Negotiators from 158 countries reached basic agreement Friday on rough targets aimed at getting some of the world's biggest polluters to reduce emissions of the greenhouse gases blamed for global warming. A weeklong UN climate conference concluded that industrialized countries should strive to cut emissions by 25 to 40 percent of their 1990 levels by 2020. Experts said that target would serve as a loose guide for a major international climate summit to be held in December in Bali, Indonesia. "We have reached broad agreement on the main issues," said Leon Charles, a negotiator from Grenada who helped oversee the Vienna talks. Delegates worked into Friday evening to overcome resistance from several countries, including Canada, Japan and Russia, that preferred a more open approach rather than setting emissions targets. The 2020 targets are not binding, but they were seen as an important signal that industrialized nations are serious about slashing the amount of carbon dioxide and other dangerous gases in an effort to avert the most catastrophic consequences of global warming.