How much will a doubling of CO2 change Earth temperatures?

How much will temperatures go up, if we double the amount of CO2 in the atmosphere? That has turned out to be a tricky question to answer scientifically. In this podcast, climate scientist Anna von der Heydt explains how initial conditions turn out to be important for getting the right answer: When the Earth system is in a cold state it does not react to a doubling as it does in a warm state. Which state are we in now, then?