On April 4, the Senate Energy & Natural Resources Committee hosted a climate-change summit with major U.S. companies for ideas to incorporate into climate-change legislation that Sens. Pete Domenici (R-N.M.) and Jeff Bingaman (D-N.M.) are expected to introduce this year. Officials of several corporations—including Duke Energy, General Electric, PNM Resources, Sempra Energy, Shell Oil, and Wal-Mart—said it was time for a mandatory cap on U.S. greenhouse gas emissions. They said Congress should pass legislation setting up a cap-and-trade program that would give firms emissions allowances for CO2 and other greenhouse gases. Those with emissions lower than their targets could sell allowances to other firms. Several companies called for a financial "safety valve" that would allow more allowances to be allocated if energy prices spike beyond certain limits. And they said that the U.S. effort to reduce greenhouse gas emissions should start soon. Southern Co. was the only firm at the conference that advocated voluntary action.