Can anyone explain how the IPCC deals with the following numbers?

Question

I have seen several articles and models now that try to pin down what percentage of the greenhouse effect can be pinned down to what gas.

This article is as good as any of them:
http://www.realclimate.org/index.php?p=142

Notice it says water vapour could account for 36 percent of the effect up to 66 percent (not including clouds). This is a rather staggering uncertainty. Also, this is only the variance that could be argued for water vapour. The cumulative statistical uncertainty is higher if you include the other greenhouse gases. If other scientists in other fields formed conclusions based off of numbers that were that uncertain then I think they would have a lot to answer for.

Can someone tell me if these numbers are true and why many climatologists are so sure about what is happening with global warming if these numbers are true?

As always, this is not meant to take sides but to better understand the issue. Sources are always a plus.

I should add that I am not wondering if the water vapour is there because of forcing or not. None of the gases are pinned down very well and they can't all be the result instead of the cause. If we understand them so poorly I wonder how we can say they are the cause at all with absolute certainty.

Answer 1

The uncertainty lies in the way in which these values are estimated, using a gas column in a tube to simulate the atmosphere. Unfortunately, this leads to a relatively homogeneous gas column (ie, gases distributed with relative uniformity) simulating an atmosphere that is heterogeneous both vertically AND laterally. And clouds are only studied through computer models that to date cannot account for all cloud effects.

Here's one take on some of the difficulties of assessing the Greenhouse effect:
http://www.vho.org/tr/2003/2/Nettesheim131-135.html

and others:
http://ncwatch.typepad.com/media/files/D-Evans2007.pdf
http://www.lavoisier.com.au/papers/Conf2007/Carter2007.pdf
http://pubs.acs.org/subscribe/journals/ci/31/special/may01_viewpoint.html

I know that's a lot of info, and I wouldn't necessarily take these sources, or the realclimate source you provided, as gospel truth, but it does illustrate just how indefinite our knowledge is in trying to nail down dynamic climate phenomena.

Answer 2

Water vapor is the most important greenhouse gas. But it is highly variable in a not easily predictable way. CO2 is an important greenhouse gas too, and it is not highly variable, at least not on a short time scale. And looking at the graph in the source tells me that people are causing an important long term change in the CO2 level in the air. It may or not be a problem, but it must at least be considered. There are all kinds of contradictory theories out there, but this CO2 measurement is very reliable and agree on by all. And the trend sure does look alarming. So far the concentration is still small, but the change is becoming non-trivial.

Answer 3

You need to be rigorously correct in your use of the terminology. It is true that there is a very high uncertainty in determining the fraction of the observed increased in global mean temperature that is due to a particular radiative forcing. However, it is not true that there is very high uncertainty in all the radiative forcings themselves. In strict terms, the radiative forcings should be called the "greenhouse effects," not the rise in global temperature. The rise in temperature is a response of the climate system to the change in the radiative forcings.

The IPCC assiduously avoids apportioning the fraction of the rise in temperature (i.e., the response) to the known particular forcing mechanisms. However, they do try to estimate the uncertainties in the magnitudes of the forcings themselves. So, the radiative forcing of anthropogenic CO2 is well known and currently around +1.6 watts/m^2 (I think) while the radiative forcing of the indirect aerosol effects is much less precisely known: in fact it is unclear if the number is positive or negative. But nowhere in any of the IPCC documents will you find a statement such as: "47% of the observed increase in global mean temperature is due to the radiative forcing of CO2." The reason they do not do this is elegantly spelled out by the realclimate article.

The bottom line is that while the forcings themselves are well known, their interactions in such a tightly coupled nonlinear system are not easily determined. However, the net effect of all of the forcings, positive and negative, are easily observed as a warming arctic and steep rise in global mean temperature. Climate physicists are convinced that they have the forcings pretty much correct because when they run the models they do a very good job simulating the observed increase in temperature and the known regional effects (e.g., the warming arctic).

Answer 4

The key point is simple:

Water vapor in the air and liquid water reach equilibrium very rapidly.

So the effect of water vapor on greenhouse warming is relatively constant. Uncertainty about the precise contribution doesn't matter, whatever the contribution is it changes little year to year.

By contrast CO2 in the air is in a really slow equilibrium with plant matter and with oceans. So the increase in CO2 due to burning fossil fuels is a continual process, which is responsible for the present warming trend.

From the site you listed:

"While water vapour is indeed the most important greenhouse gas, the issue that makes it a feedback (rather than a forcing) is the relatively short residence time for water in the atmosphere (around 10 days)."

The residence time for CO2 is years.

Perhaps a clearer explanation is here:

http://environment.newscientist.com/channel/earth/climate-change/dn11652

But the bottom line is that your question focuses on static factors, while it's the kinetics that's important in the warming of the Earth over time.

Answer 5

Check out page 4 of the IPCC Summary for Policymakers:

http://ipcc-wg1.ucar.edu/wg1/wg1-report.html

It's got a chart that answers your question. Basically there's high certainty regarding the radiative forcing of the largest effects - the various greenhouse gases. Water vapor is more uncertain, but its concentration is dependent on the atmospheric temperature. In other words, it amplifies global warming, but does not initiate it.

There is uncertainty about some other forcing effects, but they're either small contributors or create cooling rather than warming. As you can see at the bottom of the chart, the total net anthropogenic warming is known to be between 0.6 and 2.4 W/m^2, most likely with a value of 1.6 W/m^2.

Answer 6

Did you have a point? I know that site well. They have numerous other articles on greenhouse gases including water, and account quite precisely for the role of water, which has been studied by scientists now, for a very long time.