HadCRUH specific humidity is found to correlate strongly with global temperature records from the HadCRUT3 dataset. Statistically significant increases in specific humidity are widespread, with a global mean trend of 0.07 g kg-1 10yr-1 (given reported temperature trends, the expected specific humidity trend at a constant relative humidity of 70% is ~0.08g kg-1 10yr-1). Trends are strongest in the Tropics and Northern Hemisphere Summer, regions of higher ambient temperature and thus consistent with the Clausius-Clapeyron relation. Relative humidity trends are statistically indistinguishable from zero on the largest spatial and temporal scales, but can be significant at smaller scales.
A strong positive bias is present in marine humidity data prior to 1982, most likely relating to a known change in observing practice for dewpoint temperature at this time. Thus, trends in both specific and relative humidity are likely to be underestimated over the oceans.
The Northern Hemisphere specific humidity signal is detectable above HadCM3 climate model expectations of natural variability. The HadCM3 anthropogenic forcings only ensemble provides a far better explanation than the natural forcings ensemble across a range of space scales pointing to a primarily anthropogenic origin of recently observed changes.