The idea that climate change has played a role in the fall of the Roman Empire has a long history itself. Gibbon mentions it as a possibility in Chapter 9 of the Decline and Fall of the Roman Empire. But I have looked in vain for actual information on what exactly the climate was actually like during the time of the empire.
The best I could find was this summary in the New Scientist which basically concedes that we aren't really sure.
Hopefully at some stage we will be able to piece together a more detailed set of data for actual localities in Europe that will enable us to get a clearer picture of what conditions the people of the time had to live through.
But in the meantime nobody is going to let the lack of data get in the way of a good argument. The latest proposal is a paper in Science that looks specifically at tree ring data and uses that to infer rainfall and temperature variability in Central Europe. The point they pick up on is that the climate was more variable between 250 and 600, when there was a prolonged political crisis. By contrast during periods of prosperity in the Roman Empire and in later Medieval times the climate was a lot more settled and was warmer and wetter.
I have to say that I can't get too excited about all this simply because it is just that bit too vague. The timescale of political events is simply a lot shorter than the periods this study is working on. But it does show just how interesting it would be if we could plot more precise data both in time and in area. In particular, the question I would love to know the answer to would be, if the people of the time had known what was going on could they have done anything about it?
I picked this up on the New Scientist website, and there is a link through to the Science paper at the bottom of it.