See Brave New Climate on 21 November 2009, for a post on global temperatures by James Hansen.
If you ever find yourself at a dinner party with one of those annoying people who are in denial about the whole global warming thing, get their email address and send them this post. It has graphs showing temperatures, and explains where the data is from. It offers insights into the sorts of problems that arise when analysing large data sets, and how mistakes can be made, and exploited by those hoping to cast doubt in people's minds about whether the world is warming and what is causing it, and how they are inundated with "Freedom of Information" requests from people who aim to dispute the conclusions drawn from their data. This research group now publishes their data in a private domain first, so that they can check the data thoroughly before going public on the internet - they learned the hard way that their mistakes are siezed upon.