[topher1181]

I haven't been able to understand why global warming, due to human causes, is still being questioned.

Most scientists agree that global warming is due to human causes. I would also add that whether you indifferent towards, love, or hate Al Gore, An Inconvenient Truth (his documentary on global warming) is excellent.

Here's one stat that stands out to me:
Of 928 articles dealing with climate change in scientific journals in the past 10years, 0% had any doubt as to the cause of global warming.

Of 636 articles in the press about climate change over the past 10 years, 53% cast doubt.

I'll go with the scientific journals because that's where the studies are published, and it supports what I have read myself. I have seen web sites that try to argue global warming is not caused by humans, but I haven't been able to find many that use credible sources that either rely on studies funded by oil and energy companies or have a mission to promote a "conservative" agenda.