The Bush administration says a new United Nations report on the dangers of global warming is very valuable, but stopped short of calling for mandatory government controls on greenhouse gasses.

The Intergovernmental Panel on Climate Change released a study Friday warning that human activities are "very likely" to blame for global warming, and that the world can expect the trend to continue.

U.S. Energy Secretary Samuel Bodman said the Bush administration welcomed the report, but repeated the administration's opposition to mandating caps on the emission of greenhouse gasses. Greenhouse gasses are produced by fossil fuels and carbon-intensive industries

Bodman called for a global discussion of the issue because it is a global problem. U.S. Democratic Senator Barbara Boxer urged President Bush to convene a summit of 12 of the world's biggest producers of greenhouse gasses.

The head of the U.N. Environmental Program Achim Steiner said the report shifts debate from whether mankind is contributing to global warming to acting on it.

French President Jacques Chirac warned the world is "on the verge of the irreversible."

South African Environment Minister Marthinus van Schalkwyk said failure to act would be indefensible. His British counterpart David Miliband said it is another "nail in the coffin" of the climate change deniers. And, European Environment Commissioner Stavros Dimas agreed on the need for international action.

The U.N. report, "Climate Change 2007: the Physical Science Basis," is aimed at pressing governments and companies around the world to adopt policies that will curb the emission of greenhouse gases released into the air. It was reviewed by 2,500 scientists from 113 countries.

Some greenhouse gases are naturally occurring, while others are made-made pollutants, such as emissions from burning fossil fuels. The gases trap energy from the sun that would otherwise reflect back into space.

Some information for this report was provided by AP, AFP and Reuters