9/11 research challenged: contrails aren't turning up the heat
Since the 9/11 terrorist attacks, it has been widely believed that the pretty white lines in the sky created by aircraft at altitude - otherwise known as 'contrails' - are a significant contributor to global warming.
The theory goes that contrails can become cirrus clouds that trap 'outbound' longwave radiation (heat) from the Earth's surface and atmosphere. They do this better than they reflect 'inbound' solar radiation (sunlight) back into space, so their net effect is warming.
In order to prove this, you might use the 'diurnal temperature range' (DTR), which is the difference between peak daytime temperatures and minimum night-time temperatures. You would compare DTR when aircraft are flying, with DTR when they are not flying. If the range increased, that is, night-time got colder and daytime warmer, it would demonstrate that contrails were indeed trapping outbound heat and reflecting inbound sunshine.
The problem here is that air traffic never actually stops. But for three days after 9/11, that's exactly what happened when all commercial flights were grounded. A team led by David J Travis of the University of Wisconsin-Whitewater took the necessary measurements, crunched the data and published the findings in the journal Nature.
The result? DTR did indeed widen by a full 1°C during those three days, in distinct contrast to the three days before the grounding and the three days after flights resumed.
But now a US study by Dr Gang Hong of Texas A&M University has found that DTR variations of 1°C during September aren't all that unusual and that the change in 2001 was probably attributable to low cloud cover. Elsewhere, a team at Leeds University, working with the Met Office Hadley Centre, ran contrails through its climate models and found that you'd need about 200 times the quantity of flights over America to produce a significant effect on DTR.
So while climate warming contrails join endangered polar bears on the list of flawed factoids, it begs the question of why the idea gained so much traction. The three-day grounding was an unprecedented scientific opportunity, yes, but the sample size was arguably far too tiny to have ever produced anything but indicative findings and certainly nothing approaching definitive proof.
© 2012~RS~q~RS~~RS~z~RS~36~RS~)
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"...why the idea gained so much traction?"
Perhaps you should ask one of your colleagues on Horizon.
http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_qa.shtml
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Nobody's perfect! (And to be fair to the Horizon team, the findings of the Texas A&M and Leeds/Hadley studies weren't available to them when this doc was made.)
Also, if anyone clicks on SheffTim's link, there is an idiosyncracy in the our blog engine that adds <br /> after an underscore, so the link will break. To follow the link, either cut-and-paste the whole URL from SheffTim's comment, or click on the link and delete the <br /> from the URL when it appears in the address bar. In the meantime, I'll endeavour to get it fixed...
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1. At 3:19pm on 21 May 2009, SheffTim wrote:
"...why the idea gained so much traction?"
Also...
Ground-based astronomy could be impossible in 40 years because of pollution from aircraft exhaust trails and climate change, an expert says.
And...
9/11 effect
The first clue came from an unlikely and tragic event.
In the days after 9/11, aircraft across the USA were grounded for three whole days.
Surprisingly, there appeared to be a change in the weather.
Props to Mr Cable for once again daring to report upon climate-related issues in a way that the BBC's front-line big-name environmental correspondents would, I'm sure, rather he didn't.
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Its strange to think that something seemingly so seemingly insignificant can be an influence on climate change. But are they also taking into account the emissions of planes?
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I think it's fair to say that emissions are a separate issue to contrails. As mentioned in a couple of other blogs (see below), the airline industry is generally used as lazy benchmark for emissions-based dreadfulness:
- The internet: never mind the emissions, just look at those savings
- Who's a sucker for carbon dioxide?
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Richard,
Thanks for posting this. And the link to the polar bear story.
Regarding forecasts and computer models, have a look at these BBC stories:
1. September 2008
Milder and drier winter predicted
This winter will be milder than average and drier than last year in the UK, Met Office forecasters have predicted.
... the winter months will see relatively high temperatures.
2. February 2009
Heavy snow disrupts London travel
Thousands of people have been unable to travel in London as snow disrupted the city's transport network.
...
London has seen the heaviest snowfall in 18 years, weather experts said, with an accumulation of 20cm (8in) in some areas.
Maybe you could write a post about the success rate of these predictions. They always seem to forecast some kind of global warming - mild winters or hot summers and the BBC eagerly run the story. Is it time to call them out ?
The latest one predicts a "barbecue" summer. This is great - I could predict that and charge a lot less for the service. But it's not very scientific.
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You can´t predict local weather from a climate change model. Climate is a median measure oriented thing. Weather forcast models are even more complicated then climate models. And any calculation will diverge from the real weather after a while because the mathematics involed are deterministcly chaotic in nature. Read Benuit MAndelbrot and others about fractals and dynamical systems.
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