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Water Vapor

Water vapor: It is only a small component of the atmosphere, accounting for 1.2x1016 kg, or 2.4 hPa in terms of surface pressure and thus about 0.24% of the atmosphere as a whole. Yet it is vital to climate and how it varies and changes. Water vapor is the most important greenhouse gas on the planet and accounts for about 60% of the current greenhouse effect, warming the planet enough to make it habitable. It provides the moisture that is captured by all weather systems to produce clouds and precipitation (rain and snow). It changes rapidly from hour to hour, and year to year, and its changes are vital for climate change as it provides the strongest positive feedback in the climate system. For instance, a doubling of carbon dioxide in the atmosphere from human fossil fuel burning, would increase global mean temperatures by about 1.2°C, but increased evaporation and water vapor roughly doubles that value.

Figure from: Rockstrom, J., L. Gordon, C. Folke, M. Falkenmark, and M. Engwall. 1999. Linkages among water vapor flows, food production, and terrestrial ecosystem services. Conservation Ecology 3(2): 5.

In spite of its importance, changes in water vapor are poorly known. Measurements are difficult, and only since 1988 are values reliable over the ocean as new microwave measurements from SSM/I have come available.

Scientists in CAS have carefully evaluated many observations and datasets on water vapor and come up with new definitive answers on its mean, variability and trends.

See Trenberth, K. E., J. Fasullo, and L. Smith, 2005: Clim. Dyn.DOI 10.1007/s00382-005-0017-4. http://www.cgd.ucar.edu/cas/trenberth.papers/waterVapTrendsCoCDr.pdf

Fig. 11. Linear trend in precipitable water for 1988-2003in %/decade and the time series for the integral over the global ocean, which has a linear trend of 1.3 mm/decade.

Precipitable water variability for 1988-2001 is dominated by the evolution of El Niño and especially the structures that occurred during and following the 1997-98 El Niño event. The evidence from SSM/I for the global ocean suggests that recent trends in precipitable water are generally positive and, for 1988 through 2003, average 0.40±0.09 mm decade-1 or 1.3±0.3% decade-1 for the ocean as a whole, where the error bars are 95% confidence intervals. Over the oceans, the precipitable water variability relates very strongly to changes in SSTs, both in terms of spatial structure of trends and temporal variability (with a regression coefficient for 30°N-30°S of 7.8% K-1) and is consistent with the assumption of fairly constant relative humidity. These relationships allow estimates of changes in water vapor for the 20th Century to be about 5% over the global oceans, and this indeed suggests a radiative forcing equivalent to that of carbon dioxide increases.

At the same time, the increasing abundance of water vapor fuels stronger storms and makes for heavier rainfall events, which is also observed to be happening around the world.

This research by CAS scientists is not only important scientifically but also in practical terms as to how the climate, precipitation and water resources change.

For additional information, visit Climate Analysis (CAS).