I've often heard it said that there is a consensus of thousands of scientists on the global warming issue, that humans are causing a catastrophic change to the climate system, Well I am one scientist, and there are many, that simply think that is not true. -- John Christy, Professor and Director of the Earth System Science Center, NSSTC University of Alabama.As for what does cause climate change, if the human influence is not so large:
Solar activity over the last hundred years, over the last several hundred years, correlates very nicely, on a decadal basis, with temperature. --Ian Clark, Professor of Isotope hydrogeology and paleoclimatology at the Dept of Earth Sciences, University of OttawaSeparately, Russia has been developing more accurate methods to measure solar activity and Habibullo Abdussamatov, head of the St. Petersburg's Pulkovo Astronomical Observatory in Russia, believes that solar activity accounts for most of the observed climate changes both here on Earth and on Mars. This result seems to be in line with a Danish study noted here.
UPDATE: A list of 76 scientists willing to go on the record as skeptical of global warming is here.
UPDATE: Another physics professor, Bjarne Andresen of University of Copenhagen, has expressed doubts about the UN interpretation about the usefulness of the "global warming" concepts and warming's affects on the climate. His study will be published in the Journal of Non-Equilibrium Thermodynamics.