Is two degrees too much?

Today on Post Carbon, Juliet Eilperin writes:

Back in the mid-1990s, the U.N. Intergovernmental Panel on Climate Change projected this would give the world a decent shot at avoiding dangerous climate impacts. Most leaders have signed off on this goal, including members of the G-20. However some scientists, environmentalists and leaders from vulnerable island nations--including the IPCC's chair, Rajendra K. Pachauri--now say it's too modest. Any new climate deal should identify 1.5 degrees Celsius--or 2.7 degrees Fahrenheit--as the upper limit of a global temperature increase, according to this camp. There's even a billboard on Copenhagen's streets featuring a polar bear and the slogan, "2 Degrees is Too Much."

Unfortunately, she also points out that there's very little chance we'll hit that goal:

We're headed for an increase of 3.8 degrees Celsius, or 6.8 degrees Fahrenheit, if every nation fulfilled its current climate pledge.