permaculture

[lbsaltzman@aol.com]

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.

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.

Orbital Controls over Greenhouse Gases

Natural variations in the earth's orbit, such as those related to precession 
(diagrams), redistribute the sunlight that reaches the globe over long 
timescales. For the past million years, these subtle changes have driven 
major dips and swells in atmospheric concentrations of methane and carbon 
dioxide (graphs). Although scientists do not fully understand why, global 
concentrations of these greenhouse gases respond mainly to changes that occur 
during summer in the Northern Hemisphere, the time of year when the North 
Pole is pointed most directly at the sun.

MORE TO EXPLORE

Plagues and Peoples. William McNeill. Doubleday, 1976.

Ice Ages: Solving the Mystery. John Imbrie and Katherine Palmer Imbrie. 
Enslow, 1979.

Guns, Germs, and Steel: The Fates of Human Societies. Jared Diamond. W. W. 
Norton, 1999. Earth's Climate: Past and Future. William F. Ruddiman. W. H. 
Freeman, 2001.

The Anthropogenic Greenhouse Era Began Thousands of Years Ago. William F. 
Ruddiman in Climatic Change, Vol. 61, No. 3, pages 261-293; 2003.

Deforesting the Earth: From Prehistory to Global Crisis. Michael A. Williams. 
University of Chicago Press, 2003.

Plows, Plagues, and Petroleum: How Humans Took Control of Climate. William F. 
Ruddiman. Princeton University Press [in press].

GRAPH: Methane concentrations rose and fell over the past 250,000 years in 
near harmony with the precession-induced ups and downs of solar radiation in 
the Northern Hemisphere. The highest temperatures stimulated extreme methane 
production in wetlands, which are the atmosphere's primary natural source of 
this greenhouse gas.

GRAPH: CO2 concentrations, which fluctuated in cycles over the past 350,000 
years, varied in response to precession as well as to shifts in the tilt of 
the earth's rotational axis and in the shape of its orbit. These other cycles 
occur every 41,000 and 100,000 years, respectively.

GRAPH: GREENHOUSE EFFECT from human activities has 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 toward 
the next glaciation--now thousands of years overdue.

DIAGRAM: Wobble in the earth's axis of rotation, known as precession, is one 
of the three orbital cycles that account for sunlight variations in the 
Northern Hemisphere. Like a toy top about to fall, the earth's axis traces 
imaginary circles in space, making one revolution every 22,000 years.

DIAGRAM: Summer warmth in the Northern Hemisphere peaks once every 22,O00 
years, when the yearly northern summer coincides with the earth's closest 
passage to the sun and the Northern Hemisphere receives the most intense 
sunlight.

DIAGRAM: Summer heat bottoms out 11,000 years later, after the earth's axis 
has shifted (precessed) to the opposite position. The Northern Hemisphere 
then receives the least summer sunlight, because the earth is farthest from 
the sun.

PHOTO (COLOR): AGRICULTURAL TERRACES have been constructed for some 2,000 
years. Those on the opposite page are in Guizhou Province, China.
~~~~~~~~

By William E. Ruddiman

WILLIAM F. RUDDIMAN is a marine geologist and professor emeritus of 
environmental sciences at the University of Virginia. He joined the faculty 
there in 1991 and served as department chair from 1993 to 1996. Ruddiman 
first began studying records of climate change in ocean sediments as a 
graduate student at Columbia University, where he received his doctorate in 
1969. He then worked as a senior scientist and oceanographer with the U.S. 
Naval Oceanographic Office in Maryland and later as a senior research 
scientist at Columbia's Lamont-Doherty Earth Observatory.

Overview/Early Global Warming

    * A new hypothesis challenges the conventional assumption that greenhouse 
gases released by human activities have perturbed the earth's delicate 
climate only within the past 200 years.

    * New evidence suggests instead that our human ancestors began 
contributing significant quantities of greenhouse gases to the atmosphere 
thousands of years earlier by clearing forests and irrigating fields to grow 
crops.

    * As a result, human beings kept the planet notably warmer than it would 
have been otherwise-and possibly even averted the start of a new ice age.

Human Disease and Global Cooling

Concentrations of CO2 in the atmosphere have been climbing since about 8,000 
years ago. During the past two millennia, however, that steady increase at 
times reversed direction, and the CO2 levels fell for decades or more. 
Scientists usually attribute such CO2 drops--and the accompanying dips in 
global temperature-to natural reductions in the sun's energy output or to 
volcanic eruptions. These factors have been regarded as major drivers of 
climate change over decades or centuries, but for the CO2 patterns, such 
explanations fall short--which implies that an additional factor forced CO2 
levels downward. Because I had already concluded that our human ancestors had 
caused the slow rise in CO2 for thousands of years by clearing forests for 
agriculture [see main article], this new finding made me wonder whether some 
kind of reversal of the ongoing clearance could explain the brief CO2 drops.

The most likely root cause turns out to be disease--the massive human 
mortality accompanying pandemics. Two severe outbreaks of bubonic plague, the 
single most devastating killer in human history, correlate well with large 
CO2 drops at approximately AD 540 and 1350 [graph]. Plague first erupted 
during the Roman era, with the most virulent pandemic, the Plague of 
Justinian, in A.D. 540 to 542. The infamous "Black Death" struck between 1347 
and 1352, followed by lesser outbreaks for more than a century. Each of these 
pandemics killed some 25 to 40 percent of the population of Europe. An even 
worse catastrophe followed in the Americas after 1492 when Europeans 
introduced smallpox and a host of other diseases that killed around 50 
million people, or about 90 percent of the pre-Columbian population. The 
American pandemic coincides with the largest CO2 drop of all, from 1550 to 
1800.

Observers at the time noted that the massive mortality rates produced by 
these pandemics caused widespread abandonment of rural villages and farms, 
leaving untended farmland to revert to the wild. Ecologists have shown that 
forests will reoccupy abandoned land in just 50 years. Coupled with estimates 
of human population and the acreage cultivated by each farmer, calculations 
of forest regrowth in pandemic-stricken regions indicate that renewed 
forest's could have sequestered enough carbon to reduce concentrations of CO2 
in the atmosphere by the amounts observed. Global climate would have cooled 
as a result, until each pandemic passed and rebounding populations began 
cutting and burning forests anew.

-W.F.R.

GRAPH: MOST DRAMATIC DROPS in atmospheric CO2 concentrations during the past 
2,000 years (as recorded in two Antarctic: ice cores) occurred around the 
same periods that disease outbreaks were taking the greatest toll on human 
life [yellow bars].









 













________________________________________________________________________
AOL now offers free email to everyone.  Find out more about what's free from 
AOL at AOL.com.