Global Warming's Early Roots
Plows, Plagues, and Petroleum: How Humans Took Control of
Climate. William F. Ruddiman. xiv + 202 pp. Princeton
University Press, 2005. $24.95.
In Franconia in southern Germany (where I grew up), there are plenty
of towns with names ending in reuth, the best known of
these being Bayreuth, famous for performances of operas inspired by
Teutonic mythology. Reuth denotes a clearing—that is,
a place ridden of trees. To the south we find town names with the
endings ried and reid, whereas going north we note
rode, roda and rade, until in Denmark we
encounter roed. Typically, the settlements so named are
more than 1,000 years old. In each instance the meaning of the
suffix is the same. Settlers would have had to clear forest not just
for the towns but for farmlands to support them.
The necessary amount of clearing can be figured from the caloric
requirements for food, if one can estimate the number of settlers.
Considering that we know that Roman generals complained about the
hazards associated with the impenetrable forests north of the Alps,
it is easily deduced that the clearings took place within the past
2,000 years or so. Presumably, beginning with Roman colonization and
accelerating during the centuries after the collapse of the Roman
Empire, the rate of forest removal in Europe has followed the shape
of a logistic curve, exponentially increasing until one half of the
forest was gone, in the late Middle Ages, and decreasing since then
toward the present situation, in which we find that patchy forests
remain only on soil that is poorly suited for farming.
This much is general knowledge. What William Ruddiman has done in
Plows, Plagues and Petroleum, an attractive,
well-written new book aimed at a popular audience, is to explore the
geochemical and climatological implications of worldwide
deforestation over the past several thousand years. He concludes
that the clearing of forests on a global scale resulted in the input
of significant amounts of heat-trapping carbon dioxide to the
ocean-atmosphere system. In addition, he points out as possible
greenhouse drivers the warming effects of the methane supplied by
expanding rice fields and increasingly numerous cattle herds. In
short, his thesis is that anthropogenic climate change began long
before the Industrial Revolution. What's more, Ruddiman argues that
this meddling with climate was probably a good thing, because it
prevented the general cooling he says one should otherwise expect
according to the predictions of Milankovitch theory.
Milankovitch theory posits that Earth's climate changes as a result
of cyclic variations in the way our planet orbits the Sun (which
affect the way sunlight gets distributed over the globe). Although
some aspects of the theory remain controversial, it has proved
largely successful in accounting for the dramatic swings in climate
that have taken place in the past on the scale of thousands of years.
Whether or not one accepts Ruddiman's Milankovitch-based arguments
for what should have happened, the basic proposition that human
activities have affected climate during much of the Holocene seems
entirely reasonable. After all, there are strong arguments that
humankind had a crucial role in the extinction of mammoths, woolly
rhinoceroses, cave bears and dozens of other types of Pleistocene
mammals. It would be surprising indeed if our activities had
not influenced patterns of vegetation, erosion, albedo (the
fraction of sunlight reflected back into space) and the chemistry of
the atmosphere. The question is not whether we have had an effect,
but whether one can quantify it, and whether this quantification
warrants the conclusion that global climate was substantially altered.
Ruddiman, to his credit, makes the effort. With great ingenuity he
digs up clues to the growing numbers of humans on the planet, their
impact in terms of forest removal, the addition of trace gas to the
atmosphere resulting from that removal, and the positive feedbacks
within the climate system that amplify the effect. His main
rationale for searching for human impact on atmospheric chemistry
over the past several thousand years (rather than the past two
centuries) is that there is a perceived discrepancy between the
carbon dioxide content seen in ice cores and what might be
reasonably expected by comparison with earlier interglacial periods.
According to Ruddiman, the discrepancy can be quantified, and the
results support the claim that were it not for human emissions of
carbon dioxide (and methane), northern ice masses would have begun
to build up beginning some 5,000 years ago. He further maintains
that the apparent stability of the climate for most of the Holocene
is the result of a natural trend moving the system toward cooling
being coincidentally canceled out by a human-made trend moving it
Ruddiman's line of argumentation seems compelling in its major
aspects. However, there is room for disagreement and discussion. For
example, invoking "coincidence" is rarely a good strategy.
It is better, I think, to look for feedback keeping the Holocene
climate system in its preferred state. In my opinion, Ruddiman has
identified an important factor that has, against expectations, kept
this feedback operating.
There are other places where one might argue as well. The focus on
plows trivializes the impact of burning before farming
spread. I suspect that pyromania may be equal in importance
Ruddiman's contention concerning plagues is that they
depopulated many previously deforested regions, allowing forests to
grow and bind carbon, thus reducing the greenhouse effect—an
interesting proposal, but one that presumably implies a sensitivity
of climate to carbon dioxide that is toward the high end of
possibilities. The result, according to Ruddiman, was a short-term
cooling, manifested as the well-known "Little Ice Age." A
more traditional view would be that the rise of this period
featuring high numbers of nasty winters and wet summers was caused
by a dimming of the Sun. This era produced poor harvests and
widespread hunger, making people more susceptible to disease. The
evidence supporting a role for the Sun in climate change has grown
stronger over the past several decades.
The petroleum section seems more solid. Experience shows
that reserves expand with increased efforts in exploration as the
price of oil rises in response to supply falling behind demand. As
the cost of producing oil rises, we shall likely turn increasingly
to other energy sources, including coal. Ruddiman's projections of
the resulting increase in atmospheric carbon dioxide seem
conservative in a "business as usual" context.
What might be the general impact of Ruddiman's propositions on the
global-warming debate? Most likely, as Ruddiman realizes, his thesis
will please no one. Those alarmed by present trends will object to
the notion that anthropogenic warming saved us from glacial advances
and associated climate deterioration. And those who, when it comes
to energy use, place comfort over concern or who want to protect
special interests will resist the implication that climate is indeed
quite sensitive to human additions of carbon dioxide and methane to
the atmosphere. Above all, Ruddiman's argument makes it clear that
there is no "natural" baseline of climate in the late
Holocene from which to reckon the human impact of the past two
centuries. Ecologists refer to this sort of conundrum as a
"shifting baseline." The application of this concept to
climate change is a major contribution.
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