Following on from the line of investigation started here, I examine the quality of evidence the IPCC presents for global warming due to the enhancement of the greenhouse effect. The summary for policy makers apparently sums up the current state of knowledge, referencing the relevant chapter section (3.4):

The average atmospheric water vapour content has
increased since at least the 1980s over land and ocean
as well as in the upper troposphere. The increase is
broadly consistent with the extra water vapour that
warmer air can hold. {3.4}

Turning to the summary of Chapter 3 of the AR4: Chapter 3 Observations: Surface and Atmospheric Climate Change where the evidence for this claim is presented it is stated:

Similar upward trends in upper-tropospheric specific humidity, which considerably enhance the greenhouse effect, have also been
detected from 1982 to 2004. (Chapter 3 p4)

Turning to section 3.4 there is an extended discussion of the problems
of direct measurements of specific humidity via radiosondes (weather balloons)
and of controversy within the field e.g.

Changes in upper-tropospheric water vapour
in response to a warming climate have been the subject of
significant debate. (Chap 3 Sect 3.4 p39)

However, despite acknowledging a variation in the literature on this issue, and the availability of direct data that does indicate a detectable trend in upper-atmosphere, the conclude:

To summarise, the available data do not indicate a detectable
trend in upper-tropospheric relative humidity. However, there
is now evidence for global increases in upper-tropospheric
specific humidity over the past two decades, which is consistent
with the observed increases in tropospheric temperatures and
the absence of any change in relative humidity. (Chap 3 sect 3.4 p40)

The evidence for this conclusion is from the following line of reasoning:

In the absence of large changes in relative humidity, the
observed warming of the troposphere (see Section 3.4.1) implies
that the specific humidity in the upper troposphere should have
increased. As the upper troposphere moistens, the emission level
for T12 increases due to the increasing opacity of water vapour
along the satellite line of sight. In contrast, the emission level
for the MSU T2 remains constant because it depends primarily
on the concentration of oxygen, which does not vary by any
appreciable amount. Therefore, if the atmosphere moistens,
the brightness temperature difference (T2 − T12) will increase
over time due to the divergence of their emission levels (Soden
et al., 2005). This radiative signature of upper-tropospheric
moistening is evident in the positive trends of T2 − T12 for the
period 1982 to 2004 (Figure 3.21). If the specific humidity in
the upper troposphere had not increased over this period, the
emission level for T12 would have remained unchanged and
T2 − T12 would show little trend over this period (dashed line
in Figure 3.21).

In other words, if we make some obviously false assumptions of no large change in relative humidity (its falling) and increasing temperatures in the upper atmosphere (also falling), then we can conclude that the specific humidity is increasing in the upper troposphere.

As further evidence, direct measurements of specific humidity are falling, here
is the actual specific humidity measured at the upper troposphere of 300hPa, courtesy of Anthony Watts.

Does anyone else find AR4 Section 3.4 breathtaking in its misrepresentation of the evidence? The claim that “the available data do not indicate a detectable trend in upper-tropospheric relative humidity” contradicts direct evidence, and goes on to use false assumptions to make its unqualified claims. It should be noted that Brian Soden (who has published a guest commentary at RealClimate.org), the cited reference for the in the passage above, is a lead author on the chapter.

I will add it to my examples of bias I have found from my examination of evidence related to the AR4 so far.

These include:

1. Rahmstorf, who claimed climate responding faster than expected on the basis of
   a dubious graph with no statistical test;
2. Harries who claimed to detect the greenhouse effect from CO2 spectral brightening
   but whose later (unreported) publications were much more equivocal;
3. Soden, who claims to have detected increase in specific water vapor from
   spectral brightening using blatantly false assumptions.

Even though the AR4 report has been applauded by the major scientific institutions of the world, a cursory review appears to reveal huge biases.
At the very least, the literature deserves to be reviewed in a truly systematic way, focussed on quality of evidence, by an unbiased panel of experts. Such a review would be commissioned with emphasis on meta-questions such as: Is it systematic? What is the system of guidance given to the authors? Is it unbiased?