From AGU highlights, measurements from 2002 to 2009 show short term feedback still subject to short-term climate variability, long term feedback still in the realm of models.
Measuring the effect of water vapor on climate warming
Water vapor is a potent greenhouse gas. In the atmosphere, the concentration of water vapor increases with the temperature, setting up a powerful positive feedback loop. This water vapor feedback is the strongest known positive feedback, with the potential to roughly double the effect of warming caused by other sources. Determining the exact strength of the water vapor feedback, then, is incredibly important to limiting uncertainty in future climate change projections.
From 2002 to 2009, an infrared sounder aboard NASA’s Aqua satellite measured the atmospheric concentration of water vapor. Combined with a radiative transfer model, Gordon et al. used these observations to determine the strength of the water vapor feedback. According to their calculations, atmospheric water vapor amplifies warming by 2.2 plus or minus 0.4 watts per square meter per degree Celsius. This value, however, is only the “short-term” feedback—the strength of the feedback as measured during the observational period. This value is subject to short-term climate variability. The true value of the feedback, the “long-term” value, is what the short-term observed values should trend towards when given enough time.
Using a series of climate models, the authors estimate the strength of the long-term water vapor feedback. Extrapolating from their short-term observations they calculate a long-term feedback strength of 1.9 to 2.8 watts per square meter per degree Celsius. They find that most models get to within 15 percent of their long-term value within 25 years. The accuracy of calculations, then, could be improved with a longer set of observations.
Source: Journal of Geophysical Research-Atmospheres, doi: 10.1002/2013JD020184, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013JD020184/abstract
Title: An observationally based constraint on the water-vapor feedback
Authors: N. D. Gordon: Lawrence Livermore National Laboratory, Livermore, California, USA; A. K. Jonko: National Center for Atmospheric Research, Boulder, Colorado, USA; P. M. Forster: School of Earth and Environment, University of Leeds, Leeds, UK: K. M. Shell: College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA.
The increase in atmospheric concentrations of water vapor with global warming is a large positive feedback in the climate system. 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. In most of the climate models we analyze, the short-term feedback converges to within 15% of its long-term value after 25 years, implying that a longer observational record is necessary to accurately estimate the water vapor feedback.