|
Robert A. Berner
Jeffrey Park
Photos: Yale / Park
|
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 Wesleyan University, who did his
graduate work in geology at Yale, the collaboration simulated 10,000
variations in the carbon-cycle processes such as the sensitivity of
plant growth to extra CO2 in the atmosphere. They evaluated
these variations for a range of atmospheric warming conditions, using
the agreement with the geologic data to determine the most likely
warming scenarios. The model-estimated atmospheric CO2
variations were tested against data from ancient rocks.
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." |
