Appliance of Science: Is seasonal allergy on the increase?

Now we can add seasonal allergies to the list of reasons why we need to tackle global warming.

I thankfully don’t suffer from any seasonal allergy, but I know a lot of people who do and I have noticed them mention that the problem is getting worse and lasting longer each year. Is this simply their reaction to yet another year with these highly irritating symptoms or is seasonal allergy really on the increase?

What causes it?