On January 27, 1986, the night before the Space Shuttle Challenger was to be launched, a phone conference took place between NASA managers and Morton Thiokol, the manufacturer of the shuttle's solid rocket motors. Engineers from the rocket company told NASA that it would be too cold (26ºF) to launch since the previous coldest launch (53ºF) showed burn-through problems with the O-ring seals and therefore there was no data to show that it was safe to launch. The NASA managers asked if they could prove that the rockets would fail at low temperatures and, of course, it could not be proved. NASA then held a private call with the rocket company's managers, with the engineers excluded, and got them to agree to say it was OK to launch. The Challenger exploded the next day, 73 seconds after launch.
Of course, the NASA managers had asked the wrong question given that it was a life and death matter. Rather than asking if there was proof that the launch would fail, they should have asked if there was proof that the launch would succeed.
The discussion of climate change is following a similar course. Climate scientists are telling us that we are headed for catastrophe if we keep emitting CO2 and other greenhouse gases. But instead of heeding their warnings, we are asking for proof of the impending disaster. We harp on minor errors in otherwise overwhelming evidence and we rail against scientists when they express their frustration about the ability of deniers to confuse the public.
The fact that increased CO2 in the atmosphere leads to warming isn't really debatable... you can measure it in the laboratory. CO2 and other greenhouse gases are good at absorbing infrared radiation (heat) and then re-radiating it in all directions. They act like a blanket around the earth and without it the Earth would be a frozen snowball. But just like adding a second comforter to your bed makes you warmer, adding CO2 to the atmosphere heats up the Earth. And we know where the extra CO2 comes from. Every year, we dig up gigatons of fossil fuels and burn them, releasing CO2. So far, we have increased CO2 levels in the atmosphere by about 38% (from a pre-industrial 280 "parts per million" to 387 ppm today) and we will likely double it this century. CO2 levels haven't been as high as they are today for at least 15 million years. And the CO2 doesn't go away quickly. Some of it stays in the atmosphere for hundreds and even thousands of years.
It is interesting to listen to people who deny that climate change will happen when it is already is happening. The summer Arctic sea ice is already 40% less than it was 30 years ago and it may be all gone in as little as 10 years. And, in a few years, you will be able to sail a regular boat (not an ice breaker) over the North Pole for the first time in human history. Every decade in the last 40 years has been hotter than the previous decade. And 2010 will likely be the hottest year since we started measuring temperature (2005 is the hottest so far and 2009 is tied for second place). Glaciers are melting around the world. Floods, droughts, and extreme weather are increasing every decade. We are in the middle of a mass extinction (species are going extinct at 1000 times the "normal" rate). And worldwide food production has been dropping for the past five years due to climate change. While some people claim these effects are due to "natural cycles", if it wasn't for the greenhouse gases we have put in the atmosphere, the world would actually be cooling slightly (due to subtle changes in the Earth's orbit around the Sun).
If after examining an airplane, an aircraft mechanic expressed even a fraction of the concern that climate scientists are expressing, you would never get on that plane and you certainly would not put your children on it. Why do we treat climate change differently?
Like the rocket engineers that told NASA that 53ºF was the lowest safe temperature, climate scientists are telling us 350 ppm is the highest possible safe level of CO2. We are already at 387 ppm, though it is still possible to get back to 350.
But only if we ask the right questions.