An observationally based constraint on the water-vapor feedback

Thus, even relatively small errors in its magnitude can lead to large uncertainties in predicting climate response to anthropogenic forcing. This study incorporates observed variability of water vapor over 2002-2009 from the Atmospheric Infrared Sounder instrument into a radiative transfer scheme to provide constraints on this feedback.

We derive a short-term water vapor feedback of 2.2 ± 0.4 Wm⁻²K⁻¹. Based on the relationship between feedback derived over short and long timescales in twentieth century simulations of 14 climate models, we estimate a range of likely values for the long-term twentieth century water vapor feedback of 1.9 to 2.8 Wm⁻²K⁻¹. We use the twentieth century simulations to determine the record length necessary for the short-term feedback to approach the long-term value.