Over the weekend a clever article in the Times of London provided a useful reminder that climate change isn't just a question of carbon dioxide emissions. After counting the greenhouse gas emissions from the agricultural and animal husbandry supply chains that deliver our food--much of which involves gases other than CO2--it suggested that driving to the store might generate fewer emissions than walking there, at least in Britain. I haven't verified the details of the walk vs. drive calculation for the UK, let alone converted it to a US basis, but the basic principle is correct: virtually everything people do on a large scale contributes to climate change, and focusing exclusively on the CO2 emissions from energy--as with a carbon tax--could lead us down an expensive and inefficient path to addressing global warming.
The reason the comparison described in the Times article isn't as silly as it might sound rests firmly on the science of climate change and the wide variation in impact (GWP) of different greenhouse gases. When you examine the net impact of the increases in atmospheric concentration of all these gases since pre-industrial times, expressed as "increased radiative forcing"--the equivalent amount of extra energy heating up each square meter of the earth's surface--you find that CO2 accounts for just over half of the problem. Methane and nitrous oxide (N2O), both of which have large agricultural components, along with ozone and halocarbons (e.g. freons) contribute most of the rest.
Now, I might question the author's choice of a calorie source on which to compare the walker's emissions to those of a motorist. 100 grams of beef probably creates the worst-case scenario, from a climate change perspective, given the rate at which rainforest is being cut down to graze cattle, which produce lots of "enteric emissions,"as they are euphemistically described. And considering the changes in the British diet since the advent of Mad Cow Disease, the marginal calories seem as likely to come from a can of Coke or a pint of beer, as from a burger or steak. That could tip the balance back to walking, especially if the car in question were a Range Rover. However, it's not obvious--or at least it shouldn't be--without running the numbers, and that's something that most people aren't equipped or motivated to do.
I suppose there's a point here about the need for better education and tools to help people wade through choices like this, but this issue really goes to the heart of the ongoing climate policy discussion in this country. Legislation that ignores the impact of greenhouse gases other than carbon dioxide, or that focuses mainly on the energy sector, won't be nearly as effective as regulations that encompass all of the sources of anthropogenic global warming. Energy accounts for roughly 80% of US CO2-equivalent emissions, but because the gases associated with farming and cattle have such high GWPs, measures to reduce them offer much more leverage--and thus generally lower costs--than brute-force attacks on CO2 such as carbon capture and sequestration. That's one reason why cap-and-trade looks better than a simple carbon tax on energy. Even if we end up having to do all of these things--as appears likely--our priorities need to recognize the timing, sequence, and cost of our various options.