I don't think that the holocaust or the war in Iraq had much to do with climate.
But it would be a wild oversimplification to even say that Hitler
caused the Holocaust. For instance Stalin caused a famine in the Ukraine that killed millions of people in the 1930s, but his propaganda machine blamed the Jews for the famine. When the Nazis invaded the Ukraine in 1941, the Ukrainian population was already so anti-Stalin and anti-Jew that many of them participated in the enormous massacres of Jews (and many went so far as to become guards in the camps in Poland). So while Hitler bears supreme and central responsibility for the Holocaust, it was a near infinitude of complementary conditions that made it happen -- it wasn't him alone.
Without taking a good look at it academically, though, (I don't pretend to know the answer to this), I can at least entertain that the popular uprisings and revolutions of the late 1910s-1930s had to do with poverty and marginalization, and these may well have had to do with environmental conditions (and one can accept this without ignoring all the other influences, i.e. the rise of fascism and communism, the global depression, etc). That doesn't make them directly causal
, but that's not really the question here.
What is meant by high point, is that societies thrive because of changes in climate, crops grow better, and warmer weather creates opportunities, change and progress.
But the difficulty here is in deciding what is really a high point. If Islamic civilization and Mayan civilization had their high points at the low point of Christian civilization, then how do you possibly label that historical era? West Africa had a high point of its civilization in the 12th-14th century, and it's probably having a low point now... but are we even counting places like Africa, the Pacific Islands, the peoples of the Arctic, etc when we judge high and low points?
I'm saying that this does not discredit the theory, because it does not make the assumption that climate does have to influence good and bad behavior of humans.
Climate almost certainly influences behavior. Climate change led to a westward migration of rats in Asia during the 14th century, and this coincided with seafaring commerce on the Mediterranean -- and this led directly to the Black Death. Human migrations and political upheavals and economic transitions certainly are influenced by climactic conditions.
But it's probably not fair to blame a warm or a cold climate on a good or bad 'point' in civilization, as if there is some direct correlation between global temperature and human social sophistication.
(of any kind) that leads to low points, because that's what creates dysequilibrium. And you CAN blame the crises of the 20th century on dysequilibrium. Read "War of the World" by Niall Fergusson for much on this.