Terrapass.com carbon calculator debate

I'm having a bit of an argument with the folks at Terrapass.com about the fact that they carbon calculators ignore the radiative forcing effect of flying.

Here's what Tom Arnold wrote in response to my original email.

Hi Carl:

We’re actively tracking this issue, and the most recent research from FAA and others suggest that 2.7 number is actually not the right way to look at things. 2.7 is simply the average of four studies in the 90’s, and now scientists are proposing a different methodology.

We’re expecting a good survey paper out in a few weeks, which we’ll blog on. Thanks for your patience, we do want to get this right, but we want to do it with the science community’s support.

In the meantime, our store system easily allows you to purchase more, just increase the quantities associated with your flight offsets.

I'm not convinced they will publish my response to it so I'll repeat it here...

Hi Tom,

Huge thanks for your reply.

You are suggesting that the science I quoted is a bit out of date and that the Federal Aviation Authority has some better research. Do you think they may have a vested interest in coming up with a better number? I do.

Which are the studies you are referring to which give rise to the 2.7 figure? Even with data collected in the 1990s, as you suggest, since most of the commercial air fleet dates back to the 1970s I don't expect much has changed since then.

The science is clearly complex and slightly uncertain but it should be noted that the 2.7 figure comes from the IPCC 1999 report 'Aviation and the Global Atmosphere'.

Sir David King, former Chief Scientific Advisor to the UK government and a professor of Chemistry at Cambridge University puts this at "up to 3x". I think he's probably in a good position to understand the science and has no vested interest in mis-reporting on the issue. He quotes his source as The Stern Review p388 box 15.6 and the IPCC WGIII chapter 5.

David King also explains, quite clearly, what Radiative Forcing is all about. I'm sure he won't mind if I quote him directly. "Aviation is directly responsible for about 700 million tonnes of carbon dioxide each year, which is just 1.6 per cent of the global greenhouse emissions. However, molecule for molecule the emissions count much more than they would on the ground because planes are very efficient at causing greenhouse warming. High-altitude deliveries of nitrogen oxides (which form ozone another greenhouse gas), as well as the water in contrails that can go on to form cirrus clouds, together enhance the direct effect of carbon dioxie by up to a factor of three". (David King and Gabrielle Walker, 2008, 'The Hot Topic').

As you can see, I think the science community has long been in support of understanding the Radiative Forcing effect of carbon released from planes. It seems that what you are really saying is that you don't want to move on this without the aviation industry being in support of the figures.

As an individual, sure, I can increase my carbon offsets with you. My object is that each and every one of the estimates you currently give to people are severely wrong.

We need some bold moves to tackle climate change within the 20 year window we have to do something about it. I hope you will be bold, read the science, and fix your calculator (and in the process explain why). One of your charts currently makes flying a better option than driving - that is just misleading on a number of levels.

Carl