Some scientists and environmental groups promote 350 ppm as a safe level for CO2, but scientists acknowledge they don't really know what levels would stop the effects of global warming.

NPR: Experts: CO2 Record Illustrates 'Scary' Trend

Some scientists and environmental groups promote 350 parts per million as a safe level for CO2, but scientists acknowledge they don't really know what levels would stop the effects of global warming.

NPR: Greenhouse Gas Milestone; CO2 Levels Set Record

Begun in 1996 as a body to coordinate Arctic policy, the council was for many years seen as a platform for scientists to swap research on environmental issues.

WSJ: Arctic Body Weighs China's Bid to Observe

The number 400 has been anticipated by climate scientists and environmental activists for years as a notable indicator, in part because it's a round number not because any changes in man-made global warming happen by reaching it.

NPR: Greenhouse Gas Milestone; CO2 Levels Set Record

While the precautionary principle enjoys considerable support from environmental activists, politicians, and indeed, some scientists, as a law, it is inherently vague.

FORBES: Scientists Recommend Canadian Regulation Be More Reasonable; Government Listens

As investigative journalist Donna Laframboise reports, most of the 13 senior scientists controlling the draft report are prominently associated with environmental activist groups.

FORBES: Is It Hollywood Or Washington? Global Warming Activists Control Government Climate Report