Anthropogenic sources cause rising water vapor concentrations in the troposphere

Water vapor is one of the principle greenhouse gases which contributes to global warming by trapping heat within the atmosphere.  Water vapor in the troposphere can be from both natural and anthropogenic sources.  Scientists have observed an increase in the amount of water vapor in the troposphere, but have not been able to distinguish whether this increase is due to natural or anthropogenic sources.  Being able to distinguish between these sources is helpful in determining the potential warming effects of the additional water vapor.

The ScienceDaily article Global warming amplifier: Rising water vapor in upper troposphere to intensify climate change summarizes a recently published PNAS journal article, Upper-tropospheric moistening in response to anthropogenic warming, which shows that the increased moistening of the upper-troposphere is due to anthropogenic sources.

Illustration of annual mean T2-T12 field that provides a direct measure of the upper-tropospheric water vapor. Purple = dry and Red = moist.

Credit: Eui-Seok Chung, Ph.D. Assistant Scientist - UM Rosenstiel School of Marine and Atmospheric Science

 From ScienceDaily article Global warming amplifier: Rising water vapor in upper troposphere to intensify climate change

The Science Daily article states that not only is the level of water vapor in the troposphere increasing, but the cause of this rising level has been found to be due to the direct effect of humans.  This was done by comparing a series of climate models with water vapor data measured by NOAA satellites.  By comparing the predictions from the climate models with the data from the NOAA satellites, the researchers found that without including the anthropogenic sources, like other greenhouse gases, the observed data does not match the predictions from the models.

The journal article in PNAS explains in much greater detail the results of the climate models and the methods to confirm the results.  The primary method the scientists used to determine that anthropogenic effects were causing the increased moistening was by comparing the channel 2 brightness temperatures (T2) with the T12 emissions.  The difference between the brightness of these measures shows the extent of the moistening of the troposphere.  To demonstrate that these results were not due to bias, the scientists also ran a model keeping the concentration of water vapor constant over time. The results of this model showed that the T2-T12 value stayed at 0, even as the amount of water vapor in the model changed due to natural sources. (Seen below in the figure from the PNAS article)  This represents that the additional water vapor from the anthropogenic sources is  needed in order to match changes seen using the NOAA data and climate models.

I think that the ScienceDaily article did a great job of explaining the PNAS article.  The PNAS article was extremely technical and requires a lot of knowledge about the field to fully understand.  While the ScienceDaily article does not go into a lot of the details of all of the 20 climate models that were used and the bias elimination studies, the overview of the methods section is an accurate representation.  This is an important finding which I think was well represented in the ScienceDaily article, therefore I would give it an 8/10.

ScienceDaily:

http://www.sciencedaily.com/releases/2014/07/140728153933.htm

PNAS:

http://www.pnas.org/content/111/32/11636.full.pdf+html