Total U.S. emissions in 2007. (Chart: FHWA)

The Senate is taking its first public steps toward combating climate change — and while the U.S. DOT was absent from this morning’s hearing, the chiefs of the Energy Department and Environmental Protection Agency reminded lawmakers that transportation must play a key role in any emissions reduction plan.

Under questioning from Sen. Benjamin Cardin (D-MD), who lamented that "transit hasn’t gotten the attention it needs in America," both Energy Secretary Steven Chu and EPA Administrator Lisa Jackson agreed that climate legislation should help promote alternatives to single-passenger car travel.

Jackson even low-balled the carbon footprint of the nation’s transportation choices, describing cars, trucks, and aircraft as "about 20 percent" of total U.S. emissions when the total number is closer to one-third (see above chart).

Many of those fuel-burning trips, Jackson said, are commuters who drive alone "often because they have no choice," making transportation reform a "quality of life" issue as well as an environmental one.

Chu seconded his fellow Cabinet member: "Increasing public transportation use, especially in urban areas, would do a lot in terms of decreasing our greenhouse gas emissions." Moreover, Chu added, rail is not just a way to move people more sustainably — moving goods via freight rail can achieve fuel efficiency greater than 400 miles per gallon, blowing trucks out of the water.

The nod to transportation during today’s hearing signals that the Senate may not relegate transit and smart growth to the third- and fourth-fiddle roles that they played in the House climate bill. Indeed, Environment and Public Works Committee Chairman Barbara Boxer (D-CA), who will take the lead on Senate climate legislation, recently advised transportation reformers to "work with me on my global warming bill."

Sen. Thomas Carper (D-DE) revisited that theme, asking Chu and Jackson to "think differently" on transportation in order to avoid political missteps made after the energy crisis of the late 1970s prompted the nation’s first fuel-efficiency rules. The query to focus on, as Carper put it, is "how do we get ourselves to drive less?"

Of course, the biggest unknown in the Senate climate debate is whether enough centrists from both parties can be won over to break an all-but-certain GOP filibuster of the final legislation.

Climate change denier Sen. Jim Inhofe (OK), the environment committee’s senior Republican and Boxer’s erstwhile ally on transportation, seemed to warn his California colleague that his party would throw the kitchen sink at the climate measure. GOPers on the panel released a joint letter to Boxer requesting a series of hearings on actual legislative language, an attempt to cut short the intense negotiating that sparked numerous last-minute revisions to the House climate bill.

"When will we see the bill that you intend to mark up?" Inhofe asked Boxer rhetorically as the hearing opened. Advocates and ordinary voters across the country may well be asking the same question.