The spectral signature of global warming

Simulations are presented of the change in the spectrum of the clear-sky outgoing long-wave radiation (OLR) associated with the global warming produced by increases in greenhouse-gas concentrations. The input data for the present day and for the middle of the next century were taken from a recent climate-prediction run of the Hadley Centre Climate Model. The simulations focus on the spectral signature of the warming as opposed to that of the forcing. Stratospheric cooling causes decreases of the OLR in the carbon-dioxide and ozone bands, whilst surface warming increases the OLR in the region of the atmospheric window. The signal in the water-vapour bands is more subtle, due to a near cancellation between the effects of changes in atmospheric temperatures and specific humidities that have little impact on the relative humidity. The residual signal is shown to be related to small changes in upper-tropospheric relative humidity, although at some latitudes this relationship breaks down. It is suggested that satellite observations in the water-vapour bands could be used to provide a quantitative measure of the water-vapour feedback during global warming.