In reality more than half its height is covered by the sea; taken in sum, Mauna Loa is far higher than Mount Everest. So too with the greenhouse gas record—recent rise is like the subaerial parts of Mauna Loa, but the rises of the past 10,000 years, covered by the seas of time, are as much a part of the story as the more visible recent rises.

Ruddiman breaks climate history into three phases. The first extends from the earliest times on Earth to about 8,000 years ago. Over this staggeringly long time, as Ruddiman brusquely puts it, nature was in control. Then, at the mark of 8,000 years before the present, give or take some centuries, for the first time good old Mother Nature had some competition for creating atmospheric change. (Of course, other organisms have been involved in atmospheric change. But we humans are the first to do it with technology, rather than our own physiology.) The final phase, beginning about two centuries ago, marks an acceleration of trends from the 8,000-year mark onward. Thus, in this description of climate through time, Ruddiman maintains that humans have been changing the atmosphere far longer than is generally supposed.

What is the support for this interesting view? The details come from one of those interesting mergers of scientific fields that occasionally occur (too rarely, actually) but that often result in important insights derived from the edges of scientific fields. In this case it is the merging of anthropology and climatology that has led climatologists such as Ruddiman and Brian Fagan, in his delightful 2003 book The Long Summer, to think outside the box, as well as lending power to Jared Diamond's Guns, Germs, and Steel and his recent Collapse. The subtitle of Fagan's book is How Climate Changed Civilization, and that is the theme of Diamond's books as well. Ruddiman, however, would say that the subtitle should be How Civilization Changed Climate—and as we all now know, civilization continues to change climate.

At the heart of this new history is the timeline of human agriculture. The earliest evidence discovered to date comes from the Fertile

Crescent region of Mesopotamia, and in the Yellow River region of

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