The second category of the attribution of the warming is the various feedback processes caused by the forcing agents on the climate. Evaporation of water might be the most evident feedback effect that contributes to global warming. The initial warming generated by CO2 will cause more water to be evaporated into the atmosphere. Then, water vapor acts as a greenhouse gas and causes more global warming. As a result, the global warming causes still more water vapor to be evaporated until the concentration of water vapor has reached the dynamic equilibrium, with a much larger greenhouse effect than that due to CO2 alone. This increases the amount of the absolute moisture in the air, but the relative humidity of the atmosphere rarely changes because the air is warmer.
Cloud feedback effects are an area of ongoing research. In the air, clouds emit infrared radiation from the ground back to the surface and enforce the warming effect. The same clouds reflect sunlight and emit infrared radiation from sun to space and enforce the cooling effect. Because the type, altitude, and other variables are important factors in deciding the net effect of clouds on cooling or warming, it is very difficult to represent in climate models. Nevertheless, cloud feedback is reported as the second largest positive feedback that contributes to the global warming models that were used in the IPCC Fourth Assessment Report.
Ice albedo feedback is another feedback process that contributes to global warming. As global temperatures increase, the melting process of the ice near the poles increases rapidly. The resulting land and open water are, on average, less reflective than ice, and, thus, absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues. Positive feedback due to release of CO2 and CH4 from thawing permafrost is an additional mechanism contributing to warming. Possible positive feedback due to CH4 release from melting seabed ice is a further mechanism to be considered.
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