there are three ways to try to find out the climate sensitivity. the first way is to look at the temperature measurements made each year since 1750 and compare them with measurements of how much greenhouse gas was in the atmosphere at the same time. the second way is to try and work out how much greenhouse gas there was and what the temperature was in the distant past before people started measuring them. for example by measuring how much co2 is in tiny bubbles of air trapped deep in ice sheets thousands of years ago. the third way is to make models of the climate system in computers.