Because basic science suggests CO2 would warm the planet about 1C-1.5C, and experience suggests Nature would respond in a way that moderates the warming.

Meanwhile the theorists have produced models that are built on positive feedbacks that make the warming multiply to create scary scenarios of 2C,3C,4C, or more.

I don't doubt global climate change. Climate is dynamic. Warming happened in the years since 1750 because it was cold in the Little Ice Age. Conditions have changed which caused that cooling, and warming occurred, and rightfully so. Global Warming, by contrast, is the exaggeration of natural variation that has been exploited for political and financial profit, and it is propagated by a willing media that thrives on sensation. We're being played for suckers. Go ahead and accept the invitation, if you want.

Those still capable of critical thinking recognize that the burden of proof rests with those advocating a theory; and those promoting the global warming hypothesis have failed to abide by the principles of the scientific method. Also, the continuing use of ad hominem arguments against those raising dougbts about the global warming hypothesis is a patently unscientifitc development that should concern every devotee of the scientific method. But, the attacks continue unabated (denier, etc) which suggests that the global warming movement is not scientific, but, rather, a sociological movement.

CO2 warming was suppose to be linear at the onset. Arrhenius was one of the original CO2 warming thinkers who mathematically calculated it. He showed in 1896 that a doubling of CO2 in our atmosphere would cause a 5C - 6C temperature rise. This is where the original scare came from. Unfortunately, temperatures dropped continuously for him and he recalculated the warming 14 years later in 1910. This is when it became apparent that CO2 warming was non-linear. He then came up with a much lower figure 1.6C - 3.0C with a doubling of CO2.

There are many variables that affect the temperature and cause it to fluctuate up and down, but CO2 does not have as big as an effect as once thought by Arrhenius and all of his followers. The "runaway effect" of CO2 warming is non-existent simply due to it being a trace gas and the infrared (IR) frequency from which CO2 gets its warming energy from is already saturated and can only warm the atmosphere at very small (trace) amounts. That's the real science of why "Anthropogenic (man-made) Global Warming" has very little effect on the climate

It's not just some groups, but some of the brightest most respected scientists in the world. and many don't just question but deny the validity of the whole process used to come up with these types of results that are endlessly talked about in the media.

Here's why.

Top climate scientists say there is no man-made Global Warming.

The Great Global Warming Swindle