At first glance this article by Tim Case looks well argued and worth the attention of anyone interested in the causes of the fall of Rome. An extract:
If we look at history we find some very interesting events surrounding temperature change and agriculture during the last years of the Roman Empire.

For much of the history of the Roman Empire, ca. 500 BC until the Empire fell apart just prior to 500 AD the Roman Empire (including England) flourished owing to mild weather conditions. Warm weather allowed grapes and olives to be grown further north, and good rains allowed the Romans to buy abundant crops of grain from across the Mediterranean and in North Africa.

The three most important agricultural products traded in the Roman world – grain, wine and olive oil – were abundant and they created a very wealthy class of merchants. Great care was also taken to secure the routes needed to maintain a constant supply of corn from Egypt and Africa to feed the population of Rome.

However, by the close of the second century AD and early part of the third century, the Empire’s monetary policies were playing havoc with the Empire’s agriculture production. These monetary problems were nothing compared to what transpired when weather became a factor after 235 AD and the end of Severan dynasty.

The period from 235 AD to 284 AD was a half-century of unmatched calamity which nearly brought the Roman Empire crashing down on itself and was the result of constant unrepressed statism which had matured on the corpse of individualism and self-reliance with the passing of the Roman Republic.

The rigidity of the Roman psyche at this time, would not allow anything to exist in Roman territory that didn’t fit the Roman ideal of the Empire’s status quo. So when the Franks, Jutes, and Germanic Alemanni crossed the Rhine River and began to move back onto their ancient lands, and the Vandals, with the Goths, crossed the Danube River settling in the empire’s northeastern providences, there was nothing the Roman State could do but "suppress the uprisings." The question is; were these migrations really uprisings against a failing Roman Empire or was something else the cause of these migrations?

There is a growing body of evidence that suggests the third century AD was the beginning of one of the coldest periods in European history. If the data is correct then it would go a long way toward explaining those migrations from the north that the Romans ineptly called uprisings.