Response to Forcing of the Climate System

Over a time span of a few years the heat balance of the earth can generally be considered to be in balance, which means that the incoming solar radiation, S, is balanced by the outgoing long wave radiation, F. What happens then when suddenly there is a change in either S or f Let us assume, for example, that there is a sudden increase in CO2 concentration to twice the present value. 

The immediate response is a reduction in the outgoing longwave radiation at the tropopause of about 3.1 W m and an increase in the downward emission from the stratosphere by about 1.3 W m . The sum of the two, 4.4 W m , is the net instantaneous forcing at the tropopause. 

Following this immediate shock the stratosphere cools. The increased CO2 in the stratosphere enhances the thermal emission. Because the stratospheric temperature increases with altitude, this has the effect that the cooling into space is larger than the absorption from layers below. This is in fact the fundamental reason for the COi-induced cooling in the stratosphere. After stratospheric cooling a new radiative equilibrium develops with the new doubled CO, concentration. This reduces the increased downward emission at the tropopause by about 0.2 W in - and the tropopause forcing is adjusted accordingly. 

The surface-troposphere system will continue to warm until the entire system reaches a new equilibrium. This may take a considerable time due to the very high heat capacity of the ocean and it will certainly last several decades before an equilibrium is reached, if at all. 

However, this cannot happen in the present atmosphere so the direct warming effect at the surface, assuming no feedback, would amount to about 1.3 K (Ramanathan, 1981). Now it appears that the atmosphere is close to conserving relative humidity, so a warming would increase the water vapor in the atmosphere and hence further increase the warming, thus creating a positive feedback effect. It is interesting to note that even Arrhenius (1896) included the feedback from water vapor. 

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