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HUMANS
BEGAN ALTERING GLOBAL CLIMATE THOUSANDS OF YEARS AGO, STUDY SHOWS
December 9, 2003 --
A new hypothesis suggests that humans began altering
greenhouse-gas concentrations and global climate thousands of years
ago, long before the 1800s date widely
assumed. In a paper to be presented at the December American Geophysical Union
meeting in San Francisco and published in the December issue of Climatic Change,
climate scientist Bill Ruddiman, professor emeritus of environmental sciences
at the University of Virginia, concludes that human-caused emissions of carbon
dioxide (CO2) since 8,000 years ago, and methane (CH4) since 5,000 years ago
have combined to prevent a significant natural cooling of Earth’s climate.
Cyclic
changes in these two greenhouse gases and in the size of ice sheets have occurred
over hundreds of thousands of years for natural reasons. These natural
cycles are driven by small variations in Earth’s orbit that cause rhythmic
changes in the amount of solar radiation received at every location on the
planet. The changes in solar radiation in turn cause predictable changes in
climate that
drive the greenhouse-gas cycles. But within the last several thousand years,
these natural cycles were over-ridden by human activities that resulted from
the early spread of agriculture in Eurasia, Ruddiman said.
Highlights
of the new study include:
- Beginning
8,000 years ago, humans reversed an expected decrease
in CO2 by clearing forests
in Europe, China, and India for croplands and pasture
(page 2).
- Beginning
5,000 years ago, humans reversed an expected decrease in
methane by
diverting water to irrigate rice and by tending large herds of
livestock (page 3).
- In
the last few thousand years, the size of the climatic warming
caused by these early greenhouse emissions may
have grown large enough
to prevent a glaciation
that climate models predict should have begun in northeast
Canada (page 4).
- Abrupt
reversals of the slow CO2 rise caused by deforestation
correlate with bubonic plague and
other pandemics near 200-600, 1300-1400
and 1500-1700 A.D.
Historical records show that high mortality rates caused
by plague led to massive abandonment of farms. Forest
re-growth on the untended
farms pulled CO2 out of
the atmosphere and caused CO2 levels to fall. In time,
the plagues abated, the farms were reoccupied, and the
newly re-grown forests were
cut, returning the
CO2 to the atmosphere (page 5).
Humans
Reversed the Natural Methane Trend 5,000 Years Ago
Natural
controls on methane.
Bubbles of ancient air preserved in cores drilled from
the Antarctic ice sheet show that atmospheric methane
concentrations
over the
last 350,000
years varied
at a cycle of 22,000 years. The reason for this
rhythm is well understood. Changes in Earth’s orbit
at this cycle cause variations in summer solar radiation.
When a stronger summer sun heats the land, hot
air rises, and air flowing in from the ocean to replace
the rising air carries moisture that falls in summer
monsoons. The monsoon rains then fill up natural
wetlands that emit methane.
The result is a predictable 22,000-year methane
cycle
resulting from tropical monsoons driven by summer
solar radiation. Breakdown
of Natural Controls.
Near 11,000 years ago, methane concentrations reached
a natural maximum predicted by the most recent
peak in summer solar
radiation. The subsequent
drop in
methane until 5,000 years ago is expected from
the slow decrease in solar radiation.
But the reversal and slow increase of methane after
5,000 years ago is anomalous, because solar radiation
continued
to decrease
and tropical monsoon
sources
were drying out. This methane increase violates
the ‘rules’ followed for
350,000 years.
Human
Origin of the Methane Increase.
Irrigation was first used to grow wet-adapted strains
of rice in Southeast Asia about 5,000 years ago.
Because flooded
rice
fields are a major source
of methane,
irrigation is probably the main factor behind
the anomalous methane increase that began at that
time.
Later, irrigation
spread to
hillside rice paddies.
Other methane emissions resulted from growing
human populations, increased tending
of livestock and biomass burning.
Humans Reversed the Natural CO2 Trend 8,000 Years
Ago
Natural controls on CO2.
Air
bubbles in ice show that natural CO2 concentrations in the
atmosphere have varied at regular
cycles
related to changes
in
Earth’s orbit. The main
CO2 cycle is at 100,000 years. In each cycle,
peak CO2 values occurred late in the intervals
when
ice sheets were rapidly melting, and just as
warmer interglacial
intervals began. For the three CO2 peaks that
occurred prior to the most recent one, CO2 concentrations
then fell for at least the next 15,000 years
during
the
early stages of interglacial warmth.
Breakdown of Natural Controls.
Near ten thousand years ago, at the end of the
most recent ice-melting interval, CO2 levels
rose to the
same kind of
natural peak they
had reached in the
three previous cycles. The CO2 drop from 10,500
to 8,000 years ago also resembles the earlier
decreases. But the reversal
of this trend and the
slow increase
to higher
concentrations after 8,000 years ago is anomalous
compared
to the earlier trends.
Human Origin of the CO2 Increase.
The start of clearance and burning of European
forests for agriculture 8,000 years ago is documented
by the
slow westward
spread of
remains of wheat,
barley, peas and lentils from source areas in
the ‘fertile crescent’ area
of the Near East. Agriculture also began in naturally forested areas of China
near 9,500 years ago and in India after 8,500 years ago. By 2,000 years ago,
extensive clearance of forests from hill slopes in Mediterranean Europe allowed
erosion of mud that clogged river mouths and forced relocation of coastal port
cities. CO2 emissions from early forest clearance in Eurasia are large enough
to explain the anomalous increase in atmospheric CO2 since 8,000 years ago.
Human Greenhouse Emissions May Have Stopped
a Glaciation
Natural
Controls on Ice Volume.
For almost three million years, changes in the
size of Northern hemisphere ice sheets have occurred
at
the natural cycles
of Earth’s solar orbit. Changes
in solar radiation received in summer at high northern latitudes control the
rate of growth and melting of ice sheets. Models have been devised to mimic the
way solar radiation controls the size of ice sheets over thousands of years,
including the delayed response of the ice to solar heating. The ‘target
signal’ used to evaluate the models is
an index of ice volume found in ocean sediments
(the marine oxygen-isotope signal).
Breakdown of Natural Controls.
The models successfully match most features in
the target ice-volume signal over the last several
hundred
thousand
years. But several
models indicate
that ice
should have again begun accumulating in the last
5,000 years as a delayed response to the summer
radiation
decrease that
has been underway since
11,000 years
ago. The failure of ice to reappear suggests that
the natural controls on ice sheets
broke down in the last few thousand years. Humans
as the Cause of Overdue Glaciation.
A plausible explanation for the overdue glaciation
is the build-up of CO2 and methane resulting
from human activities
during the
last several thousand
years.
Releases of these gases should have warmed global
climate by ~0.8oC, and high-latitude climate
by ~2oC, enough
to have kept
snow and ice from accumulating
in extensive
areas of high terrain in northeast Canada.
Diseases
Explain Short-Term CO2 Drops
Natural Controls on Short-Term CO2.
Over shorter intervals of centuries to decades,
variations in climate have been linked to factors
such as changes
in Sun strength
and sporadic volcanic
eruptions.
Several large drops in CO2 have been detected
in ice-core layers that span the last 2,000
years. The cause of
these drops has
been evaluated with
models that
include physical, chemical and biological parts
of the climate system. These models provide
estimates of the associated
changes in temperature
and CO2
that should occur in response to changes in
solar radiation
and volcanic eruptions.
Inadequacy
of Natural Controls.
To
match the full amplitude of the observed drops in
CO2,
the models
require such
large decreases
in northern hemisphere
temperature
that they violate reconstructions of actual
temperature trends. The implication is
that ‘natural’ factors
(solar and volcanic changes) cannot explain
the size of these CO2 decreases.
Human Origin of the Abrupt CO2 Decreases.
The first two low CO2 pulses correlate
with bubonic plague pandemics. Historical
records
document
mortality rates of
25-45 percent
in western Eurasia during
these pandemics, with massive abandonment
of farms and rural villages for decades
to centuries. Ecologists estimate that
forests
can reclaim farmland within just 50 years.
Rapid
reforestation and sequestration
of carbon in trees
and shrubs
growing on abandoned farms can pull enough
CO2 out
of the atmosphere to explain both the speed
and amount of the CO2
declines during
plague pandemics.
For
the interval between 1500 and 1750 A.D.,
90
percent mortality caused by first exposure
of native American populations to European
diseases appears to have been to the major
cause of the
last CO2 drop.
Contact:
William Ruddiman, Professor Emeritus of Environmental Sciences,
University of Virginia
Phone: 540-348-1963 Email: wfr5c@virginia.edu, rudds2@ntelos.net
Author staying at Powell Hotel in San Francisco, Dec. 8-12; Phone#
415-398-3200
AGU Session PP22D-01 (Emiliani Lecture) on Dec. 9 at 5 p.m. in
Room MCC 3001-3003 |
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