What remains to be proven is whether the human race produces enough green house gases to have any influence on the global tempature, and if it does, does nature have no compensating factors that might take that excess green house gas back out of the air.
The last ice age ended around 10,000 BC. Since then the Earth has warmned. Antartica started melting around 7,000 BC. The Earth remains much colder than it was during the age of dinosaurs; at the end of the Mesozoic there were no ice caps at the poles. The warming part isn't the part that's in doubt. What's in doubt is whether humans have, since 1850, added anything to overall warmning trend.
This is how they put it at climatesolutions.org (and they very much believe that humans add to global warming, but they admit the evidence is suggestive, not conclusive):
" In 1990, over 2,000 of the world's top climate scientists working under the auspices of the United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC) concluded that the surface of the Earth had warmed over the last century. The evidence was not clear enough, however, to conclude that this global warming was human-caused and not natural in origin. The evidence for a human role grew stronger in the next five years, and in 1995 the IPCC issued its second state-of-the-science report5, adopted by 157 national governments. It concluded that, "The balance of evidence suggests a discernible human influence on global climate." This is one reason why Dr. Jane Lubchenco, past president of the American Academy for the Advancement of Science, says, "In the last few decades, humans have become a force of nature." " http://climatesolutions.org/global_warming_is_here/index.html#sure
Scientific American also has a good article up on the web, which also uses the conditional when stating that humans may influence the climate. http://www.sciam.com/0597issue/0597karl.html and linked from that article: http://www.gcrio.org/gwcc/part1.html
All these article are in favor of the idea that human behavior is effecting the planet's tempature. But they admit there is room for doubt. If you want to read that case against human's having an effect, there is this up over at the Cato Institute: http://www.cato.org/pubs/regulation/reg15n2g.html by Richard S. Lindzen of MIT.
--lk
-- krubner -- helping web designers program -- http://www.krubner.com/ -- netscape im screenname: lkrubner