That's interesting. I just looked it up and the crusades do pretty much coincide with the Medieval Warm Period.
Makes sense. Islam's strength was in hotter climes less likely to benefit from warmer temperatures. Christianity's strength was in colder climes more likely to benefit from warmer temperatures. Thus Europe enjoyed better yield increases with more surviving children and more surplus, which can be used to fund things like warfare. So Christianity takes back part of the Middle East (and also learns that taking something /= holding it.)
Sh! We must all respect the mantras. Climate was constant. The sun does not affect temperatures.
As a skeptic of CAGW I'm also skeptical of all claims that warming caused the failure of some civilization. I'm sure it has happened, I'm just skeptical because such claims, right or wrong, are also used to "prove" what a danger CAGW presents to modern civilization. In the case of the Roman Empire, such a connection seems very tenuous and convenient to me, which granted is my own learned bias rather than an examination of the actual evidence.