The atmospheric concentration of CO2 has not been higher than 300 ppm in
at least the last 425,000 years, prior to 1900.
Since 1900, CO2 concentration levels in the atmosphere have risen from
295 ppm to 379 ppm, or roughly 22%. Most of that increase is likely due
to human's burning fossil fuels, with the remainder (<10%) due to
deforestation (which increases the CO2 levels because of reduced
photosynthesis). There are no natural causes identified in the literature
for these increases in CO2 (and other greenhouse gases such as methane
and nitrous oxide). Therefore, they must be human-caused.
According to NOAA, the global warming rate in the last 25 years has risen
to 3.6 degrees F per century, which tends to confirm the predictions of
temperature increases made by international panels of climate scientists
(IPCC). The rate of CO2 increase is also accelerating steeply, and is
likely to continue to do so, even if significant reduction in CO2
emissions begin now, since their is a lag effect in their buildup in the