The influence of water vapor and lapse rate feedbacks on surface temperature

A systematic approach toward better understanding the influence of water vapor and lapse rate feedbacks on surface temperature is proposed here based on both theoretical modeling and climatic data analysis. In particular, we make the following studies: (1) a radiative-convective model study of water vapor feedback as a function of altitude, as forced by a doubling of ;In spite of a much higher radiative sensitivity of the upper tropospheric water vapor, our results clearly show the importance of the lower tropospheric feedback due to the combination of water vapor and lapse rate feedbacks; in most climate conditions, the combined feedback from the lower troposphere is close to double that from the upper troposphere. Continental climate, compared to maritime climate, exhibits clearly different feedback features: the former is significantly enhanced by a positive lapse rate feedback while the latter is slightly weakened by a negative lapse rate feedback. On the average, the overall continental feedback is about 50% stronger than the corresponding maritime feedback, and this difference is primarily from the lower troposphere.;The spectral dependence of water vapor feedback is subtle but can provide more fundamental information and guidelines for projecting future change. Based on its distinctive spectral heating rate changes, the terrestrial climate can be classified into three basic categories: (1) cold climates exhibit most of the water vapor feedback contribution from the upper troposphere, due entirely to changes in water vapor rotational band absorption; (2) moderate climates have their water vapor feedback almost evenly split between the upper and lower troposphere, due to rotational absorption in the upper troposphere and the window region continuum absorption in the lower troposphere; (3) warm and humid climates have much stronger water vapor feedback from the lower troposphere, dominated by the window region continuum absorption. In spite of the large contribution of water vapor feedback from the lower troposphere, several mechanisms are found to weaken this water vapor feedback: these are the lower lapse rate, the reduction of radiative sensitivity at high water vapor concentrations, and the "blocking effect" which is due to adding water vapor to upper levels and thus blocking the lower level emission. However, the blocking effect is found in the water vapor rotational band only, which is unable to affect the window region absorption even for the present-day tropical climate. We therefore conclude that further warming due to lower tropospheric water vapor feedback is still possible. (Abstract shortened by UMI.).