The Union of Concerned Scientists is a nonprofit partnership of scientists and citizens combining rigorous scientific analysis, innovative policy development and effective citizen advocacy to achieve practical environmental solutions.
Established in 1969, we seek to ensure that all people have clean air, energy and transportation, as well as food that is produced in a safe and sustainable manner. We strive for a future that is free from the threats of global warming and nuclear war, and a planet that supports a rich diversity of life. Sound science guides our efforts to secure changes in government policy, corporate practices and consumer choices that will protect and improve the health of our environment globally, nationally and in communities throughout the United States. In short, UCS seeks a great change in humanity's stewardship of the earth.
I just stumbled across a good article by them discussing some of the arguments made by people who say that global warming isn't happening, isn't caused by humans, or isn't a big deal. The article systematically explains the fallacious thinking in claims such as "today was cold, that means no global warming" (that's weather, not climate), "humans aren't doing it" (see thier arguments, I tire of reiterating them), "a couple degrees isn't a big deal" (I learned something today: it only took a difference of 9ºF to go from the last ice age to today), and "there's no scientific concensus on global warming" (asides from a couple misleading, unsound polls, yes there is).
Go read it and see the UCS's arguments for yourself.