Stabilizing carbon levels

In 1996, the European Union said the world should limit the global temperature rise to 2C above preindustrial levels, and that this meant limiting carbon dioxide in the atmosphere to 550ppm CO2[e]. In reality, the relationship between carbon dioxide levels and eventual temperature rise is less precise. Scientists now talk only about likely ranges. The 2006 Stern Review said that 550ppm CO2[e] would drive temperatures up by between 1.5 and 4.5C, with a median rise of 3C. It said the world should aim for a stabilisation goal of between 450 and 550ppm CO2[e].
The lower end of that range (450ppm CO2[e]) is now commonly associated with a 2C rise, and the upper end (550ppm CO2[e]) with a 3C rise. In fact, scientists say that at 450ppm CO2[e] there is still a 50% chance that the 2C target will be exceeded.
Jim Hansen, Nasa's top climate scientist, says that even a 2C rise may not avoid dangerous large scale impacts such as melting of ice sheets, and says world CO2 should not be allowed to remain above 350ppm CO2, significantly below its current level – that means removing CO2 from the atmosphere and burying it.