Realclimate has a new post about some peer-reviewed research up regarding some of the research into how accurately we can estimate number of storms before the introduction of modern meteorological data, which is understandably a big question in the debate. Their summary:
In summary, according to current knowledge, the best estimate for the underreporting bias in the hurricane record seems to be about one tropical cyclone per year on average over the period 1920-1965 and between one and three tropical cyclones per year before 1920. With only a few years of data available, the influence of Quikscat analyses after 2002 as discussed by Landsea, is difficult to as yet meaningfully estimate.
Go read the whole thing. This is an exciting area of science, one which has clear political applications, and which is very much in dispute. Personally, I don't think that the science is quite there in terms of definitively determining whether or not global warming is causing an increase in hurricanes, but I'm open to being convinced.
(And of course, none of this means that the overall scientific conclusion that global climate change is anthropogenic is in any way in doubt. The science on that has been in for a while now -- it's only denialists and those not in possession of the evidence who doubt that essential conclusion.