Climate sensitivity is a measurement within a climate system that indicates the change in the amount of energy reaching the surface of the Earth. Climate sensitivity is used to indicate global average surface warming due to radiative forcing. A positive forcing tends to warm a system (introduces energy into the system); while a negative forcing (takes energy out of the system) tends to cool it. A radiative forcing can stem from solar radiation (insolation), or concentrations of greenhouse gases in the atmosphere, including carbon dioxide (CO2), water vapor, methane, nitrous oxide and ozone.
It was noted a long time ago that water vapor plays a major role in global warming. Water vapor depends on temperature for it to accumulate in the atmosphere. In a water system, when temperature is increased, water evaporates. If a radiative forcing, such as the burning of fossil fuels increases CO2 in the atmosphere, more water evaporates, increasing the quantities of this greenhouse gas in the atmosphere. More water vapor in the atmosphere raises the temperature even further (positive feedback).
Many studies have been done to understand climate sensitivity, which relates to the way in which the Earth will respond to increases of CO2 in the atmosphere. According to the Intergovernmental Panel on Climate change (IPCC), climate sensitivity is likely to be in the range of 2-4.5° C (36-40° F) with the most approximate estimate in 3° C (37° F). Other studies have placed climate sensitivity either below or above these estimates; however, most studies are consistent with the climate sensitivity range of 2-4.5° C.
If CO2 is doubled in the atmosphere, global temperature can rise from between 2-4.5° with a central estimate of 3° C. Increased global temperatures introduce more heat in the system, producing more water vapor in the atmosphere. Water vapor is closely related to temperature. An increase in temperature increases the amounts of water vapor. Studies show that water vapor feedback doubles the warming effects produced by CO2. According to studies made by scientists at NASA's Jet Propulsion Laboratory in Pasadena, California, “Water vapor is the big player in the atmosphere as far as climate is concerned.
Studies showed that water vapor present in the atmosphere can double the warming effect caused by increased levels of CO2 in the atmosphere. A research team of scientists using data from the Atmospheric Infrared Sounder (AIRS) on NASA's Aqua satellite measured with accuracy humidity levels in the lowest atmosphere. This data was compared with other observations in shift temperatures taken around the world, allowing scientists to consider with more amplitude the interrelation between water vapor, carbon dioxide and other greenhouse gases.
Climate sensitivity due to water vapor creates more water evaporation, which absorbs even more heat. If the temperature increases 1° C (34° F) due to high levels of CO2, water vapor in the atmosphere will cause the temperature to increase another 1° C, that is to say 2° C (36° F); however, when other radiative forcing factors are taken into account, the total warming due to CO2 may go up to as much as 3°C (37° F). Because observations agree with existing evaluations, researchers predict that water vapor will contribute to climate sensitivity before the end of the century.
According to a recent study by the Massachusetts Institute of Technology, the planet will reach the doubling CO2 atmospheric levels anywhere between mid and the end of the 21st century. Twice as many levels of CO2 in the atmosphere mean more water evaporation, resulting in the accumulation of heat, which adds to climate sensitivity due to positive radiative forcing. Unless CO2 levels fall down or become stable, rapid global warming will likely occur in this century.