Monday, January 6, 2014
This is of somewhat local interest, though perhaps it's also an example that may be educational elsewhere.
It's common around here to have long-time residents say that the winter weather is both warmer and more volatile than it used to be, and that this must be due to global warming. After hearing a particularly dramatic story of this ilk recently, I thought it would be interesting to look at the data.
I found data for the Cornell weather station here and took the average January low as my metric of winter conditions. The graph is above, with the blue line being the data, and the red line a centered moving average. I have made no corrections for heat island effects and the like - if anything the Cornell campus has grown over time which might have been expected to create a (probably very slight) warming trend.
You can see immediately what people are referring to - it was four or five degrees colder in the seventies and eighties than recently - certainly enough to notice. However, attribution of this local change to global warming is made problematic by the fact that it was considerably warmer than now back in the thirties and forties. The data as a whole do not show an upward trend.
Also, the degree of volatility is not obviously more pronounced than in the past. The climate here has always been very volatile year-to-year with swings of as much as fifteen degrees Fahrenheit not unknown.
Conclusion: there is multi-decadal noise in local (and no doubt regional) temperature signals that means a clear global warming signal cannot be extracted from them (yet, anyway). You have to look at larger scale averages to confidently see anthropogenic climate change.
And the corollary would be that you cannot be confident that the climate will warm in your particular location in the next decade or two - it probably will, but not necessarily. Don't stake your credibility on it.