Scientists Ken Caldeira, Atul K. Jain, and Martin I. Hoffert offer some sobering findings: Once past the challenge of defining "safe" in terms of interference with the climate system, "the sensitivity of global mean temperature to increasing CO2 is known perhaps only to a factor of three or less." The research by scientists at Lawrence Livermore, University of Illinois, and New York University shows "how a factor of three uncertainty in climate sensitivity introduces even greater uncertainty in allowable increases in atmospheric CO2 concentration and allowable CO2 emissions." The result? "Unless climate sensitivity is low and acceptable amounts of climate change are high, climate stabilization will require a massive transition to CO2 emission-free energy technologies," they write. "Even if climate sensitivity is at the low end of the accepted change, by the end of this century over three-quarters of our primary power will need to come from sources that do not release CO2 into the atmosphere. We do not have CO2 emission-free energy technologies that can be applied cost-effectively today at the required scale. Given the long lead times needed to bring new energy technologies to implementation, we need to develop appropriate energy technologies now."