Not to contradict your statement, but this is an often misinterpreted letter. If you read the letter in its entirety, it is very clear that Landsea is talking about the very specific issue of hurricanes and global warming. He mentions global warming several times and never once questions the reality of global warming. It certainly seems to me like he's just taking global warming as a given. The reason he gives for resigning is:
"My view is that when people identify themselves as being associated with the IPCC and then make pronouncements
far outside current scientific understandings that this will harm the credibility of climate change science and will in the longer term diminish our role in public policy."
In other words, he thinks global warming science should influence policy debate. In this instance, he feels the IPCC made a statement it shouldn't have, and it will hinder future efforts to affect policy.