The key to this narrower – albeit much higher – estimate is found in the real world observations around the role of water vapor in cloud formation. Observations show when water vapor is taken up by the atmosphere through evaporation, the updrafts can either rise to 15 km to form clouds that produce heavy rains or rise just a few kilometers before falling back to the surface without forming rain clouds. The researchers discovered that climate models that exhibit a low global temperature response to carbon dioxide do not include enough of this lower-level water vapor process. In its place, they simulate nearly all updrafts as rising to 15 km and forming clouds. However, when the procedures in climate models are adjusted to match the observations in the real world, the models produce cycles that take water vapor to a wider range of heights in the atmosphere, causing fewer clouds to form as the climate warms. Consequently, this increases the volume of sunlight and heat entering the atmosphere and increases the sensitivity of our climate to carbon dioxide or any other disturbance. The result is such that, when water vapor processes are correctly represented, the sensitivity of the climate to a doubling of carbon dioxide – which will happen in the next 50 years – means that we can expect a temperature increase of at least 4 degrees Celsius by 2100.