A couple of weeks ago I was having lunch with local science writer Catherine Dold at an outdoor table of the South Side Walnut Cafe. While we sat there, a large delivery truck pulled up in front of the restaurant. While the driver got out to make the delivery, that truck sat idling -- emitting considerable noise and smelly fumes -- for about 20 minutes. No kidding.

This annoyed Cathy and I, and it got us wondering what impact lengthy bus and truck idling has on local air pollution -- including, of course, greenhouse gas emissions (GHGs).

Then last week, as I sat at a Denver airport bus stop waiting for a Skyride back to Boulder after a long flight home, I chatted with an older gentleman who was also waiting for a bus. As we sat there, RTD bus after RTD bus would pull up and sit idling for anywhere from three to 12 minutes. He started coughing and grumbled, "You know, we're paying for all that diesel fuel they're burning up just sitting there."

Fumes from large idling diesel vehicles are something you can't help but notice, even in Boulder -- especially if you use mass transit regularly. They're an attention-getting annoyance. But how much do they really contribute to local greenhouse gas emissions?

I'm just starting to look into this angle, but I'd like to share my digging process with you. Here's what I've learned so far...

Around the US, several state and local governments have enacted "idling restrictions." In Colorado, I've learned through a secondary source (I need to verify it) that Denver, Colorado Springs, and Aspen have idling restrictions.

Currently, Boulder has no idling ordinance. Beth Powell, who handles marketing and communications for the City of Boulder Environmental Affairs Dept., told me yesterday that this idea has been discussed in city government but didn't move forward. "Overall the decision was that an idling ordinance wouldn't have a significant impact on emissions within Boulder, compared to other source," Powell said.

According to Boulder's 2004 GHG emission inventory (conducted by Ecoenergy International and summarized in the city's Climate Action Plan), the transportation sector is Boulder's second-largest GHG emission source (at 28%; the largest is the commercial sector at 30%).

That may sound like a big deal -- and perhaps imply that the city should make vehicle emissions a major focus of its climate action plan. However, when you look at Boulder's emissions by energy source, electricity and natural gas (used primarily by buildings) account for 68% of GHG emissions -- more than twice the emissions produced by vehicle fuels (28%).

This is one reason why, at least in 2007, far more of the city's climate action dollars (from the carbon tax) will be spent on commercial and residential programs, rather than on transportation efforts.

In Boulder, the biggest energy consumers (and hence, biggest GHG emission culprits) tend to be quiet, unmoving buildings -- rather than moving, smelly, noisy vehicles. When you walk around town, buildings might not grab your attention as much as cars. But in terms of energy-saving opportunity and potential emissions impact, they're the true low-hanging fruit.

So if you're pondering Boulder's contribution to global warming, it's important not to get too distracted by what your senses tell you. Much of the big picture of energy and GHG emissions is still and quiet. Yes, long-idling buses and trucks do pollute. Yes, traffic jams and an overabundance of SUVs are not good for the local or global environment. But that doesn't necessarily mean they're the logical place for climate change programs to start.

...That said, Powell said the climate change action plan budget for 2007 does include $10,000 for transportation-related programs. That number is expected to increase significantly in coming years. (More on the climate action plan budget in a subsequent post)

OPEN QUESTION: Which greenhouse gas emission sources have you been noticing lately in Boulder? Do you think they're significant enough for the city to spend climate action plan funds on? Comment below, and let's discuss!