- 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.
- Gordon, N. D., A. K. Jonko, P. M. Forster, and K. M. Shell (2013), An observationally based constraint on the water-vapor feedback, Journal of Geophysical Research: Atmospheres, 118, 12,435–12,443. doi:10.1002/2013JD020184.
|Funding Statement (additional comments about funding)
- Support of this data set is provided by the Office of Science, U.S. Department
of Energy. PF and NG were supported by NERC grant NE/E016189/1.
NG was funded, in part, by the LLNL Institutional Postdoc Program.
AJ was supported by NASA Grant NNX09AN92H and by the Office of
Science (BER), U.S. Department of Energy, and KS by the National Science
Foundation under Grant ATM 0904092, by NASA Grant NNH08CC72C,
and by NorthWest Research Associates Inc.