Wednesday, October 17, 2007

First Frost and Global Warming

We haven’t had a frost yet (and we did not turn the heat on for the first time until Sunday evening, although on some cool nights we’ve had a fire in the fireplace). I’ve been thinking that it was unusual if not unprecedented to be frost-free this late into the fall -- another sign of global warming. For six or so years in the late 1990s I kept track of weather information – high and low temperatures, big storms, first frosts, etc. This morning I pulled out the notebooks in which I jotted down those records. It turns out that in 1995, we didn’t have a frost until October 18. I was under the impression that the first frost always hit in late September or early October, but this might be because the third week of September is when you have to start worrying about your tomatoes getting zapped – not that they will, but that they could.

Here are the first-frost dates from my notebook:

1994 – October 3

1995 – October 18

1996 -- October 4

1997 -- September 25

1998 -- September 24

1999 -- October 8

During those years we lived in New Canaan, on the Pound Ridge border, about 200 yards from a reservoir, which might have protected the immediate area from an early frost. In any case, a first frost during the third week of October isn’t that unusual. Of course the third week of October is half over and there’s no frost in sight yet, so who knows how long it will last. So maybe it is global warming; and since no one with any knowledge is claiming that we're just starting to feel the effects of global warming now, maybe the 1995 record should be attributed to it as well.

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Blogger Sam said...

Hey Tom, I've been learning a lot about climate stuff lately and yes, you're in a warm period now as defined as departure from the 30-year climactic average.

To really see the differences over time, you need to look at over 100 years of data - National Climate Data Center (NCDC) has some stations reporting back to 1895. It takes a LOOONG time to do this for a specific region, state, or city because there are so many observations - but the pattern is basically a steady rise in mean monthly temperatures, with a blip about 1938 or so when there was an "abnormal" hot period.

Another thing I've seen might be called the "push back phenomena" where the spring and fall seasons seem to start later in the year. I have not tested this against any NCDC data, however. This takes some fancy statistics because the overall trend is warming over time, and you want to estimate if there is a lag in RELATIVE cool versus warm temperatures. But it makes sense to me. Seems spring and fall come about 40 days later than they used to.

That too is an effect of global warming mainly because latent heat, especially near large water bodies.

I'd be careful about year-to-year comparisons because the picture becomes so clear when you look at the long range, beyond 30 years. The graphs will show peaks and troughs but the trend is unmistakable. Using a simple regression model you can strike a perfectly straight line through the graph that clearly shows warming temperatures.

My last point is that global mean temperatures don't mean squat. If you look at a single station or region, let's say Connecticut with part of the Long Island Sound (surface land, water, separate and combined) you'll see temperature increases of perhaps several degrees, not fractions of a degree. You just have to be careful to know how to "bin" the data, since averages of averages will smooth out things too much. It is truly alarming once you see it. /sam

12:30 PM  

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