I just thought I would add my own plots to Joe's.
Recently I came across NOAA's severe weather tracker, where they have the historical data broken down by month (and by day too, if you dig far enough) for severe weather events in the US. As a scientist my first thought was, "Cool! Data! I should plot that and see if I can see any trends." So I did.Looking at this data you should keep in mind a previous post of mine about interpreting graphs. So looking at this all I see is 2008 was a bad year, 2006 as well but 2009 was a relatively calm year. I would like to point out two things that I found interesting. The first has to do with the global warming political fight that is going on right now, the other deals with a more scientific question about the actual data.
Some where along the way certain elements in the news media found this data and were quick to point it out and say, "If there is supposed to be global warming, how come 2009 had the lowest level of severe weather we've seen in a long time?" To which the pro-global warming/environmental camp replied, "Weather is not climate!" But the problem is (and I say this at the risk of sounding childish, but it is true), they (the global warming/environmentalists) started it (as in, they were the first ones to confuse weather with climate). A few weeks ago I read a paper written (in 2004) by a couple of clear minded, rational environmentalists that were discussing the best tactic to take to push for environmental reform and legislation (I'll try and find the paper and post a link to it in a comment). While the paper was interesting I found the forward to be very interesting. The forward was written by the editor of the journal the paper was published in, and in the forward, the editor got on his soap box and said how the the environmental movement needs to emphasize more the connection between severe weather events and climate change. Essentially he was saying that the idea of slow climate change over 20-100 years was too far removed from people and that they needed to make it more real for people. Two years later in 2006 this idea of the connection between severe weather events and climate change was used extensively by Al Gore (in-)famous movie. The problem is, weather is not climate. For several years the environmental/global warming movement have been jumping on every case of severe weather and yelling "Look! Global Warming!" So it should come as no surprise when in 2009 the number of severe weather incidents dropped to their lowest level since NOAA began keeping accurate records (2000), the other side began yelling, "Look! No Global Warming!"
In my opinion they are both wrong. Weather is not climate, and the data we have is woefully insufficient. In terms of severe weather events we only have 10 years worth of (accurate) data, which is not enough to justify a comment about climate, which requires at least 20 years of observations. And the data is only for the US, not the world. Anyway, I guess I'll get off my soap box for now (for anyone who is interested I did find a well reasoned and sane article dealing with the climate change debate on Der Spiegel. This is probably the only news article you will find out there explaining the problems with the global warming political fight and where the debate went wrong).
So if you only look at the data by year there might be a few interesting observations you can draw, but things get more interesting if you look at the break down according to month.
Now that is an interesting pattern. It would appear that severe weather happens only in the summer and hardly ever in the winter. Which lead me to look closer at the data. It turns out that this particular data set contains only tornado reports, hail damage, and high wind damage, essentially it only contains "warm weather" severe weather. This is what you might call a biased sample. This data effectively hides "cold weather" severe weather, which also might be an indication of climate change, but again I should emphasize that we need AT LEAST 20 years of good data before we can begin to say anything about climate, right now we only have 10.
The other thing is where the data was taken. If you look at a map of where the events took place you may notice an interesting trend.
The severe weather happens mostly in the East. This may again be a consequence of the type of severe weather sampled (i.e. tornadoes, hail and wind), or it may be an indication of climate, which means if you look at it over a sufficiently long time (i.e. 100 years or so) you may see a trend. Again we only have 10 years worth of data. Or this could just be an indication of where people live. In the West there is more empty space with no people so it may just be that no one is there to report the severe weather. So many uncertainties! (uncertainties that you will never hear reported in a news story).