It's odd that a question that lies in the realm of observation and science, and which can be answered by collecting and analyzing data, has turned into a question of political debate. These people who argue for the absence of climate change must not do any gardening, because if they did, they would see the changes in their gardens over the years. When I first moved to the southeast almost 20 years ago, we were barely into climate zone 8. This past winter, the temperature never did drop into the teens, meaning that climate zone 9 has moved north into our area.
If you want to see a good graphic of this, the folks at the Arbor Day Foundation have this graphic of climate zone changes from 1990 to 2006: http://www.arborday.org/media/mapchanges.cfm
This is also the sort of thing that you can verify for yourself with historical temperature data from places like wunderground.com.
What importance does being included in climate zone 9 mean for me and my garden? For one thing, it means I can look forward to better chances of getting fruit from my loquat tree. If you read any horticultural bulletins about growing loquats, you may see them say that winters are too cold north of Jacksonville, FL for them to set and bear fruit. Well that may have been true in 1990, Jacksonville used to be the northern limit of zone 9, but not any more. A couple weeks ago, when I went on a visit to Columbia, SC, I happened upon a loquat tree that was loaded with ripe fruit. It was part of the landscaping in the parking lot of an office building, and I was able to sample a few. Quite tasty!
There are a couple of loquat trees outside my local library, and they bore fruit last year, but I figured that their microclimate was warmer. They are on the south side of a two story brick wall, so they already have an advantage and could be considered to be in zone 9, unlike the rest of the area. Or so I thought. I have since seen other loquats around town with fruit on them, so I guess we really did have a zone 9 winter. My little loquat tree is still not as tall as I am (over 6'), but with any luck, maybe it will put on some good growth this year and flower and set its first crop of fruit next year.
Oh, and if you have no idea what a loquat is, check out this page from Purdue University: http://www.hort.purdue.edu/newcrop/morton/loquat.html