[tt] NS: The ice age that never was

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The ice age that never was
http://environment.newscientist.com/article.ns?id=mg19926721.600&print=true
8.9.3

THE romantic notion that early humans lived in harmony with their
environment has taken quite a battering lately. Modern humans may
have started eliminating other species right from the start: our
ancestors stand accused of wiping out megafauna - from giant
flightless birds in Australia to mammoths in Asia and the ground
sloth of North America - as they spread across the planet.

Even so, by around 6000 years ago there were only about 12 million
people on Earth - less than a quarter of the current population of
Great Britain. That's a far cry from today's 6.6 billion, many of us
guzzling fossil fuels, churning out greenhouse gases and messing
with our planet's climate like there's no tomorrow. So it may seem
far-fetched to suggest that humans have been causing global warming
ever since our ancestors started burning and cutting forests to make
way for fields at least 7000 years ago.

Yet that's the view of retired climate scientist William Ruddiman,
formerly of the University of Virginia, Charlottesville. Ancient
farmers were pumping climate-warming carbon dioxide and methane into
the atmosphere long before recorded history began, he says. Far from
causing catastrophe, however, early farmers halted the planet's
descent into another ice age and kept Earth warm and stable for
thousands of years.

Hugely controversial

Could a few primitive farmers really have changed the climate of the
entire globe? If you find this hard to believe, you're not the only
one. Ruddiman's idea has been hugely controversial ever since he
proposed it in 2003. "Most new ideas, especially controversial ones,
die out pretty fast. It doesn't take science long to weed them out,"
he says. Yet five years on, his idea is still not dead. On the
contrary, he says the latest evidence strengthens his case. "It has
become clear that natural explanations for the rise in greenhouse
gases over the past few thousand years are the ones that are not
measuring up, and we can reject them," he claims.

There is no doubt that the soaring levels of carbon dioxide and
other greenhouse gases we see in the atmosphere today - causing a
0.7 °C rise in average global temperature during the 20th century -
are the result of human activities. In the late 1990s, however,
Ruddiman started to suspect that our contribution to the global
greenhouse began to become significant long before the industrial
age began. This was when an ice core drilled at the Vostok station
in Antarctica revealed how atmospheric CO[2] and methane levels have
changed over the past 400,000 years. Bubbles trapped in the ice
provide a record of the ancient atmosphere during the past three
interglacials.

What we see is a regular pattern of rises and falls with a period of
about 100,000 years, coinciding with the coming and going of ice
ages. There is a good explanation for these cycles: periodic changes
in the planet's orbit and axis of rotation alter the amount of
sunlight reaching the Earth (see "The ice ages"). We are now in one
of the relatively brief, warm interglacial periods that follow an
ice age.

Regular patterns

Within this larger pattern there are regular peaks in methane every
22,000 years that coincide with the times when the Earth's orbit
makes summers in the northern hemisphere warmest. This makes sense,
because warm northern summers drive strong tropical monsoons in
southern Asia that both encourage the growth of vegetation and cause
flooding during which vegetation rotting in oxygen-poor water will
emit methane. Around the Arctic, hot summers thaw wetlands for
longer, again promoting both vegetation growth and methane emission.

In recent times, however, this regular pattern has changed. The last
methane peak occurred around 11,000 years ago, at about 700 parts
per billion, after which levels began to fall. But instead of
continuing to fall to what Ruddiman says should have been a minimum
of about 450 ppb today, the atmospheric methane began to climb again
5000 years ago. "It just went the wrong way," Ruddiman says.

Intrigued, he turned to records for CO[2]. No one understands
exactly why the level of atmospheric CO[2] varies naturally - a
complex combination of factors is involved, from the growth of
vegetation to volcanic activity. The Vostok core, however, shows
that CO[2] levels correlate closely with temperature, peaking at 280
to 300 parts per million as an ice age ends, then falling to lower
values. (Today the level is 384 ppm and rising ever faster.)

This pattern was repeated until about 7000 years ago, when CO[2]
bucked a downward trend and started to rise again. Just before the
industrial era began it was already around 40 ppm higher than would
be expected based on the last three previous interglacial periods.

Canadian ice sheet

The effect of the rising levels of these greenhouse gases on Earth's
climate would have been significant. The extra 250 ppb of methane
and the extra 40 ppm of CO[2] would have kept the average global
temperature nearly 0.7 °C warmer than it would otherwise have been,
equalling the warming during the 20th century due to industrial
emissions.

Working with climate modellers Stephen Vavrus and John Kutzbach of
the University of Wisconsin-Madison, Ruddiman has shown that if the
levels of these gases had continued to fall rather than rising when
they did, ice sheets would now cover swathes of northern Canada and
Siberia. The world would be heading into another ice age.

So why did both methane and CO[2] rise over the past few thousand
years? In other words, why has this interglacial been different from
previous ones. Could humans be to blame?

Forest clearing

Agriculture emerged around the eastern Mediterranean some 11,000
years ago, then shortly afterwards in China, and several thousand
years later in the Americas. Farming can release greenhouse gases in
various ways: clearing forests liberates lots of stored carbon as
the wood rots or is burned, for instance, while flooded rice paddies
release methane just as wetlands do.

To find out more about early farming, Ruddiman began to dig around
in studies of agricultural history. These revealed that there was a
sharp rise in rice cultivation in Asia around 5000 years ago, with
the practice spreading across China and south-east Asia. Here at
least was a possible source for the unexpected methane rise.

In Europe, people had begun clearing forests to grow cereal crops
such as barley and wheat 7500 years ago (see graphic). There is no
firm figure for the total extent of this forest clearance, but
Ruddiman says it could have been vast. One pointer to this is the
Domesday Book, which documents the 11th-century census of England
ordered by William the Conqueror. "There were 1.5 million people and
they had cleared 85 per cent of the forest," Ruddiman says.

When he published his theory in 2003 (Climatic Change, vol 61, p
261), there was no shortage of criticism. The most devastating came
from Fortunat Joos at the University of Bern, Switzerland, who
pointed out that there was simply not enough forest on Earth to
account for the anomalous rise in CO[2]. Cutting down every tree on
the planet wouldn't explain the CO[2] rise seen in ice cores over
the past 7000 years.

Amplified

Ruddiman has conceded that Joos was right, and now argues that
deforestation and rice growing released enough CO[2] and methane to
cause a little warming, which then triggered feedback mechanisms
that released more CO[2], causing further warming and so on,
amplifying the influence of early farmers.

That such feedback mechanisms exist is not in doubt. Climate
scientists all agree that the magnitude of the temperature changes
as the Earth has gone in and out of ice ages cannot be explained by
orbital changes alone. The issue is whether early farmers could have
released enough CO[2] and methane to trigger feedback effects large
enough to explain the total observed rise in CO[2].

With the help of Vavrus and Kutzbach, Ruddiman is using climate
models to explore whether this is plausible. The models suggest that
the early human influence might have been great enough to keep the
oceans warm compared with previous interglacials. This could have
boosted CO[2] levels in two ways. "Carbon dioxide is less soluble if
the oceans are warmer," says Ruddiman. "It's a bit like a soda drink
bubbling off its gases on a warm summer's day."

Warmer oceans

Warmer southern oceans would also mean less sea ice around
Antarctica, increasing CO[2] levels by boosting gas exchange between
the atmosphere and seawater. "Both the observational data and our
model suggest these two factors are players," Ruddiman says.
"Whether they fill the entire gap in the CO[2] budget we can't say,
but they look very promising."

The feedback idea also overcomes another objection to the "early
anthropogenic hypothesis". During photosynthesis, plants take up a
slightly larger proportion of the carbon-12 in the atmosphere than
of the heavier isotope, carbon-13. This means that if all the CO[2]
came from deforestation - which reverses the process - there should
have been a gradual rise in the proportion of carbon-12 in the
atmosphere relative to carbon-13. Yet the ice cores show no such
thing, a fact pointed out in a 2006 article by renowned climate
scientist Wally Broecker of Columbia University's Lamont-Doherty
Earth Observatory in Palisades, New York.

If a feedback mechanism was at work, however, 75 per cent of the
extra CO[2] would have come from the oceans, and the relatively
small rise in the proportion of carbon-12 would be consistent with
the only isotope record published so far, Ruddiman says.

What's more, since Ruddiman first suggested his hypothesis, ice core
records of atmospheric methane and CO[2] levels have been extended
back to 800,000 years ago, covering another five interglacial
periods, three similar to the present one. So we now have data for
six comparable interglacials, all of which suggest the rise in the
gases over the past few thousand years is unprecedented, he claims.
"Natural explanations for the rise have been tested six times and
they failed six times - 12 times if you allow for the two gases."

Insane argument

Compelling? Emphatically not, says Broecker, who is Ruddiman's most
fervent critic. "It's an insane argument," he says. "It's total and
utter nonsense."

"If Ruddiman wanted to argue that over the last thousand years there
had been an effect, that would be harder to argue against because
maybe there was," adds Broecker. "But if you look at that record
during the last thousand years, CO[2] sort of flattens out. Just at
the time you'd think that man's effect would be the strongest, you
see the least effect."

Ruddiman argues that this is because deforestation had levelled out.
"There's good evidence that most of Eurasia was deforested by the
time of Christ," he says.

Broecker reckons there's a natural explanation for the CO[2] rise.
The deep oceans store lots of carbon as solid calcium carbonate, and
deep-sea sediments record a drop in carbonate concentrations that
could account for the rise in atmospheric CO[2].

Previous interglacials

But if the CO[2] rise before the industrial age is natural, why did
it not happen in previous interglacial periods? It did, Broeker
claims, during the interglacial period 400,000 years ago (see
graphic). He compares this interglacial with the present one by
aligning them from the point at which the previous ice age ended,
which shows CO[2] naturally staying high, above 270 ppm, for about
28,000 years. Ruddiman aligns the interglacials on the basis of the
point in the orbital cycle at which incoming solar radiation is at a
minimum, signalling the end of the interglacial. This paints a very
different picture.

Other climate experts say it's not possible to match the two cycles
exactly because the three orbital parameters (see graphic) were
different, so any choice is somewhat arbitrary.

Another of Broecker's complaints is that Ruddiman has handed
global-warming deniers some cheap ammunition: if ancient farmers
headed off a looming ice age, isn't that a good thing? "It confuses
the situation and we don't need that," says Broecker. "People use it
in some curious ways to speak against the effects of global
warming," he adds. "I get very emotional about it, yes, because I
think it's very bad science."

A lot of progress

Some climate-change deniers did pounce on Ruddiman's hypothesis when
it was first publicised, but largely dropped it when it was pointed
out that they were effectively conceding that humans really do cause
climate change. Many researchers, by contrast, remain intrigued by
Ruddiman's idea. "We know that humans have had a lot of influence,"
says Richard Alley, a geologist at Penn State University in
University Park, Pennsylvania. "Bill's idea is that you see it in
the atmosphere. He may be right."

Alley is impressed by a paper that Ruddiman and his colleagues
published this year in Quaternary Science Reviews (vol 27, p 1291)
which shows that the number of new rice-growing sites appearing in
China from 6000 to 4000 years ago was 10 times that in previous
millennia. "I would not say Ruddiman wins, it's over, let's go home
- but I think that he's making a lot of progress with the methane."

Geochemist Ed Brook at Oregon State University in Corvallis isn't so
sure. He says the methane rise over the past 5000 years could have
arisen from wetlands in the southern hemisphere. "There are some
nice records from caves in South America that show that region was
getting wetter."

Circumstantial

On CO[2], things are even more uncertain. "It's a tough one, because
the evidence is primarily circumstantial," Brook says. "I'm just as
willing to think that there are some natural mechanisms that we
don't understand." Subtle changes in the Earth's orbit might drive
feedback that makes the atmospheric chemistry very different from
one interglacial period to the next.

Despite the doubts, Brook welcomes Ruddiman's research. "He has
questioned the conventional wisdom in a way that I think is
productive," he says. "It makes us really stop and think."

Future results should help resolve matters. Measurements of carbon
and hydrogen isotope ratios in methane from ice cores could provide
subtle clues to the methane's origin, while comparisons of methane
levels in cores from Antarctica and Greenland might reveal where the
extra methane came from geographically. The existing measurements
are not conclusive but suggest the extra methane came from low
latitudes, which is consistent both with Ruddiman's hypothesis and
with natural methane from tropical South America.

Ruddiman, meanwhile, plans to continue working on climate models and
researching the history of human land use. "If we don't understand
something as basic as why greenhouse gases have increased in the
past several thousand years, then there's a huge gap in our
knowledge of the climate," he says.

For now, it is still an open question: did ancient farmers kick off
global warming or not? The answer is in a sense academic, because it
has no bearing on the far more rapid warming caused by
industrialised societies. If it's true, though, it could be seen as
a warning. If a few million people wielding stone axes averted an
ice age, just imagine what the legacy of their 7 billion
oil-guzzling descendants will be.

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A chilling tale

Did ancient plagues and pestilence that killed millions of people
also alter the planet's climate?

Slight dips in the level of carbon dioxide in the atmosphere over
the past 2000 years are usually put down to natural causes, such as
lower emissions from volcanoes. But when human populations were
decimated by disease, large areas of farmland would have been
abandoned to nature. As forests reclaimed the land, huge quantities
of CO[2] would have been sucked out of the atmosphere.

Ruddiman has shown that the timing of one of the larger dips in
CO[2]matches a series of plagues that peaked around AD 540. Another
coincides with the "black death" of the 14th century. In both cases
Europe's population may have fallen by a third or more. Worse still
was the effect of European settlers bringing smallpox and other
diseases to the Americas, causing populations to fall by as much as
90 per cent. This coincides with a relatively cool period known as
the "little ice age".

Historians have suggested that the little ice age caused famine,
disease and depopulation - but was disease the trigger for the
little ice age, rather than the upshot?

The ice ages

The coming and going of ice ages is driven by periodic changes in
Earth's orbit and rotation axis, known as Milankovich cycles, caused
by the pull of other bodies in the solar system.

The tilt of the Earth's axis varies in a cycle lasting about 41,000
years. In the more "upright" position, both poles have colder
winters, making the Earth more prone to glaciations.

Another periodic change is precession, in which the pointing
direction of the Earth's axis rotates, going full circle in 22,000
years. Because the Earth's orbit is slightly elliptical, or
eccentric, this alters the season in which the Earth is farthest
from the sun. The eccentricity of Earth's orbit also varies every
100,000 years, though it's not clear how this is linked to changes
in climate.

The rapid swings between ice ages and interglacials began in earnest
only around 2.5 million years ago - before this the Earth had been
much warmer. A long-term cooling trend in the past 55 million years
could be due to the rise of the Tibetan plateau increasing chemical
weathering of rocks, which removes CO[2] from the atmosphere.

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Weblinks

William Ruddiman, University of Virginia
http://www.evsc.virginia.edu/faculty/people/ruddiman.shtml
Richard Alley, Penn State University
http://www.geosc.psu.edu/people/faculty/personalpages/ralley
Ed Brook, Oregon State University
http://geo.oregonstate.edu/people/faculty/brooke.htm