cc: Phil Jones
date: Wed, 24 Sep 2008 11:54:27 +0100 from: firstname.lastname@example.org subject: Re: Downward trend in relative humidity over land? to: Adrian.Simmons@ecmwf.int
Over land I found non-significant and very small decadal trends, with the Northern Hemisphere trend actually being slightly positive. (G = -0.03, NH = 0.07, T = -0.10 and SH = -0.34 - SH data is very sparse and likely of low quality). In contrast, the Marine data showed very significant negative trends but I'm highly suspicious about the pre-1982 data which has a strong positive bias relative to the rest of the timeseries.
Steve McIntyre at Climate Audit (the global warming skeptic's counter-site to the Real Climate project) points to a new paper that purports to show that the relative humidity of the upper atmosphere is declining rather than increasing as one might expect based on climate models. The implication pertains to how much feedback there is with increase in atmospheric CO2 - a positive feedback (increasing relative humidity) would imply greater temperature change per doubling of CO2 concentration increase, ie making global warming worse, whereas a negative feedback would imply the opposite, that global warming is less of a threat. Note that the debate is only about the magnitude of the increase in temperature change with increased CO2, not a repudiation of global warming outright. In fact the whole issue of relative humidity essentially validates the main assumption of global warming as caused by CO2 in the first place.