Atmospheric Carbon Dioxide Levels For The Last 500 Million Years: No Correlation With Climate

In this article the author concludes that he can not detect any relationship between past atmospheric carbon dioxide levels and climate variations. Note that he attributes varying carbon dioxide levels to the completely natural geologic processes of weathering of rocks, magmatism (volcanism) and the burial of organic carbon.

This raises the obvious question, (again) if there is no correlation between atmospheric carbon dioxide levels and climate change for the past 500 million years of Earth history, how can a sudden, yet minor (the past 100 years), increase of man-caused carbon dioxide emissions be causing global warming?

Something tells me this scientist was not consulted by the United Nations, the IPCC, and of course not by Al Gore.
Peter



From: http://www.pnas.org/cgi/reprint/99/7/4167?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=carbon+dioxide&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT

Atmospheric carbon dioxide levels for the last
500 million years
Daniel H. Rothman†
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Communicated by Paul F. Hoffman, Harvard University, Cambridge, MA, January 30, 2002 (received for review October 9, 2001)

The last 500 million years of the strontium-isotope record are
shown to correlate significantly with the concurrent record of
isotopic fractionation between inorganic and organic carbon after
the effects of recycled sediment are removed from the strontium
signal. The correlation is shown to result from the common dependence
of both signals on weathering and magmatic processes.

Because the long-term evolution of carbon dioxide levels depends
similarly on weathering and magmatism, the relative fluctuations
of CO2 levels are inferred from the shared fluctuations of the
isotopic records. The resulting CO2 signal exhibits no systematic
correspondence with the geologic record of climatic variations at
tectonic time scales.

The long-term carbon cycle is controlled by chemical weathering,
volcanic and metamorphic degassing, and the burial of
organic carbon (1, 2). Ancient atmospheric carbon dioxide levels
are reflected in the isotopic content of organic carbon (3) and,
less directly, strontium (4) in marine sedimentary rocks; the
former because photosynthetic carbon isotope fractionation is
sensitive to CO2 levels, and the latter because weathering and
degassing are associated with extreme values of the abundance
ratio 87Sr
86Sr.

However, attempts to use these geochemical
signals to estimate past CO2 levels (5–8) are hindered by the
signals’ additional relationships to various tectonic (9, 10) and
biological (11) effects. Moreover, the strontium signal has
proven especially difficult to parse (12–15).

Here, I attempt to resolve these ambiguities in the isotopic
signals of carbon and strontium. First, it is shown that the last
500 million years of the strontium signal, after transformation
to remove the effects of recycled sediment (16, 17), correlate
significantly with the concurrent record of isotopic fractionation
between inorganic and organic carbon (3). This empirical
result is supplemented by the theoretical deduction that the
two records are linked by their common dependence on rates
of continental weathering and magmatic activity.

The assumption that CO2 levels fall with the former and rise with the latter
then indicates that an appropriate average of the two records
should ref lect the long-term fluctuations of the partial pressure
of atmospheric CO2. The CO2 signal derived from this
analysis represents fluctuations at time scales greater than
about 10 million years (My). Comparison with the geologic
record of climatic variations (18) reveals no obvious
correspondence.

(Continued)