When do the years make a difference?

In the last little while, I've been reading with interest the reactions to how temperatures haven't been changing in the last 7-10 years. Since the spike in 1998, which can be attributed to a massive El Nino, temperatures around the world don't seem to have done much of anything. 2007 has seen a downward drop, which can largely be attributed to the large La Nina we're in right now.

For the AGW crowd, this is bad news. While they are quick to attribute the 1998 spike to global warming, predicting increases of such large El Nino events, along with any flood, drought, heat wave, cold spell, etc., any suggestions that things are cooling down has lead to either denials that temperatures are cooling (how ironic!), or that one year doesn't make climate.

They're right, of course. One year doesn't make a climate. 2007 doesn't represent climate any more than 1998 did. Nor does two years, or three. But what about 5? Or 7? Or 10? When does it stop being "just weather" and start being climate?

Generally speaking, climate is weather over a long period of time, which is something I've discussed before. More specifically, it's the average weather in a geographical region over the space of about 30 years. The choice of 30 years was pretty arbitrary. Most weather stations have been around for at least 30 years, so we have the data to work with.

Every now and then, that 30 year time period needs to be adjusted. I'd wondered how that worked, and finally found out. When it's adjusted is, again, pretty arbitrary. There are no hard and fast rules. It's not done every 30 years - that would be pretty useless - but every 5 or 10 years. Years that are added to, then eventually taken out of, the 30 year time period. That means climate is actually weather over 30 years, plus or minus 5 to 10 years.

So if the average global temperature has pretty much flat lined for almost a decade, it's no longer "just a year." It's not even "just a trend." We're looking at almost a full third of the time period used to determine climate for a region. It has to be adjusted - we can't just keep using the 70's as a starting point. Yet it seems to me that that's what the AGW proponents are trying to do by denying that the last decade's lack of warming means anything.