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  • From: Silvio Marquardt <marquardtsilvio@yahoo.com>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: Re: [permaculture] Farming and global warming
  • Date: Tue, 5 Jun 2007 17:00:19 +0100 (BST)

The same article illustrated.
How Did Humans First Alter Global Climate? (Scientific American)
http://marukuwato.multiply.com/journal/item/149





lbsaltzman@aol.com wrote:
HOW DID HUMANS FIRST ALTER GLOBAL CLIMATE? , By: Ruddiman, William E.,
Scientific American, 00368733, Mar2005, Vol. 292, Issue 3

Database: Academic Search Premier

HOW DID HUMANS FIRST ALTER GLOBAL CLIMATE?

A bold new hypothesis suggests, that our ancestors" farming practices kicked
off global warming thousands of years before we started burning coal and
driving cars

The scientific consensus that human actions first began to have a warming
effect on the earth's climate within the past century has become part of the
public perception as well. With the advent of coalburning factories and power
plants, industrial societies began releasing carbon dioxide (CO2) and other
greenhouse gases into the air. Later, motor vehicles added to such emissions.
In this scenario, those of us who have lived during the industrial era are
responsible not only for the gas buildup in the atmosphere but also for at
least part of the accompanying global warming trend. Now, though, it seems
our ancient agrarian ancestors may have begun adding these gases to the
atmosphere many millennia ago, thereby altering the earth's climate long
before anyone thought.

New evidence suggests that concentrations of CO2 started rising about 8,000
years ago, even though natural trends indicate they should have been
dropping. Some 3,000 years later the same thing happened to methane, another
heat-trapping gas. The consequences of these surprising rises have been
profound. Without them, current temperatures in northern parts of North
America and Europe would be cooler by three to four degrees Celsius--enough
to make agriculture difficult. In addition, an incipient ice age--marked by
the appearance of small ice caps--would probably have begun several thousand
years ago in parts of northeastern Canada. Instead the earth's climate has
remained relatively warm and stable in recent millennia.

Until a few years ago, these anomalous reversals in greenhouse gas trends and
their resulting effects on climate had escaped notice. But after studying the
problem for some time, I realized that about 8,000 years ago the gas trends
stopped following the pattern that would be predicted from their past
long-term behavior, which had been marked by regular cycles. I concluded that
human activities tied to farming--primarily agricultural deforestation and
crop irrigation-must have added the extra CO2 and methane to the atmosphere.
These activities explained both the reversals in gas trends and the ongoing
increases right up to the start of the industrial era. Since then, modern
technological innovations have brought about even faster rises in greenhouse
gas concentrations.

My claim that human contributions have been altering the earth's climate for
millennia is provocative and controversial. Other scientists have reacted to
this proposal with the mixture of enthusiasm and skepticism that is typical
when novel ideas are put forward, and testing of this hypothesis is now under
way.

The Current View

THIS NEW IDEA builds on decades of advances in understanding long-term
climate change. Scientists have known since the 1970s that three predictable
variations in the earth's orbit around the sun have exerted the dominant
control over long-term global climate for millions of years. As a consequence
of these orbital cycles (which operate over 100,000, 41,000 and 22,000
years), the amount of solar radiation reaching various parts of the globe
during a given season can differ by more than 10 percent. Over the past three
million years, these regular changes in the amount of sunlight reaching the
planet's surface have produced a long sequence of ice ages (when great areas
of Northern Hemisphere continents were covered with ice) separated by short,
warm interglacial periods.

Dozens of these climatic sequences occurred over the millions of years when
hominids were slowly evolving toward anatomically modern humans. At the end
of the most recent glacial period, the ice sheets that had blanketed northern
Europe and North America for the previous 100,000 years shrank and, by 6,000
years ago, had disappeared. Soon after, our ancestors built cities, invented
writing and founded religions. Many scientists credit much of the progress of
civilization to this naturally warm gap between less favorable glacial
intervals, but in my opinion this view is far from the full story.

In recent years, cores of ice drilled in the Antarctic and Greenland ice
sheets have provided extremely valuable evidence about the earth's past
climate, including changes in the concentrations of the greenhouse gases. A
three-kilometer-long ice core retrieved from Vostok Station in Antarctica
during the 1990s contained trapped bubbles of ancient air that revealed the
composition of the atmosphere (and the gases) at the time the ice layers
formed. The Vostok ice confirmed that concentrations of CO2 and methane rose
and fell in a regular pattern during virtually all of the past 400,000 years.

Particularly noteworthy was that these increases and decreases in greenhouse
gases occurred at the same intervals as variations in the intensity of solar
radiation and the size of the ice sheets. For example, methane concentrations
fluctuate mainly at the 22,000-year tempo of an orbital cycle called
precession. As the earth spins on its rotation axis, it wobbles like a top,
slowly swinging the Northern Hemisphere closer to and then farther from the
sun. When this precessional wobble brings the northern continents nearest the
sun during the summertime, the atmosphere gets a notable boost of methane
from its primary natural source--the decomposition of plant matter in
wetlands.

After wetland vegetation flourishes in late summer, it then dies, decays and
emits carbon in the form of methane, sometimes called swamp gas. Periods of
maximum summertime heating enhance methane production in two primary ways: In
southern Asia, the warmth draws additional moisture-laden air in from the
Indian Ocean, driving strong tropical monsoons that flood regions that might
otherwise stay dry. In far northern Asia and Europe, hot summers thaw boreal
wetlands for longer periods of the year. Both processes enable more
vegetation to grow, decompose and emit methane every 22,000 years. When the
Northern Hemisphere veers farther from the sun, methane emissions start to
decline. They bottom out 11,000 years later--the point in the cycle when
Northern Hemisphere summers receive the least solar radiation.

Unexpected Reversals

EXAMINING RECORDS from the Vostok ice core closely, I spotted something odd
about the recent part of the record. Early in previous interglacial
intervals, the methane concentration typically reached a peak of almost 700
parts per billion (ppb) as precession brought summer radiation to a maximum.
The same thing happened 11,000 years ago, just as the current interglacial
period began. Also in agreement with prior cycles, the methane concentration
then declined by 100 ppb as summer sunshine subsequently waned. Had the
recent trend continued to mimic older interglacial intervals, it would have
fallen to a value near 450 ppb during the current minimum in summer heating.
Instead the trend reversed direction 5,000 years ago and rose gradually back
to almost 700 ppb just before the start of the industrial era. In short, the
methane concentration rose when it should have fallen, and it ended up 250
ppb higher than the equivalent point in earlier cycles.

Like methane, CO2 has behaved unexpectedly over the past several thousand
years. Although a complex combination of all three orbital cycles controls
CO2 variations, the trends during previous interglacial intervals were all
surprisingly similar to one another. Concentrations peaked at 275 to 300
parts per million (ppm) early in each warm period, even before the last
remnants of the great ice sheets finished melting. The CO2 levels then fell
steadily over the next 15,000 years to an average of about 245 ppm. During
the current interglacial interval, CO2 concentrations reached the expected
peak around 10,500 years ago and, just as anticipated, began a similar
decline. But instead of continuing to drop steadily through modern times, the
trend reversed direction 8,000 years ago. By the start of the industrial era,
the concentration had risen to 285 ppm-roughly 40 ppm higher than expected
from the earlier behavior.

What could explain these unexpected reversals in the natural trends of both
methane and CO2? Other investigators suggested that natural factors in the
climate system provided the answer. The methane increase has been ascribed to
expansion of wetlands in Arctic regions and the CO2 rise to natural losses of
carbon-rich vegetation on the continents, as well as to changes in the
chemistry of the ocean. Yet it struck me that these explanations were doomed
to fail for a simple reason. During the four preceding interglaciations, the
major factors thought to influence greenhouse gas concentrations in the
atmosphere were nearly the same as in recent millennia. The northern ice
sheets had melted, northern forests had reoccupied the land uncovered by ice,
meltwater from the ice had returned sea level to its high interglacial
position, and solar radiation driven by the earth's orbit had increased and
then began to decrease in the same way.

Why, then, would the gas concentrations have fallen during the last four
interglaciations yet risen only during the current one? I concluded that
something new to the natural workings of the climate system must have been
operating during the past several thousand years.

The Human Connection

THE MOST PLAUSIBLE "new factor" operating in the climate system during the
present interglaciation is farming. The basic timeline of agricultural
innovations is well known. Agriculture originated in the Fertile Crescent
region of the eastern Mediterranean around 11,000 years ago, shortly
thereafter in northern China, and several thousand years later in the
Americas. Through subsequent millennia it spread to other regions and
increased in sophistication. By 2,000 years ago, every crop food eaten today
was being cultivated somewhere in the world.

Several farming activities generate methane. Rice paddies flooded by
irrigation generate methane for the same reason that natural wetlands
do--vegetation decomposes in the stagnant standing water. Methane is also
released as farmers burn grasslands to attract game and promote growth of
berries. In addition, people and their domesticated animals emit methane with
feces and belches. All these factors probably contributed to a gradual rise
in methane as human populations grew slowly, but only one process seems
likely to have accounted for the abruptness of the reversal from a natural
methane decline to an unexpected rise around 5,000 years ago--the onset of
rice irrigation in southern Asia.

Farmers began flooding lowlands near rivers to grow wet-adapted strains of
rice around 5,000 years ago in the south of China. With extensive floodplains
lying within easy reach of several large rivers, it makes sense that broad
swaths of land could have been flooded soon after the technique was
discovered, thus explaining the quick shift in the methane trend. Historical
records also indicate a steady expansion in rice irrigation throughout the
interval when methane values were rising. By 3,000 years ago the technique
had spread south into Indochina and west to the Ganges River Valley in India,
further increasing methane emissions. After 2,000 years, farmers began to
construct rice paddies on the steep hillsides of Southeast Asia.

Future research may provide quantitative estimates of the amount of land
irrigated and methane generated through this 5,000-year interval. Such
estimates will be probably be difficult to come by, however, because repeated
irrigation of the same areas into modern times has probably disturbed much of
the earlier evidence. For now, my case rests mainly on the basic fact that
the methane trend went the "wrong way" and that farmers began to irrigate
wetlands at just the right time to explain this wrong-way trend.

Another common practice tied to farming--deforestation--provides a plausible
explanation for the start of the anomalous CO2 trend. Growing crops in
naturally forested areas requires cutting trees, and farmers began to clear
forests for this purpose in Europe and China by 8,000 years ago, initially
with axes made of stone and later from bronze and then iron. Whether the
fallen trees were burned or left to rot, their carbon would have soon
oxidized and ended up in the atmosphere as CO2.

Scientists have precisely dated evidence that Europeans began growing
nonindigenous crop plants such as wheat, barley and peas in naturally
forested areas just as the CO2 trend reversed 8,000 years ago. Remains of
these plants, initially cultivated in the Near East, first appear in lake
sediments in southeastern Europe and then spread to the west and north over
the next several thousand years. During this interval, silt and clay began to
wash into rivers and lakes from denuded hillsides at increasing rates,
further attesting to ongoing forest clearance.

The most unequivocal evidence of early and extensive deforestation lies in a
unique historical document--the Doomsday Book. This survey of England,
ordered by William the Conqueror, reported that 90 percent of the natural
forest in lowland, agricultural regions was cleared as of A.D. 1086. The
survey also counted 1.5 million people living in England at the time,
indicating that an average density of 10 people per square kilometer was
sufficient to eliminate the forests. Because the advanced civilizations of
the major river valleys of China and India had reached much higher population
densities several thousand years prior, many historical ecologists have
concluded that these regions were heavily deforested some two or even three
thousand years ago. In summary, Europe and southern Asia had been heavily
deforested long before the start of the industrial era, and the clearance
process was well under way throughout the time of the unusual CO2 rise.

An Ice Age Prevented?

IF FARMERS WERE responsible for greenhouse gas anomalies this large-250 ppb
for methane and 40 ppm for C02 by the 1700s--the effect of their practices on
the earth's climate would have been substantial. Based on the average
sensitivity shown by a range of climate models, the combined effect from
these anomalies would have been an average warming of almost 0.8 degree C
just before the industrial era. That amount is larger than the 0.6 degree C
warming measured during the past century-implying that the effect of early
farming on climate rivals or even exceeds the combined changes registered
during the time of rapid industrialization.

How did this dramatic warming effect escape recognition for so long? The main
reason is that it was masked by natural climatic changes in the opposite
direction. The earth's orbital cycles were driving a simultaneous natural
cooling trend, especially at high northern latitudes. The net temperature
change was a gradual summer cooling trend lasting until the 1800s.

Had greenhouse gases been allowed to follow their natural tendency to
decline, the resulting cooling would have augmented the one being driven by
the drop in summer radiation, and this planet would have become considerably
cooler than it is now. To explore this possibility, I joined with Stephen J.
Vavrus and John E. Kutzbach of the University of Wisconsin-Madison to use a
climate model to predict modern-day temperature in the absence of all
human-generated greenhouse gases. The model simulates the average state of
the earth's climate--including temperature and precipitation--in response to
different initial conditions.

For our experiment, we reduced the greenhouse gas levels in the atmosphere to
the values they would have reached today without early farming or industrial
emissions. The resulting simulation showed that our planet would be almost
two degrees C cooler than it is now--a significant difference. In comparison,
the global mean temperature at the last glacial maximum 20,000 years ago was
only five to six degrees C colder than it is today. In effect, current
temperatures would be well on the way toward typical glacial temperatures had
it not been for the greenhouse gas contributions from early farming practices
and later industrialization.

I had also initially proposed that new ice sheets might have begun to form in
the far north if this natural cooling had been allowed to proceed. Other
researchers had shown previously that parts of far northeastern Canada might
be ice covered today if the world were cooler by just 1.5 to two degrees
C--the same amount of cooling that our experiment suggested has been offset
by the greenhouse gas anomalies. The later modeling effort with my Wisconsin
colleagues showed that snow would now persist into late summer in two areas
of northeastern Canada: Baffin Island, just east of the mainland, and
Labrador, farther south. Because any snow that survives throughout the summer
will accumulate in thicker piles year by year and eventually become glacial
ice, these results suggest that a new ice age would have begun in northeast
Canada several millennia ago, at least on a small scale.

This conclusion is startlingly different from the traditional view that human
civilization blossomed within a period of warmth that nature provided. As I
see it, nature would have cooled the earth's climate, but our ancestors kept
it warm by discovering agriculture.

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.


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