Astronaut physicist Ward Cunningham criticizes NASA scientist <a href=http://wattsupwiththat.wordpress.com/2008/08/01/award-winning-astronaut-slams-hansen-urges-nasa-to-debunk-the-current-hysteria-over-warming/#respond">; James Hansen as a polemicist.

Cunningham "It doesn’t help that NASA scientist James Hansen was one of the early alarmists claiming humans caused global warming. Hansen is a political activist who spreads fear even when NASA’s own data contradict him. [...] NASA should be at the forefront in the collection of scientific evidence and debunking the current hysteria over human-caused, or Anthropogenic Global Warming (AGW). Unfortunately, it is becoming just another agency caught up in the politics of global warming, or worse, politicized science. Advocacy is replacing objective evaluation of data, while scientific data is being ignored in favor of emotions and politics...

Historically, temperature increases have preceded high CO2 levels, and there have been periods when atmospheric CO2 levels were as much as 16 times what they are now, periods characterized not by warming but by glaciation. You might have to go back half a million years to match our current level of atmospheric CO2, but you only have to go back to the Medieval Warming Period, from the 10th to the 14th Century, to find an intense global warming episode, followed immediately by the drastic cooling of the Little Ice Age. Neither of these events were caused by variations in CO2 levels.

The reality is that atmospheric CO2 has a minimal impact on greenhouse gases and world temperature. Water vapor is responsible for 95 percent of the greenhouse effect. CO2 contributes just 3.6 percent, with human activity responsible for only 3.2 percent of that. That is why some studies claim CO2 levels are largely irrelevant to global warming. Without the greenhouse effect to keep our world warm, the planet would have an average temperature of minus 18 degrees Celsius. Because we do have it, the temperature is a comfortable plus 15 degrees Celsius. Based on the seasonal and geographic distribution of any projected warming, a good case can be made that a warmer average temperature would be even more beneficial for humans."