The U.S. Senate held its first hearing today to examine America’s Climate Security Act, the new climate-change bill introduced last Wednesday by Sens. Joe Lieberman (ID-Conn.) and John Warner (R-Va.). Given that the hearing was convened by a subcommittee that Lieberman chairs and on which Warner is ranking member, it should be no surprise that the expert witnesses overwhelmingly approved of the legislation.

Normally at subcommittee hearings, members of the minority party are less inclined to attend. Their voices are overwhelmed, their issues are not at stake, and their input often isn’t appreciated in any meaningful way. As today’s hearing convened, though, the Republican side of the stage was at capacity — every seat filled by its rightful senator, and staffers seated and standing behind them — while on the Democratic side, less than a handful of people showed up.

One of them was Bernie Sanders, independent of Vermont who caucuses with the Democrats; he was the sole official voice speaking up for significant strengthening of the bill. Sanders stood by the work he’d done with Sen. Barbara Boxer (D-Calif.) in crafting a much stricter climate bill. He called for incentivizing clean energies like wind, solar, and geothermal; pointed out the great opportunity a new energy regime would present for creating new jobs; and warned that insufficient action could spell calamity for billions of people. (Boxer could not attend, according to a letter distributed by her staff, because of the wildfire crisis in California.)

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On the Republican side, some senators — usual suspects like James Inhofe (R-Okla.) and George Voinovich (R-Ohio) — opposed the legislation outright. But many others simply wanted to express their concerns that the bill might hurt the American economy or that it featured too few subsidies for the nuclear and coal industries.

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In a twist that might make environmentalists happy, the two representatives of the green community invited to testify — Frances Beinecke, president of the NRDC, and William Moomaw, director of the Fletcher School Center for International Environment and Resource Policy at Tufts University — did not show up simply to provide a stamp of approval for the bill.

NRDC supports the legislation, but tepidly, and the group hopes to see it enhanced. In her testimony [PDF], Beinecke called for improvements explicitly, noting that the way emissions permits are allocated “can be substantially improved”; that “the bill should be revised to allow EPA to take all necessary actions to avoid dangerous global warming by requiring additional reductions”; and that “the bill’s coverage should be increased” to cover “commercial and residential use of natural gas.”

Moomaw raised similar objections and included a series of recommendations, suggesting that the bill should auction 100 percent of emissions permits by 2025 (as opposed to 2036), and that the bill’s effectiveness would be improved by incentivizing building efficiency, improved transportation systems, and urbanization.

Some environmental groups, notably Environmental Defense, are much more bullish on the bill as it is right now. But none of them were at the table today.

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Perhaps the most interesting question of the day came from Lieberman. He asked Beinecke and Kevin Anton, president of Alcoa Materials Management — both founding members of the United States Climate Action Partnership — what features of the bill they regarded as essential. What, he asked, would cause you to revoke your support?

For Anton, the clincher was a provision that rewards early adopters — companies that have already acted to cut emissions — with a better package of entitlements than other companies can expect. Beinecke suggested that if the emissions caps were loosened, it would be a deal breaker. Those features of the bill do seem to be secure. The caps may become more stringent, and the early adopters might benefit more, but neither provision looks likely to be weakened.

Support for the bill appears to be gradually growing among both Democrats and Republicans. Lamar Alexander (R-Tenn.) was lukewarm about it. John Barrasso (R-Wyo.) was a bit hotter. Max Baucus (D-Mont.) announced his full intent to support the bill, and Warner noted that he’d been assured of support from Montana’s other Democratic senator, Jon Tester, just days before.

Hearings like this, Lieberman noted, “are the first steps — big ones, but first steps — that this legislation will take through the Senate.” At this point, it’s hard to say how the rest of the bill’s journey will shape up.