1.   But scientists increasingly believe that it is one of the chief pollutants responsible for the greenhouse effect warming the planet.

2.   An invisible gas people exhale is warming the planet.

3.   Scientists believe that human production of carbon dioxide and other heat-trapping gases is gradually warming the planet and altering climate.

4.   Scientists increasingly agree that chemicals released from burning fossil fuels and other human activities are warming the planet, but giving the issue political urgency is an uphill fight.

5.   Critics contend that a U.S. withdrawal would probably doom the pact, since the United States emits about a quarter of the greenhouse gases warming the planet.

6.   Critics say a U.S. withdrawal would probably doom the pact, since the United States emits about a quarter of the greenhouse gases warming the planet.

7.   Environmental activists shut down a shale oil project Monday to protest the weekend collapse of international negotiations on how to cut the pollution that is warming the planet.

8.   On Monday, the politicians convened at the U.N. climate conference to make decisions on how to cut greenhouses gases that are gradually warming the planet.