2.3.8 Observations of Long-Lived Greenhouse Gas Radiative Effects

Observations of the clear-sky radiation emerging at the top of the atmosphere and at the surface have been conducted. Such observations, by their nature, do not measure RF as defined here. Instead, they yield a perspective on the influence of various species on the transfer of radiation in the atmosphere. Most importantly, the conditions involved with these observations involve varying thermal and moisture profiles in the atmosphere such that they do not conform to the conditions underlying the RF definition (see Section 2.2). There is a more comprehensive discussion of observations of the Earth’s radiative balance in Section 3.4.

Harries et al. (2001) analysed spectra of the outgoing longwave radiation as measured by two satellites in 1970 and 1997 over the tropical Pacific Ocean. The reduced brightness temperature observed in the spectral regions of many of the greenhouse gases is experimental evidence for an increase in the Earth’s greenhouse effect. In particular, the spectral signatures were large for CO₂ and CH₄. The halocarbons, with their large change between 1970 and 1997, also had an impact on the brightness temperature. Philipona et al. (2004) found an increase in the measured longwave downward radiation at the surface over the period from 1995 to 2002 at eight stations over the central Alps. A significant increase in the clear-sky longwave downward flux was found to be due to an enhanced greenhouse effect after combining the measurements with model calculations to estimate the contribution from increases in temperature and humidity. While both types of observations attest to the radiative influences of the gases, they should not be interpreted as having a direct linkage to the value of RFs in Section 2.3.