The sensitivity of Earth’s climate to increasing levels of atmospheric carbon dioxide may be less than previously estimated. Climate sensitivity is a standard measure of how much Earth’s global surface air temperature will increase in response to a doubling of CO2 compared with preindustrial levels. The current best estimate of climate sensitivity is 3.0 °C and likely falls within the range of 2.0 to 4.5 °C, according to the Intergovernmental Panel on Climate Change (IPCC). Those numbers are based on simulations primarily using data measured since 1850. They roughly correspond to the temperature increase range IPCC projects will occur by 2100. Oregon State University’s Andreas Schmittner and colleagues incorporated previously unavailable paleoclimate data from the peak of the last ice age 21,000 years ago into simulations to reestimate climate sensitivity (Science, DOI: 10.1126/science.1203513). They obtained a median sensitivity of 2.3 °C with a reduced uncertainty of 1.7 to 2.6 °C. IPCC has concluded that an increase of more than 2.0 °C would cause drastic changes in global climate (see page 7).