photo: Ann the Doc via flickr.
You want to be a better eco-citizen, minimize your air travel, and want to buy a carbon offset for those times when you do have to fly. But before you hit that calculate my emissions button you notice a small check box."Include Radiative Forcing" it says. What the heck is radiative forcing and why does it matter to the carbon emissions of my travel? And if it does, how much does (or should) it add? Here's the quick(ish) answer:The IPCC Definition of Radiative Forcing
This is how the IPCC defines it:
The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent (say, a change in greenhouse gas concentrations) is the change in net (down minus up) irradiance (solar plus long-wave; in Wm-2) at the tropopause AFTER allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values.
Which is all fine and good, and needs to be said, but probably doesn't mean much for readers in practical terms. The shorter answer is that radiative forcing is the change in the energy balance in the lower atmosphere by a climate change mechanism.
photo: Shane Halloran via flickr.
Flying Has More Impact Than Just Carbon Emissions
In terms of aviation those climate change mechanisms are not just carbon emissions, but other emissions from burning fuel as well, plus soot, contrails (which can help form cirrus clouds) and other factors as well. These factors magnify the warming effect beyond what just the effect of carbon dioxide. And since you're pumping them directly into the upper atmosphere the effect is still greater.
Radiative Forcing Takes Into Account More Than a Single Flight
It's important to note that when talking about radiative forcing, the impact does not take into account the influence of a particular flight from one place to another; instead, it figures in the historic impact over time. It's not that your particular flight has this impact, but aviation in general does.
This is unlike how carbon emissions from a flight are calculated, even those these too are a bit of an abstraction: Certain assumptions are made regarding the planes used on a given route, by particular airlines, how full the plane is (generally about 75%), et cetera, to come up with the emissions for a route that are a good representation of the flight.
How Much Greater an Impact Does Radiative Forcing Create?
In figuring how much greater an impact radiative forcing causes, a radiative forcing index is used as a multiplier. No one doubts that radiative forcing has an impact, but in calculating how large an RFI to use is where the real debate starts.
Depending on the particular study you look at an RFI of anywhere between 1.2 and 4.7 is appropriate. As of a 2000 report, the IPCC considers a good RFI figure to be 2.7. However, a more recent study indicates that a lower figure of 1.9 is probably a better estimate of radiative forcing's impact.
How does this all apply to the carbon offset you want to buy?
Different offset services use different factors, or none at all.
For example CarbonFund allows you to include radiative forcing as an option, and uses a factor of 2.7, based on that IPCC report's recommendations.
TerraPass on the other hand doesn't give you the option to include radiative forcing at all (though they do allow you to be more precise inasmuch as giving you different airline options). The analytic platform on which their calculations are based, TRX Travel Analytics, will calculate the impact of radiative forcing, however, using that same IPCC figure.
Why Err on the Side of Not Doing Enough?
So why not include it? The stock answer that TRX provides on their website (and TerraPass will tell you if you call them) is that because there is still scientific debate ongoing regarding the true impact of radiative forcing, some people may not want to include it in making their offset calculations. Which really seems to be to be a bit of a cop-out.
It's true that the debate is ongoing and there are different figures out there, but none of them say that the impact shouldn't be considered or diminishes the impact of flying.
And when it comes to doing things to offset the environmental impact of your fossil fuel consumption, it's probably better to err on the side of doing too much rather than doing too little, just because we don't know if the effect of radiative forcing is bad or really bad.
Want to dig more into radiative forcing? Check out these reports:
Oxford University: Calculating the Environmental Impact of Aviation Emissions
Max Planck Institute for Meteorology: Climate forcing of aviation emissions in high altitudes and comparison of metrics
Aircraft Emissions: Contributions of Different Climate Components to Changes in Radiative Forcing
Confused About Which Carbon Offset Service to Use: EDF Lists Eleven They Trust
ShopGreen Lets Online Retailers Incorporate Carbon Offsets into Purchases
Carbon Neutral Myth? Protesters Take on Carbon Offset Companies
Flying Carbon Emissions
Virgin America Becomes First US Airline to Report Its Greenhouse Gas Emisisons
84% Reduction in Jet Fuel Carbon Emissions Using Camelina, New Analysis Shows