Feb 172010

While it may not feel like it if you live in the US or Western Europe, this has been a very warm January, the fourth warmest on record:

It was certainly warmer than usual (and a lot dryer than usual!) up here in Ottawa. Not that I am complaining.

Some proponents of anthropogenic global warming will point at this result as proof that global warming is “scientific fact”. The plot tells a more nuanced story. In the short term (last 8 years or so) there’s no warming trend at all, if anything there has been slight cooling. Prior to that, there was a roughly 30-year warming period. One other warming period that is very similar in nature occurred in the early half of the 20th century. In between, there was some cooling, just not enough to offset the earlier gains, hence the more recent warming period started with an already higher temperature, and we see an overall warming trend across the 130-year span of this plot.

But is that really a long term trend? Or is it just another periodicity that we simply cannot discern because the data set it too short in duration? This is the question that those “proxy” temperature estimates from tree rings, ice cores and whatnot are supposed to answer. How reliable are those? How representative? That is a key question, which may determine, for instance, if the infamous medieval warm period was comparable to the current warming or not.

 Posted by at 8:55 pm