There were people, thousands in fact, that believed duodenal ulcers were caused by stress and diet, an idea that was in vogue for many decades and still has a tiny handful of adherants. Millions of dollars was spent on research in the field, too. They were wrong. The ulcers were and are caused by an infection with Helicobacter pylori.
The proof that the IIPC Report isn't reliable is that it contains virtually no dissenting views, though they abound in the scientific community. It contains virtually no research or comments on how best to take advantage of temperature increases, such as crop changes, and farming method changes.
Its sole aim is to act as a fig leaf for modern
Luddites.
Additional reference:
A Global Warming Primer
Global Warming Bugaboo