We're dealing with physics equations that no one has dis-proven, so until someone does, it makes scientific sense to believe that human added greenhouse gasses will warm the earth, and it makes scientific, moral, economic, and national defense sense to try and understand what that warming will do, and mitigate any effects effects which will be negative (which is most of them).
It even goes beyond that. We know humans are remaking the planet into something that we are unfamilier with. Isn't it logical to ask Scientists to try to determine what the short and long term effects of this are? Greenhouse gasses are only part of this complex subject.
The more brilliant minds we have studying this the more we will begin to understand it. Hell, maybe we'll find that the freon we've been dumping into the atmosphere for the last 40-50 years is beneficial. If so, then we'll know we can all start cutting the condensing tubes on our AC's and refridgerators and have us a big ol' freon party. Perhaps clear cut logging is the best thing we could do for the forests, if so...lets find out so we can cut down the remaining 3% old growth Redwood trees left. Hell, I'd love to get my hands on some "heart" redwood. I'll even bring my own chainsaw to the party. One never knows, but it may well be that the toxins we've been dumping into the Oceans are allowing fish to reproduce at rates we never imagined. If so, lets close those Waste Water Treatment Plants down. Who likes those things anyways?
I can understand someone ignoring what's happening today on this planet if they own lots of stock in Exxon, or have hundreds of acres of timber that they want to clearcut...or they have lots of money invested in one of the large Power companies and really don't want to spend the $$ to install scrubbers on their coal fired plants. I can even understand some of the developing nations who are trying to play "catch-up" with the more developed Nations.
What I don't understand are the rest who choose not to acknowledge that mankind is changing the ecosystem in ways that we've never seen before. If we don't acknowledge it how are we even going to begin to understand the ramifications, if any, of those changes?