evidence: There is a well-known correlation between temperatures and CO2 concentrations over glacial periods. The figure below shows both temperatures (red line) – reconstructed from hydrogen isotope analysis – and atmospheric CO2 concentrations (blue line) from air bubbles trapped in Antarctic ice over the past 800,000 years. [...] Both the northern hemisphere and overall global temperatures actually lagged CO2; in other words, for the world as a whole, warming happened after atmospheric CO2 concentrations increased. The reasons for this are complex and are driven in part by changes in ocean currents as ice ages end. [...] While there is a clear relationship between CO2 and temperature over the past million years, it is difficult to extrapolate to future changes to the Earth’s climate. For example, at the end of the last ice age atmospheric CO2 concentrations increased by around 50% while global temperatures increased by around 4C. If CO2 were the only factor at work, this would imply a very high sensitivity of the climate to CO2 – around 8C per doubling CO2, far higher than current estimates based on climate models and other lines of evidence.