Denying climate change, or in many cases, redefining it as a totally natural event, has been a popular exercise in right-wing politics. The most recent local example is a letter sent to the Las Cruces Sun News (http://www.lcsun-news.com/... - third letter down for January 21.) The author cites Christopher Scotese (See: http://www.scotese.com/) of the University of Texas at Arlington, and Robert A Berner (2001, American Journal of Science - I can't seem to link to this paper) a Yale Geologist, as indicating that vast changes have occurred in the past and that human activity could have no real impact on earth's climate, but fails to cite further papers such as Thomas J. Crowley (from the University of Edinburgh) and Berner's paper on CO2 and climate change in Science (http://courses.washington.edu/....) This supposed lack of human causality is an interesting concept and one I wish were true, as the other side of the coin is terrifyingly bleak. From my understanding of the papers, admittedly not complete, it seems the author(s) are arguing that the models for global climate change may be complicated by tectonics and other factors and likely are incomplete, not that there is no effect from carbon dioxide increases.
The idea that humans could possibly influence climate was "refuted,"as the author of the letter notes, in "Global Warming: A Chilling Perspective" (http://www.geocraft.com/....) However, this interpretation has been challenged (see for example: https://sites.google.com/...) He does not, of course, mention the latter analysis.
I have serious difficulty with the idea that if the Cambrian carbon dioxide level was 18 times todays todays level, there can be no damage to the biosphere, as then there was obviously a thriving biota. The lack of evidence for damage then has, as far as I can see, no bearing on the present situation. At that time there were no cities (trilobites did not build them) and the sea level was considerably higher than at present. That may not matter much to trilobites or bryozoans, but it sure does to humans. To assume that because there was a rich biodiversity in the Cambrian the modern carbon dioxide level is irrelevant is a grave error as far as I can see. I am no climate scientist (I have been a field biologist and in essence a applied entomological ecologist,) but I see no comparison. It seems to me that Berner and others are only saying that the end result may not be easily predictable, based on past earth history, not that there is no cause for concern. Weather is chaotic and climate is based on long-term weather data and is thus strongly complicated by chaotic systems, continental and ocean effects, and random events, like asteroid strikes.
In essence, the writer of the letter to the Sun News and the amateur climatologist who wrote "Global Warming: A Chilling Perspective," appear to have confused ongoing natural climate change (usually fairly gradual if not caused by a catastrophic event) with the geologically rapid warming of the atmosphere and oceans caused by human induced carbon dioxide and methane increases. I have seen in my lifetime enough changes to make me think that the empirical evidence is pointing strongly to anthropogenic climate change. I thus have to conclude that the letter is based on false assumptions and inaccurate conclusions from data not intended to address the current situation.