Seeker Blog

This highly recommended article by William F. Ruddiman is fun, thought-provoking and sure to be controversial (Dr. Ruddiman is a marine geologist at the University of Virginia). In this piece, "How Did Humans First Alter Global Climate" (subscription required), he examines a new hypothesis that advances in human agriculture explain the observed rising levels of atmospheric carbon dioxide beginning about 8,000 years ago.

This graphic depicts how key innovations, mainly deforestation and rice irrigation, warded off a glaciation that otherwise would have begun about 5,000 years ago:

Early human agricultural activities produced enough greenhouse gases to offset most of the natural cooling trend during preindustrial times (yellow), warming the planet by an average of almost 0.8 degree Celsius. That early warming effect (a) rivals the 0.6 degree Celsius (b) warming measured in the past century of rapid industrialization (orange). Once most fossil fuels are depleted and the temperature rise caused by greenhouse gases peaks, the earth will cool towards the next glaciation - now thousands of years overdue.

In addition to his exposition of this theory of human activities, Dr. Ruddiman provides and excellent overview of other significant factors which impact greenhouse gases, including excellent graphics on the impact of earth’s orbit which accounts for periodicities in solar radiation of 22,000 years, 41,000 years and 100,000 years.

Here are Ruddiman’s concluding comments:

Implications for the Future: the conclusion that humans prevented a cooling and arguably stopped the initial stage of a glacial cycle bears directly on a long-running dispute over what global climate has in store for us in the near future. Part of the reason that policymakers had trouble embracing the initial predictions of global warming in the 1980s was that a number of scientists had spent the previous decade telling everyone almost exactly the opposite—that an ice age was on its way. Based on the new confirmation that orbital variations control the growth and decay of ice sheets, some scientists studying these longer-scale changes had reasonably concluded that the next ice age might be only a few hundred or at most a few thousand years away.

In subsequent years, however, investigators found that greenhouse gas concentrations were rising rapidly and that the earth’s climate was warming, at least in part because of the gas increases. This evidence convinced most scientists that the relatively near-term future (the next century or two) would be dominated by global warming rather than by global cooling. This revised prediction, based on an improved understanding of the climate system, led some policymakers to discount all forecasts—whether of global warming or an impending ice age—as untrustworthy.

My findings add a new wrinkle to each scenario. If anything, such forecasts of an “impending” ice age were actually understated: new ice sheets should have begun to grow several millennia ago. The ice failed to grow because human-induced global warming actually began far earlier than previously thought—well before the industrial era. In these kinds of hotly contested topics that touch on public policy, scientific results are often used for opposing ends. Global-warming skeptics could cite my work as evidence that human-generated greenhouse gases played a beneficial role for several thousand years by keeping the earth’s climate more hospitable than it would otherwise have been. Others might counter that if so few humans with relatively primitive technologies were able to alter the course of climate so significantly, then we have reason to be concerned about the current rise of greenhouse gases to unparalleled concentrations at unprecedented rates.

The rapid warming of the past century is probably destined to persist for at least 200 years, until the economically accessible fossil fuels become scarce. Once that happens, the earth’s climate should begin to cool gradually as the deep ocean slowly absorbs the pulse of excess CO2 from human activities. Whether global climate will cool enough to produce the long-overdue glaciation or remain warm enough to avoid that fate is impossible to predict.