Tuesday, October 6, 2009

Did global warming cause the Atlanta flood?

Stu Ostro, leader of the Weather Channel’s team of tornado, hurricane and climate experts just outside the Perimeter, describes himself as a reformed skeptic when it comes to the topic of man’s impact on climate.

But in a post over the weekend, Ostro raised the ultimate question: “Did global warming ‘cause’ the Atlanta flood?”

The post is long and complex, with charts and graphs as tough to wade through as Pumpkinvine Creek was 10 days ago. But here’s his summary toward the end:

…There’s a straightforward connection in the way the changing climate “set the table” for what happened this September in Atlanta and elsewhere. It behooves us to understand not only theoretical expected increases in heavy precipitation (via relatively slow/linear changes in temperatures, evaporation, and atmospheric moisture) but also how changing circulation patterns are already squeezing out that moisture in extreme doses and affecting weather in other ways.

In other words, the answer is yes.

Off the chain without a 'cane

Stu Ostro, The Weather Channel

That's what the weather was in the Atlanta metro area early last week, and things were wiggy in the U.S. for much of September. Usually during that month when there's wild weather, including precipitation extremes, it's as a result of a hurricane or tropical storm. Not in 2009.

This "ex-skeptic" hasn't blogged about climate change in a while. For that matter, I haven't blogged about anything for a while! Been a bit distracted, but it's time to jump in the water again. Or maybe I should say, time to dust off my Nomex suit and put it on!

Before you fire up the flamethrower, though, let me say what this long entry is NOT about.

It's not about H.R. 2454 (more commonly known as the Waxman-Markey bill).

And I'm not telling you that you can't drive your SUV.

This blog is about the effect of climate change upon day-to-day weather. About physics and thermodynamics not politics.

It was two years ago last week that I first thoroughly laid out the basic premise.

Nothing that's gone on in the atmosphere since then has convinced me otherwise, and I've continued to add gazillions of weather events to this PDF [56MB file, and now up to 529 slides]. My goal has been and continues to be to document and objectively analyze these cases.

There have been anomalies and extremes for as long as there has been weather on the planet; the key is to assess how they are now changing as the climate changes.

To review:

--The global climate is overall warmer than it was in the 1970s. (That shouldn't be too controversial a statement!)

--Technical talk: The atmospheric warming has resulted in an increase in 1000-500 millibar thicknesses. Those increased thicknesses are manifesting themselves primarily by an increase in 500 mb heights (particularly notable in mid-high latitudes of the Northern Hemisphere), as there has not been a similar rise in 1000 mb heights. Although there is of course natural year-to-year variability, the overall trend at 500 mb has clearly been upward.

Analogy: It's like bread baking in the oven. As it warms, the dough expands in depth. Although the details of the science involved are different, the analogy works, which is that the depth (thickness) of a given layer of the atmosphere is increasing on average as that layer warms. Furthermore, in this case, the bottom of that atmospheric layer (1000 millibars) is not significantly changing, just as the bottom of the bread isn't (in that case, it's fixed by the bottom of the pan).

The rest of this fascinating post at weather.com is here.

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