High res PDF version
The climate sensitivity due to CO2 is expressed as the temperature change in °C associated with a doubling of the concentration of carbon dioxide in Earth’s atmosphere. The equilibrium climate sensitivity (ECS) refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric carbon dioxide concentration. The transient climate response (TCR) is defined as the average temperature response over a twenty-year period centered at CO2 doubling in a transient simulation with CO2 increasing at 1% per year. The transient response is lower than the equilibrium sensitivity, due to the “inertia” of ocean heat uptake.
Scientists made numerous estimates of climate sensitivity over the last few decades and have yet to determine the correct value. The figure shows the change in published climate sensitivity measurements over the past 15 years (from here). The ECS and TCR estimates have both declined in the last 15 years, with the ECS declining from 6C to less than 2C. While one cannot extrapolate from past results, it is likely that the true figure is below 2C, and may continue to decline. Based on this historic pattern we should reject the studies that falsely exaggerated the climate sensitivity in the past and remember that global warming is not the most serious issue facing the world today.