climate are two very different things. Climate is sort of long-term things. Weather is short-term- daily, hourly, monthly, whatever. And what we're talking about when we talk about the greenhouse effect is we're talking about climate. NARRATOR: James Trefil is a physicist and science writer who has followed the global warming issue closely. JAMES TREFIL: In order to say it is actually getting warmer now, you have to know what it's been in the past and you have to know that what you're seeing is not just a fluctuation. I mean, I remember when I was a kid, in the 50's, hearing stuff about the coming ice age because there was- we were in a kind of a little cold snap then. And people- I remember pictures on the covers of magazines showing glaciers moving down across American cities. So if you take any short-term timeline, any short-term database for temperature and extrapolate it, you can get crazy results. In order to make an incontrovertible case for global warming, you'd have to have a long-term temperature record, centuries, that was over a large part of the globe. And so you have to look over a long term and say "What's the average been for several hundred years, and is this a significant departure from that?" And that's what's very difficult to do. NARRATOR: Bad weather in one place at one time does not prove the climate is changing. To prove humans are warming the globe requires detailed knowledge of the