No, water vapour is not what's *causing* global warming

Tim Ball wrote at WUWT an article with a misleading headline (as is usual at WUWT)):

Thanks To The IPCC, the Public Doesn’t Know Water Vapor Is Most Important Greenhouse Gas

This is a "back to basics" article. The basics being: CO2 is a greenhouse gas. As atmospheric CO2 increases, the earth gets hotter. As temperatures rise, more water evaporates.  Water is also a greenhouse gas. More atmospheric water vapour means the world gets hotter still.

The lack of basic scientific knowledge displayed at WUWT sometimes surprises even me.

What is the most prolific greenhouse gas?

Let's look at the WUWT headline statement one part at a time. First the basic premise: is water the most important greenhouse gas? It's certainly the one in the greatest quantity in the atmosphere. And it is the one that makes the greatest contribution to the greenhouse effect. That's the effect itself.

• The net effect on radiative forcing by absorption of long wave radiation is: carbon dioxide 19%, water vapour 50%, clouds 25%, all other forcings 7%.
• For clear sky forcings (no clouds) the long wave absorption (greenhouse effect) is: water vapour 67%, CO2 24% and other forcings 9%.

Source: Schmidt et al (2010)

When it comes to global warming - an increase in the greenhouse effect - it's not water that is the primary cause of the rise in global temperatures. Without a rise in CO2, there wouldn't have been a rise in global temperatures. There is more water in the atmosphere now because it's warmer. This makes it warmer still. However it was the rise in CO2 that caused the extra warmth in the first place.

Apart from the sheer volume in the atmosphere at any time, one of the main differences between water and carbon dioxide is that water doesn't accumulate in the air over the long term. What goes up comes down. Water gets rained out. The surface has a great capacity to absorb water. The oceans are full of the stuff and can get a lot fuller.

Carbon dioxide is different in the way it circulates between the surface and the air. The properties of water means that you'll find it as solid (ice), liquid and gas all at the same time somewhere on earth. Not so CO2. In it's natural state at present earth temperatures you'll mainly find it as a gas, with some dissolved in water.

When water vapour is in the air it rises up with the rest of the air (warm air rises) and cools down again as it gets higher. When it cools down it condenses into water - as clouds. Then it rains and falls right back to the surface.

It's a lot more difficult for CO2 to get out of the air than it is for water to get out of the air.

Carbon dioxide gas has to get a lot colder before it changes state. Even at zero degrees Celsius, CO2 remains a gas. It's melting point is -57°C. Therefore, unlike water, as air rises and becomes cold, CO2 doesn't condense to form clouds and doesn't rain out of the sky as rain, snow and ice.

In the short term, on a scale of days to decades, the main ways for CO2 to be removed from the air are through plant uptake (via photosynthesis) and by dissolving in water (in the oceans, rivers and lakes). You can read about the carbon cycle in an excellent series of articles on the NASA Earth Observatory website.

There are limits to the extent CO2 can be dissolved in the bodies of water at the surface. These limits are set in part by the relationship between temperature, the partial pressure of atmospheric CO2 and the amount already dissolved in the water.

There is no such limit for adding water to the bodies of water. All it takes is more water in the air to rain out. In other words, one of the limitations to the amount of water in the air vs on the surface is the total amount of water on earth. It's either in the air, in the oceans and waterways, in cold places (ice), tied up in living organisms or dead plant matter (eg peat, coal), or in the soil and sub-soil.The main limit to how much water is on the surface compared to in the atmosphere is the temperature of the atmosphere and the total amount of water.

You can read about the water cycle in another multi-part article on the NASA Earth Observatory website.

Rising and falling sea level gives a sign of how hot or cold earth is at a particular time. When sea levels are higher, earth is warmer because there's more water in the oceans than is locked up in ice. The amount in the air rises and falls a bit with the temperature of the earth. When it's warmer, there is more water vapour in the air (on average). So it adds to the greenhouse effect, making earth warmer still.

What is the most important greenhouse gas for humans?

As far as maintaining the earth in conditions that suit human civilisation, then if I had to choose I'd say that atmospheric carbon dioxide is more important than water. On the other hand, without water we wouldn't exist anyway. And without the moderating effect of the water cycle, we'd be in deep trouble. Actually, we'd never have been here in the first place.  Just as we wouldn't exist at all if not for carbon. I'll stop trying to assign priorities. Is there any point trying to decide which is more important - water or carbon dioxide?

Remember I'm not talking about the importance of water as such. I'm only referring to the fact that it's CO2 that is the "control knob" of global temperatures. That's because it works on a very long cycle, unlike water. You can't easily remove CO2 from the air. You can add it fairly easily, as we've been doing, by mass burning of hydrocarbons that have been buried in very long term carbon sequestration processes. Unlike water, CO2 doesn't precipitate out as rain.

What the public doesn't know

I've no way of checking whether the public knows that water evaporates as temperatures rise. Nor the extent to which the public is aware that water is a greenhouse gas. What I can do is see what the IPCC says about it.

What does the IPCC say about water?

Since Tim blames what he considers public ignorance on the IPCC, what does the IPCC say about water? You can read it for yourself if you've the time. I'd say that Tim Ball hasn't read an IPCC report.

In the latest AR5 WG1 report (the one about the physical science) here are some stats to mull over:

• The word "water" appears 2017 times, including 85 as "water vapor". The word "ice" appears 5225 times. "H2O" appears 58 times. 
• The word "carbon" appears 2534 times, including 305 as "carbon dioxide". "CO2" appears 2,783 times.

There are entire chapters on clouds and ice and the atmosphere. There are entire sections on the hydrologic cycle. 

If a person reads the IPCC scientific reports and doesn't understand the importance of water and water vapour, then I'd say it is not the fault of the IPCC or the authors. And if a person reads the IPCC scientific reports and still doesn't understand the importance of carbon dioxide, then I'd say it's not the fault of the IPCC and the authors.

From the WUWT comments

markstoval hasn't bothered to read any science, let alone an IPCC report. He wrote:

February 8, 2015 at 11:16 am
Another great post. Thanks Dr. Ball.
We live on a water world and the IPCC claims that CO2 does it all. Someday this lunacy will end. (I hope)

The comments were mostly dumb denier memes. Barry injected some common sense.

February 8, 2015 at 1:17 pm
Many people are trying to better understand water vapor (indeed, the entire hydrologic cycle) and how it impacts the energy balance. Obviously it’s of critical importance. But please explain to me how humans can have a significant impact on the amount of water vapor in the atmosphere. Do you know how much water evaporates from the oceans each day?

About the best that Barry got was a conspiracy theory from highflight56433

February 8, 2015 at 2:10 pm
The point is that evil people are being deceptive for their own gain. 

Chris Aikman added some sense as well:

February 8, 2015 at 1:33 pm
Every scientist acknowledges that H20 has greater opacity in the Infrared than CO2, and is thus the stronger component to holding in the Earth’s heat. The point is the H20 precipitates out as rain and snow, CO2 does not.

Because warm air holds much more water vapor than cool air, there is a strong positive feedback effect that amplifies the blanketing of the atmosphere in the infrared when CO2 increases. If we had a water-free planet, the effect of CO2 would be much less.

But we live on a watery planet. Which means it’s worth taking care of it. 

Chris's comment attracted this piece of idiocy from Latitude, who seems to be disputing what Chris wrote. Maybe Latitude has never seen rain or snow in his life:

February 8, 2015 at 2:58 pm
Chris, if that happened…it would have already happened
We would have had run away global humidity before….
Actually, if that were possible, we wouldn’t be here right now

Tim Ball added this comment full of conspiracy ideation.

February 8, 2015 at 1:53 pm
Barry
Apparently the IPCC know the answer to your question. Confronted with problem of CO2 saturation that set an upper limit on temperature increase, even with doubling and tripling of CO2, they invoked a positive feedback mechanism?

This claimed that the small temperature increase caused by increased CO2 due to humans, resulted in increased evaporation. The higher water vapor level in turn caused a further temperature increase as a positive feedback mechanism.

So, they only include water vapor when it suits their claim of a positive feedback. This implies they do know how much water vapor varies and how much it increased due to human activity. Can anybody show me their calculations and real world proof of the claim?

Tim acts as if he's read the IPCC reports but he doesn't demonstrate that in his words. For example, if he wants real world proof, he can look at salinity observations. Salinity will increase as more water is evaporated. Figure TFE1 Figure 1 from the IPCC AR5 WG1 report has different ways of showing changes in water vapour and salinity:

TFE.1, Figure 1: Changes in sea surface salinity are related to the atmospheric patterns of Evaporation minus Precipitation (E-P) and trends in total precipitable water:
(a) Linear trend (1988 to 2010) in total precipitable water (water vapor integrated from Earth’s surface up through the entire atmosphere) (kg m–2 per decade) from satellite observations.
(b) The 1979–2005 climatological mean net evaporation minus precipitation (cm yr–1) from meteorological reanalysis data.
(c) Trend (1950 to 2000) in surface salinity (PSS78 per 50years).
(d) The climatological-mean surface salinity (PSS78) (blues <35; yellows-reds >35).
(e) Global difference between salinity averaged over regions where the sea surface salinity is greater than the global mean sea surface salinity (“High Salinity”) and salinity averaged over regions values below the global mean (“Low Salinity”).
Details of data sources: see Figure 3.21 and FAQ 3.3, Figure 1. {3.9} IPCC AR5 WG1

Is NielsZoo saying snow never melts on Everest? That rain is made by magic? Or what? I can't make head or tail of this comment of his:

February 8, 2015 at 2:07 pm
This is great and adds even more nails to the CAGW coffin with simple logic… however, my pet peeve. Water vapor is NOT a gas anywhere in our atmosphere except briefly at volcanic vents before cooling to equilibrium temps and “normal” atmospheric pressures. At the highest point on Earth the boiling point of water is ~72°C (161°F) and I don’t think it gets that hot at the top of Everest. (Hottest recorded natural temp is 134°F in Death Valley, below sea level.) Look at any phase diagram for water and our atmospheric conditions don’t fall in the gas area. The reason I bring this up is, as Dr. Ball points out, the AGW types not only omit allowing for water vapor’s sensible heat in calculations and models, they omit the massive amounts of energy in latent heat transferred in and out of the atmosphere as water vapor evaporates and condenses out of solution. Gases do NOT do this above their boiling points.</rant>

There is a whole lot more, including conspiracy theories galore from mods and commenters alike.

You can read the other comments here, if you like.

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Further reading and references

The Water Cycle: By Steve Graham, Claire Parkinson, and Mous Chahine, Design by Robert Simmon, October 1, 2010 - NASA Earth System Observatory.

The Carbon Cycle: By Holli Riebeek, Design by Robert Simmon, June 16, 2011 - NASA Earth System Observatory.

Schmidt, G. A., R. A. Ruedy, R. L. Miller, and A. A. Lacis (2010), Attribution of the present‐day total greenhouse effect, J. Geophys. Res., 115, D20106, doi:10.1029/2010JD014287

IPCC AR5 WG1 Climate Change 2013: The Physical Science Basis