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Came across a 2007 study concerning climate sensitivity.
It was published by the Journal Nature, but only as a letter.
Can't say exactly why they do that, but it probably hasn't
been scrutinized as closely as it would be otherwise. Anyhow,
in this study, atmospheric CO2 concentrations over the past
420 million years were compared to computer models. The best
fit was obtained using a climate sensitivity of 2.8C/CO2 doubling.
There are of course other factors that influence CO2 levels:
Global Co2 degassing
Organic and Carbonate burial rates
Land plant populations
Solar Radiation
Palaeogeography
Palaeolithology
Palaeohydrology
When these other factors were adjusted within physically reasonable ranges,
it was concluded that climate sensitivity is unlikely to be less than 1.5C/CO2 doubling.
The only way the computer model would reasonably fit historical data with a climate
sensitivity of less than 1.5C/CO2 doubling was by assuming that CO2 optimally fertilizes
all land plants as if there were never any limits to nutrients, water or light.
However, we know this is unlikely as there are typically large desert areas on Earth
and plants grow better with more light.
Here is a link to the paper:
http://www.gfdl.gov/~ih/jerusalem_papers/royer.pdf
Climate sensitivity constrained by CO2
concentrations over the past 420 million years
While this is an interesting study, it is IMO not particularly news worthy.
It is however, consistent with the broad range of values found by most other studies.
It was published by the Journal Nature, but only as a letter.
Can't say exactly why they do that, but it probably hasn't
been scrutinized as closely as it would be otherwise. Anyhow,
in this study, atmospheric CO2 concentrations over the past
420 million years were compared to computer models. The best
fit was obtained using a climate sensitivity of 2.8C/CO2 doubling.
There are of course other factors that influence CO2 levels:
Global Co2 degassing
Organic and Carbonate burial rates
Land plant populations
Solar Radiation
Palaeogeography
Palaeolithology
Palaeohydrology
When these other factors were adjusted within physically reasonable ranges,
it was concluded that climate sensitivity is unlikely to be less than 1.5C/CO2 doubling.
The only way the computer model would reasonably fit historical data with a climate
sensitivity of less than 1.5C/CO2 doubling was by assuming that CO2 optimally fertilizes
all land plants as if there were never any limits to nutrients, water or light.
However, we know this is unlikely as there are typically large desert areas on Earth
and plants grow better with more light.
Here is a link to the paper:
http://www.gfdl.gov/~ih/jerusalem_papers/royer.pdf
Climate sensitivity constrained by CO2
concentrations over the past 420 million years
Most estimates of climate sensitivity are based on 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 calculations tend to underestimate
the magnitude of large climate-change events and may
not be applicable to climate change under warmer conditions in
the future. Here we estimate long-term equilibrium climate sensitivity
by modeling carbon dioxide concentrations over the past
420 million years and comparing our calculations with a proxy
record. Our estimates are broadly consistent with estimates based
on short-term climate records, and indicate that a weak radiative
forcing by carbon dioxide is highly unlikely on multi-million-year
timescales. We conclude that a climate sensitivity greater than
1.5C has probably been a robust feature of the Earth’s climate
system over the past 420 million years, regardless of temporal
scaling.
While this is an interesting study, it is IMO not particularly news worthy.
It is however, consistent with the broad range of values found by most other studies.
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