"The next big emissions target is 450 ppm," Butler said. "That's the one the IPCC [The United Nations' Intergovernmental Panel on Climate Change] warns we really don't want to rise above. Based on our current rate of emissions, we could be there in 25 years. That's a scary prospect."
Watch a visual presentation of how CO2 has changed over the last 800,000 years. Video courtesy of Andy Jacobson of the University of Colorado and NOAA's Earth System Research Laboratory: