What's Known for Certain?

Scientists know for certain that human activities are changing the composition of Earth's atmosphere. Increasing levels of greenhouse gases, like carbon dioxide (CO2 ), in the atmosphere since pre-industrial times have been well documented. There is no doubt this atmospheric buildup of carbon dioxide and other greenhouse gases is largely the result of human activities

The EPA's official stance on global warming can be found at this site. Hmm. With this level of certainty, within our government, as to the effects of humans on earth's atmosphere, what is the controversial issue about adopting more environmentally friendly policies?

Decius provided me earlier with some "non-partisan" websites with more info. Here they are:
"
http://yosemite.epa.gov/oar/globalwarming.nsf/content/index.html
http://www.realclimate.org/index.php?p=238
http://en.wikipedia.org/wiki/Global_warming
http://en.wikipedia.org/wiki/Global_warming_controversy

Of course, the British Government could be viewed as "liberal:"
http://news.bbc.co.uk/1/hi/sci/tech/4660938.stm

CNN is also known as the "Communist News Network:"
http://edition.cnn.com/2004/TECH/science/01/21/climate.cooling.ap/

The author of that report is: http://www.pik-potsdam.de/~stefan/

If you're looking for a politically conservative view, you should try
the Cato Institute:
http://www.cato.org/current/global-warming/"

If everyone agrees- at least- that humans have some effect on the atmosphere, why wouldn't it be at least a low-level priority to make that effect positive? Scientists? Explain please.