The two most well qualified atmospheric scientists to cast doubt on the science of global warming are Richard Lindzen, Pubs Alfred P. Sloan Professor of Atmospheric Science at the Massachusetts Institute of Technology and member of the National Academy of Sciences, and Garth Paltridge, Pubs Visiting Fellow ANU and retired Chief Research Scientist, CSIRO Division of Atmospheric Research and retired Director of the Institute of the Antarctic Cooperative Research Centre.
In 2001, Richard Lindzen agreed that global warming is occurring and could be caused by increased carbon dioxide (CO2) levels but believed that scientists were not in a position to prove the connection. He said, "We are quite confident (1) that global mean temperature is about 0.5 °C higher than it was a century ago; (2) that atmospheric levels of CO2 have risen over the past two centuries; and (3) that CO2 is a greenhouse gas whose increase is likely to warm the earth (one of many, the most important being water vapor and clouds). But - and I cannot stress this enough - we are not in a position to confidently attribute past climate change to CO2 or to forecast what the climate will be in the future."
In 2009, Garth Paltridge agreed that there are good reasons to believe that burning fossil fuels could lead to global warming, but was uncertain as to how significant this would be. He said, "There are good and straightforward scientific reasons to believe that the burning of fossil fuel and consequent increase in atmospheric carbon dioxide will lead to an increase in the average temperature of the world above that which would otherwise be the case. Whether the increase will be large enough to be noticeable is still an unanswered question."
In September 2005 Willam M. Gray, Professor Emeritus and head of The Tropical Meteorology Project, Department of Atmospheric Science, Colorado State University stated in an interview with Discovery Magazine: "This small warming is likely a result of the natural alterations in global ocean currents which are driven by ocean salinity variations. Ocean circulation variations are as yet little understood. Human kind has little or nothing to do with the recent temperature changes. We are not that influential."[...]"I am of the opinion that [global warming] is one of the greatest hoaxes ever perpetrated on the American people."[...]"So many people have a vested interest in this global-warming thing-all these big labs and research and stuff. The idea is to frighten the public, to get money to study it more."
In 2007 Tim Patterson, Pubs paleoclimatologist and Professor of Geology at Calreton University, Canada wrote in the Financial Post: "There is no meaningful correlation between CO2 levels and Earth's temperature over this [geologic] time frame. In fact, when CO2 levels were over ten times higher than they are now, about 450 million years ago, the planet was in the depths of the absolute coldest period in the last half billion years. On the basis of this evidence, how could anyone still believe that the recent relatively small increase in CO2 levels would be the major cause of the past century's modest warming?"
Tom Segalstad, head of the Geology Museum at the University of Oslo stated in his article "What is CO2 - friend of foe?": "The IPCC's temperature curve (the so-called 'hockey stick' curve) must be in error...human influence on the 'Greenhouse Effect' is minimal (maximum 4%). Anthropogenic CO2 amounts to 4% of the ~2% of the "Greenhouse Effect", hence an influence of less than 1 permil of the Earth's total natural 'Greenhouse Effect' (some 0.03 °C of the total ~33 °C)"