The June 2010 issue of BAMS (Bulletin of the American Meteorological Society) contains the State of the Climate in 2009 report (Arndt, D. S., M. O. Baringer, and M. R. Johnson, Eds., 2010: State of the Climate in 2009. Bull. Amer. Meteor. Soc., 91 (6), S1-S224). One feature of the report is the display of key climate indicators, each being a time series which is unambiguously expected to be going in some particular direction due to global warming. You can view graphs and download the data here.
One of the interesting graphs is multiple estimates of specific humidity (the amount of water vapor in the atmosphere). In a warming world, specific humidity is expected to increase because a warmer atmophere will naturally hold more water vapor. Because water vapor is also a greenhouse gas (in fact, its the most prominent one in our atmosphere) this leads to one of the expected feedback mechanisms of man-made global warming. As the globe warms due to CO2 (and other anthropogenic greenhouse gases) more water vapor enters the atmosphere, which enhances greenhouse-gas warming due to water vapor, therefore amplifying the CO2-induced warming.
Three data sets of specific humidity (in grams H2O per kg air) are available. The first is from Dai 2006, Recent Climatology, Variability, and Trends in Global Surface Humidity, J. Climate, 19, 3589-3606, the second from Willett et al. 2008, Recent changes in surface humidity: development of the HadCRUH dataset, J. Clim..21, 5364:5383, the third from Berry & Kent, 2009, A New Air-Sea Interaction Gridded Dataset from ICOADS with Uncertainty Estimates, Bulletin of the American Meteorological Society, 90(5), 645-656 (DOI: 10.1175/2008BAMS2639.1), which is data for the marine environment only (which nonetheless is most of the surface of the earth). Plotted together (as specific humidity anomaly) they look like this:
All three data sets show increasing atmospheric humidity. The Dai (2006) data indicate increase at 0.06 g/kg/decade, the Willett et al. (2008) data indicate increase at 0.08 g/kg/decade, and the Berry & Kent data indicate increase at 0.07 g/kg/decade. These results are consistent with what’s expected given the observed global temperature increase over the same time span.
In fact the changes in humidity track the changes in global temperature surprisingly well, as can be seen by superimposing a scaled verstion of global temperature anomaly from GISS, shown here in black:
Not only is global humidity rising with the warming trend as expected, even its year-to-year fluctuations match well with those of global temperature. This is extremely strong evidence that, just as expected both from computer models and from basic physics, the dominant factor in global humidity is global temperature.
It also raises a question for those who doubt the correctness of observed global temperature increase: if (as so many denialists claim) the globe isn’t warming because the global temperature estimates are wrong, then why does the specific humidity track it so well?
Any bets on how long it takes someone to suggest that the increase in global humidity isn’t real — that the data are wrong and the apparent increase is only due to the “urban wet island” effect?