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March
28, 2007 New calculations
show that
sensitivity of Earth's climate to changes in the greenhouse gas carbon
dioxide
(CO2) has been consistent for the last 420 million years, according to
an
article in Nature by geologists at
Yale and A popular predictor
of future
climate sensitivity is the change in global temperature produced by
each
doubling of CO2 in the atmosphere. This study confirms that in the
Earth's past
420 million years, each doubling of atmospheric CO2 translates to an
average
global temperature increase of about 3° Celsius, or 5°
Fahrenheit. According to the
authors, since
there has continuously been life on the planet over this time span,
there must
be an ongoing balance between CO2 entering and leaving the atmosphere
from the
rocks and waters at Earth's surface. Their simulations examined a wide
span of
possible relationships between atmospheric CO2 and temperature and the
likelihood they could have occurred based on proxy data from geological
samples. Most estimates of
climate
sensitivity have been based on computer simulations of climate or
records of
climate change over the past few decades to thousands of years, when
carbon
dioxide concentrations and global temperatures were similar to or lower
than
today. Such estimates could underestimate the magnitude of large
climate-change
events. To keep Earth's
carbon cycle in
balance, atmospheric CO2 has varied over geologic time. Carbon-cycle
models
balance chemical reactions that involve carbon, such as photosynthesis
and the
formation of limestone, on a global scale. To better predict future
trends in
global warming, these researchers compared estimates from long-term
modeling of
Earth's carbon cycle with the recent proxy measurements of CO2. This study used 500
data points
in the geological records as "proxy data" and evaluated them in the
context of the CO2 cycling models of co-author Robert Berner, professor
emeritus of geology and geophysics at Yale who pioneered models of the
balance
of CO2 in the Earth and Earth's atmosphere. "Proxy data are
indirect
measurements of CO2 — they are a measure of the effects of
CO2," explained
co-author Jeffrey Park, professor of geology and geophysics at Yale who
created
the computer simulations for the project. "While we cannot actually
measure the CO2 that was in the atmosphere millions of years ago, we
can
measure the geologic record of its presence. For example, measurement
of carbon
isotopes in ancient ocean-plankton material reflects atmospheric CO2
concentrations." Led by Dana L.
Royer, assistant
professor of Earth and Environmental Sciences at Other proxy
measurements of soil,
rock and fossils provided estimates of CO2 over the past 420 million
years.
Calculation of the climate sensitivity in this way did not require
independent
estimates of temperature. It incorporated information from times when
the Earth
was substantially warmer and colder than today, and reflects the
sensitivity of
the carbon-cycle balance over millions of years. "Our results are
consistent
with estimates from shorter-term records, and indicate that climate
sensitivity
was almost certainly greater than 1.5, but less than 5.5 degrees
Celsius over
this period," said Park. "At those extremes of CO2 sensitivity,
[1.5°C or 5.5°C] the carbon-cycle would have been in a
'perfect storm'
condition."
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