Chances are, however, you're more polite than that. You might also want to be part of the solution. So when tree huggers come accosting, you simply make the case, again, that you drive responsibly. You probably also point out that much progress has been made to make cars cleaner, and that you will continue to support and push for more progress, until cars are so clean that we're making soup with the tailpipe vapours.

Well, that's along the lines of what I usually say. But I have been recently ruminating about another possible response -- one that comes from an even more defensible position.

I will simply state that, yes, I drive, but I don't have a fireplace, and rarely, if ever, burn wood. In other words, I have priorities.

Burning one cord of wood in a fireplace produces more smog-causing emissions than the entire lifetime emissions produced from 10 new SUVs. (I got that from a recent speech by Reid Bigland, who is president of DaimlerChrysler Canada. Yes, his company is in the business of selling new SUVs, but I'm sure the company lawyers looked at it.) Bear in mind, however, that smog-causing emissions are not the same as carbon dioxide emissions, which are the greater contributor to global warming.

I could also say that I only fly for work purposes.

Noted British commentator George Monbiot recently wrote a book called: Heat: How to Stop the Planet From Burning. In it he lists The Top 10 Things You Can Do For the Environment.

At the top of the list? "Cut your flights. Nothing else you do causes so much climate change in so short a time."

I caught Monbiot on television the other night, where he expanded on the impacts of airplane travel.

"Airline travel is not part of any solution to curb global warming...There are no good technological substitutes or innovations (looming) in the time scale we need. No super-efficient engines for instance, or low-carbon fuel."

He proposes a Carbon Rationing System. Under this scheme everyone would be entitled to produce 1.2 tons of carbon dioxide per year. (See his website, turnuptheheat.org, for details.)

"Your 1.2 tons would be used up in one trans-Atlantic trip," said Monbiot.

I guess that's a pretty strict ration, especially for someone addicted to and/or passionate about travel. But Monbiot, like many others, is convinced that now is the time to do some major adjustments in how we live and move around, especially in the developed world.

Some of his other suggestions include:

Build wind generators in the oceans, where the wind is stronger, and connect them to the grid with DC (direct current) cables, which are much more efficient than AC (alternate current) transmission lines;

Do away with big-box stores;

Build houses that can stay a constant 19 degrees centigrade without a heating system (already being done in Germany);

Move bus stations from city centres to freeway junctures, and expand urban transport systems (subway, LRT, etc.) to those freeway bus stations;

Find a better way of making concrete, which requires huge amounts of heat and chemicals.

Car travel seems to get off relatively lightly in Monbiot's recommendations. But there is this item on his Top 10 Things You Can Do for the Environment list.

"Think hard before you pick up your car keys. On average, 40 percent of the journeys made by car could be made by other means -- on foot, by bicycle, or on public transit."

And here's the rest of the list:

Organize a "walking bus," to take the children to school;

Ask your boss to devise a "workplace travel plan," which rewards people for leaving their cars at home;

Switch over to a supplier of renewable electricity. You don't have to erect your own wind turbine, but you can buy your power from someone who has;

Ask a builder to give you an estimate for bringing your home up to R2000 standards;

Ditch your air conditioner;

Turn down your thermostat in winter: 18 degrees is as warm as your house ever needs to be. You just have to dress warmer;

Make sure every bulb in your house is a compact fluorescent or LED;

Do not buy a plasma TV. They use five times as much energy as other models.

If it were up to me, I'd add a few more items to that list: Cut down the times you burn wood in an open fire; Cut down the times you burn wood in a moving vehicle (double whammy there, no good at all); Cut down the times you burn effigies (distasteful and polluting).

All this is not to put the automobile in a saintly position, relative to the other offenders. Rather, it just seems to me that with so many ways to make a difference, car people need to do some serious prioritizing. Keep driving if you love or need to, just make sure you check a few items off that list, or ones like it.

Car people tend to focus on car stuff, but maybe we should be more attuned at keeping our collective butts on the virtuous side of any carbon-dioxide line that might soon be drawn in the sand.