Global Warming and Intelligent Design share a fundamental characteristic in common, the lack of definitive, proven evidence of the causation which would make them fact rather than theory.
In Intelligent Design you can see the leaps of evolution and the bizarre anomalies in the fossil record and you can say "something must have caused this", but there is no proof of what or who is the cause. You can choose God or aliens or mutation or random unidentified forces as your explanation equally convincingly because no evidence exists to definitively identify the unknown forces driving the evolutionary process. If you believe in a conscious force behind evolution you can only reach that conclusion through faith.
In the same way, Global Warming is a clearly identifiable phenomenon which can be documented through the geological record and contemporary temperature measurements, but it is impossible to point to a specific cause and say "aha, here's THE thing which is causing global warming." Human causation is the popular choice as a cause, but human output of the gasses which cause global warming is substantially less than the cumulative yearly output from natural sources like volcanoes and forest fires and falls well within the normal variations of those gasses from year to year. Natural forces like solar activity and the earth's climate cycle also contribute to global warming and it is impossible to definitively identify one cause as specifically responsible. If your choice is to believe in human causation as the one thing that causes Global Warming, that position can only be reached by the equivalent of a leap of faith.
Because these are beliefs which can only be reached by faith, those who subscribe to them have a tendancy to be driven to fanaticism in defense of their position. They attack and harass those who disagree with them, deride them as deniers driven by a political agenda, be it secularism or corporate greed, and do their best to essentially redefine reality on their own terms. They will go to great and destructive extremes to force their beliefs on others, including imposing their agenda on the school curriculum, promoting their beliefs through advocacy groups, seeking to pass legislation enshrining it in law, and attempting to destroy and discredit those who don't agree with them.
The remarkable thing with this sort of fanatically held belief is that it doesn't matter if there's any truth or evidence behind it, or if there are facts which directly contradict it. The true believers will gladly redefine reality on the fly so that whatever facts they're presented with will fit into their worldview. Inconvenient fossil records predating the biblical creation of the earth? God created them to test our faith. Inconvenient record snowfalls and cooling trends in Europe? Well, of course, cold weather is a symptom of global warming. The great thing about faith is that it can help you rationalize anything no matter how ridiculous.
It's interesting that the people who object most to the irrationality of Intelligent Design theory are often the same people who are just as dogmatic in their adherence to Global Warming theory, and vice versa. The two are unrelated, but it's interesting that both groups have their issues of absolute belief which they hold to be inviolable with equal levels of vehemence and obsession.
What neither group considers is that maybe both positions are irrational and untennable and that both theories have flaws, are open to criticism and might need more study and analysis. Both Global Warming and Intelligent Design are still theories and neither has the stamp of absolute empirical truth on them, because both are based on assumptions which cannot be definitively proven and to some degree have to be 'taken on faith'. I find any demand to believe something which cannot be proven just because others believe in it – no matter how numerous they are – to be objectionable and an offense to reason. Reality isn't defined by a majority vote. What's worse, when a theory becomes dogma, those who subscribe to it fanatically will try to stifle further discussion, exploration and analysis, and that's a very negative force in a any kind of scientific inquiry.