The study indicates there is a decrease in water vapor levels due to cooling induced by massive volcanic eruptions, e.g. Pinatubo in 1991. The "fingerprint match" of the observed levels of water vapor is primarily due to human-caused increases in greenhouse gases, and not due to either solar forcing or volcanic forcing. The findings provide preliminary evidence of an emerging anthropogenic signal in the moisture content of Earth’s atmosphere. Water vapor feedbacks are likely to play a key role in determining the magnitude of the climate changes we experience over the next century. Data from the satellite-based Special Sensor Microwave Imager (SSM/I) show that the total atmospheric moisture content over oceans has increased by 0.41 kg/m2/decade since 1988. Results from current climate models indicate that water vapor increases of this magnitude cannot be explained by climate noise alone. In a formal detection and attribution analysis using the pooled results from 22 different climate models, the simulated "fingerprint" pattern of anthropogenically-caused changes in water vapor is identifiable with high statistical confidence in the SSM/I data. Experiments have been conducted in which forcing factors such as greenhouse gas forcing, solar forcing and volcanic forcing are varied individually.
Reference: Santer, B. D. , C. Mears, F. J. Wentz, K. E. Taylor, P. J. Gleckler, T. M. L. Wigley, T. P. Barnett, J. S. Boyle, W. Brüggemann, N. P. Gillett, S. A. Klein, G. A. Meehl, T. Nozawa, D. W. Pierce, P. A. Stott, W. M. Washington, and M. F. Wehner. 2007. "Identification of Human-Induced Changes in Atmospheric Moisture Content," Proceedings of the National Academy of Sciences, September 25, 2007.
Contact: Anjuli Bamzai, SC-23.3, (301) 903-0294
SC-33.1 Earth and Environmental Sciences Division, BER
(formerly SC-23.3 Climate Change Research Division, OBER)
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