Reuters: A climate change bill circulating in the Senate on Tuesday is slightly more ambitious than one passed in the House of Representatives, but still has many details to be worked out on how to encourage companies to reduce their emissions of greenhouse gases. The legislation by Senators Barbara Boxer and John Kerry to be formally unveiled on Wednesday aims to reduce smokestack emissions of carbon dioxide by 20 percent by 2020 and 83 percent by 2050 — from 2005 levels. A …