“Water Vapor Feedback Still Uncertain” By Marcel Crok

Marcel Crok is an outstanding climate science writer and reporter (see also his book http://www.staatvanhetklimaat.nl/2011/04/13/de-staat-van-het-klimaat-nu-ook-als-ebook/ which is expected to soon be translated into English). This past week on his weblog he has published a very important post with respect to the water vapor feedback aspect of climate. His post is

Water vapor feedback still uncertain

I have reposted below critical new information on the water vapor trends which Marcel obtained from Tom Vonder Haar [who is a  close colleague of mine at Colorado State University] as reported in Water vapor feedback still uncertain:

“We have most definitely never said the preliminary NVAP data show a negative trend and anyone who does is making a false scientific statement. All we can say at present is that the preliminary NVAP data, according to the Null Hypothesis, cannot disprove a trend in global water vapor either positive or negative.

In addition, there are good reasons based upon both Sampling / Signal Processing Theory and observed natural fluctuations of water vapor ( ENSO’s, Monsoons, volcanic events, etc. ) to believe that there are no sufficient data sets on hand with a long enough period of record from any source to make a conclusive scientific statement about global water vapor trends.

I believe discussion and informed speculation is healthy for Earth System Science when properly reported.  As you know the most recent IPCC assessment went into considerable detail about Uncertainty and I support even more of this work.  It helps focus our scientific attention on key areas where improvements should and can be made. Water vapor variability and feedback is one such area and that is the reason for the Re-analyses and improvements to the NVAP data set.  We are planning first release and discussion of our new results of NVAP-M ( sponsored by the NASA MEaSUREs research program ) at the World Climate Research Programme Science Conference in Denver, CO in October, 2011.  That will begin a period of more than one year wherein we will intercompare the NVAP-M results with independent estimates by colleagues in both the US and international community.  The checks and balances provided by such a collaborative effort should then produce a credible statement about our rapidly increasing knowledge of variability and trends of water vapor.

Now although of course he is very careful – as most scientists are – these statements are far less certain about the observational evidence for a positive water vapor feedback than IPCC was in AR4. There they wrote in the Summary for Policy Makers:

“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.”

Now this of course sounds much more certain than the remark of Vonder Haar that ‘there are no sufficient data sets on hand with a long enough period of record from any source to make a conclusive scientific statement about global water vapor trends’. So I think we all look forward to hear more about this important data set next October in Denver.”

Marcel also reported on new valuable analyses of multi-decadal precipitation trends by Demetris Koutsoyiannis. As Marcel wrote 

“Koutsoyiannis recently presented an analysis about trends in extreme precipitation at the EGU conference concluding that especially since 1970 there is no trend at all. Also at EGU he showed that models underestimate extreme rainfall for some stations around the Mediterranean up to a factor of ten…..Koutsoyiannis found no trend in floods worldwide either.”

I will add here to Marcel’s insightful comments and very effective journalism (and to Demetris’s cutting edge analysis).

The IPCC view on water vapor feedback is that the radiative warming effect is amplified by added water vapor in the atmosphere as the ocean surface warms which results in greater evaporation into the atmosphere, as Marcel discussed in his weblog post.

However, there are a number of other studies which conclude that the multi-decadal global climate models as reported by the IPCC are incorrectly simulating the water cycle which includes the amount of water vapor in the atmosphere. These include, for example,

Stephens, G. L., T. L’Ecuyer, R. Forbes, A. Gettlemen, J.‐C. Golaz, A. Bodas‐Salcedo, K. Suzuki, P. Gabriel, and J. Haynes (2010), Dreary state of precipitation in global models, J. Geophys. Res., 115, D24211, doi:10.1029/2010JD014532

who concluded that

“….models produce precipitation approximately twice as often as that observed and make rainfall far too lightly. This finding reinforces similar findings from other studies based on surface accumulated rainfall measurements. The implications of this dreary state of model depiction of the real world are discussed.”


Sun, De-Zheng, Yongqiang Yu, Tao Zhang, 2009: Tropical Water Vapor and Cloud Feedbacks in Climate Models: A Further Assessment Using Coupled Simulations. J. Climate, 22, 1287–1304.

who wrote

“…….extended calculation using coupled runs confirms the earlier inference from the AMIP runs that underestimating the negative feedback from cloud albedo and overestimating the positive feedback from the greenhouse effect of water vapor over the tropical Pacific during ENSO is a prevalent problem of climate models.”

They did write in their paper that

“We …. suggest that the two common biases revealed in the simulated ENSO variability may not be carried over to the simulated global warming, though these biases highlight the continuing difficulty that models have to simulate accurately the feedbacks of water vapor and clouds on a time-scale we have observations”.

but in the following question that I asked the De-Zheng Sun (see)

“[I]t is not clear how such a bias could be removed when the models are applied in longer term model projections. Indeed, what is the data which says that the biases are removed?”

he replied

“You are right that no data have shown that those biases will not be removed. We are just mentioning the possibility that there could be error cancellation as global warming may involve more processes that those in ENSO, and the errors may cancel in such a way that prediction of global warming by these models that have these errors may actually get the answer right.  It is just a possibility worth mentioning.”

Our analysis of observed data also indicates that on the regional scale at least, the water vapor amplification feedback is not occurring as claimed in the 2007 IPCC WG1 report.  We reported on this in our paper

Wang, J.-W., K. Wang, R.A. Pielke, J.C. Lin, and T. Matsui, 2008: Towards a robust test on North America warming trend and precipitable water content increase. Geophys. Res. Letts., 35, L18804, doi:10.1029/2008GL034564

where we found that

‘…. atmospheric temperature and water vapor trends do not follow the conjecture of constant relative humidity over North America.We found that for the domain we evaluated ……. temperatures significantly increased (0.248 ± 0.0742 K/decade) according to the 27-year monthly data, but the pr e c i p i t a b le wat e r vapor ( 0.00619 ± 0.0755 Kg/m2/decade) and total precipitable water (0.0108 ± 0.0782 Kg/m2/decade) did not.

Marcel has succinctly and clearly presented information which conflicts with the IPCC statements on our current understanding of the climate system. We need  more journalists to adopt the high investigatory standards that Marcel exemplifies.

Comments Off on “Water Vapor Feedback Still Uncertain” By Marcel Crok

Filed under Climate Change Forcings & Feedbacks, Climate Science Reporting

Comments are closed.