"Carbon Dioxide (CO2). In nature, carbon is cycled between various
atmospheric, oceanic, land biotic, marine biotic, and mineral reservoirs.
The largest fluxs occur between the atmosphere and surface water of the
oceans. In the atmosphere, carbon predominantly exists in its oxidized
form as CO2. Atmospheric carbon dioxide is part of this
global carbon cycle, and therefore its fate is a complex function of
geochemical and biological processes. Carbon dioxide concentrations in
the atmosphere increased from approximately 280 parts per million by
volume (ppmv) in preindustrial times to 367 in 1999, a 31 percent
increase (IPCC 2001). The IPCC notes that:
"[t]his concentration has not been exceeded during the
past 420,000 years, and likely not during the past 20
million years. The rate or increase over the past century
is unprecedented, at least over the past 20,000 years."
The IPCC definitively states that "the present atmospheric CO2 increase
is caused by anthropogenic emissions of CO2" (IPCC 2001). Forest
clearing, other biomass burning, and some non-energy production processes
(e.g., cement production) also emit notable quantities of carbon dioxide.
In its second assessment, the IPCC also stated that "[t]he increased
amount of carbon dioxide [in the atmosphere] is leading to climate change
and will produce, on average, a global warming of the Earth's
surface--although the magnitude and significance of the effects are not
fully resolved" (IPCC 1996).
Excerpt from Inventory of U.S. Gas Emissions and Sinks 1990-2000
From what I now know about the Paleocene-Eocene Thermal Maximum (PETM) of
55 Ma and the Eocene, 20 Ma seems about right for the time that the
concentration of CO2 was as high as it is today (and increasing ! ).
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