obviously, because global warming may lead to something very bad and very $$; if it doesn't lead to these things, not a lot of people will care.
How do we know global warming might lead to something bad , at least in a quantitative sense ?
All (all) of our detailed knowledge is from computer programs (climate models) which simulate changes in the future
However, It is an observed fact (fyfe) that over the last ten years, the surface temp of the earth has not increased as much as predicted by models; the models fail.
The models also can't reconstruct the last few thousand years (Liu), where we Know what happened.
This anomaly is the main current argument of denialists (those who think global warming is not occurring, or is not manmade, or is not important) and cause for concern among climate scientists.
Several attempts have been made to find the missing heat without great acceptance, eg Cowtan (who are not, afaik, climatologists) say that the missing heat is in the Arctic, which is not well measured by instruments.
It appears that Chen and Tung have found the answer: the earth is warming, but the heat is going into the ocean instead of the atmosphere.
SO: the models are clearly not accurate even on a 10year time scale.
so why should we take seriously alarmist views about the future ?
I guess it is probability: if there is even a X% chance that something really bad could happen, is it worth spending ~ 0.5% of global GDP (~ 850 billion dollars a year) to prevent this possible catastrophre ?
Me personally, my house is about 5 miles and 200 feet up from the Atlantic Ocean, so global warming is good for me: I get beachfront property......
chen and tung