Friday, February 20, 2009

Climate changing from rain to snow with some measurable flooding impacts

We had flood warnings over the weekend that didn't amount to much, fortunately. Yesterday the weather cleared enough to show the Mount Hamilton Range covered in snow, which helps a great deal. The higher-altitude precipitation comes down as snow instead of rain during storm events and melts slowly over a period of days, instead of surging down in a few hours with all the other rain.

That's how it's happened in the past and today, but will be less true in the future due to climate change. While more intense storms and flooding are a possible consequence of warming due to changed weather patterns, that effect is hard to quantify. Changing from snow to rain, however, isn't so hard to make some rough calculations.

The saturated adiabatic lapse rate is just under 5 degrees Celsius per 1000 meters. The standard prediction from the latest International Panel on Climate Change for warming in the next few decades is .2 degrees Celsius per decade. Putting the two figures together means that after ten years, the snow line in a typical storm would be 40 meters higher than today, or 80 meters higher after 20 years (about 130 and 260 feet, respectively). While that may not sound like a lot, it could potentially turn a great deal of snowfall into rain, and it's only going to get worse.

While this is far from the most serious climate change impact, it's fairly quantifiable. The question is whether land use agencies will take this under consideration in their new responsibilities to prepare for flooding.


UPDATE: See page 2 to see a graph of California temperatures. It's in Fahrenheit, but looks like roughly .5 degrees/decade, or about the same as the IPCC global prediction in Celsius.

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