The climatological record of clear‐sky longwave radiation at the Earth's surface: evidence for water vapour feedback?

Abstract

An increase in global surface temperature of between 1.4 K and 5.8 K is expected to occur by 2100 due to a doubling of the global concentration of CO₂. Associated with this predicted surface warming will be an increase in the downwards longwave (3–100 µm) radiation (F↓) at the Earth's surface. Observations of this quantity on a global scale are almost non‐existent. Clear‐sky estimates of F↓ can be obtained from radiative transfer calculations using temperature and moisture profiles from radiosoundings. Here long‐term (more than 25 years) mean monthly profiles obtained from globally distributed land‐based radiosonde stations are subjected to detailed radiative transfer computations and Fourier time‐series analysis. The results indicate that over the period 1964–1990, there has been a global increase in the clear‐sky longwave flux at the surface. The global trend is approximately +1.7 W m⁻² per decade, and there is a strong latitudinal pattern, with greater increases occurring in the tropics and smaller increases at both poles. There are also concomitant increases in precipitable water and the patterns appear to be highly correlated with increases in F↓. Increases in CO₂ with time were not included in the calculations and it is estimated that the radiative impact of changes in CO₂ on the F↓ trend is ∼20% per decade. A simple model of the dependence of surface air temperature and precipitable water on the downwards clear‐sky flux supports the notion that both variables are contributing to increases in F↓. It is suggested that increases in precipitable water represent a positive feedback on F↓.

Acknowledgements

I am grateful to the anonymous referees for their comments and in particular to one referee whose thorough review has helped me to improve the manuscript.