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Facts and fictions: Water vapour, CO2, and the Carbon Cycle


Fiction: CO2 is a harmless gas.

Fact: Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor. It can cause asphyxiation and irritation. When inhaled at concentrations much higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat.

These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. This sensation can also occur during an attempt to stifle a burp after drinking a carbonated beverage. Amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm (equal to 5% by volume) are considered dangerous to animal life.

Fiction: Water vapour is a bigger greenhouse gas then CO2

Fact: While there is far more water vapour in the atmosphere than other greenhouse gases, the other greenhouse gases play an important role in influencing our climate. The increase in anthropogenic greenhouse gases is largely responsible for the observed warming of 0.74°C over the 20th century. This warming has had a ‘positive feedback’ as a warmer atmosphere can hold more water vapour – enhancing human induced warming by about 50%.

In other words, water vapour itself does not cause the original heat rising in the first place, but it magnifies the results.

The other thing here is human activity does not seem directly affect the amount of atmospheric water vapour. The reason been water vapour condenses rapidly and does not exist in vapour form for very long when cooled.  At higher temperatures, the pressure rises.

CO2 on the other hand has a extremely stronger bond. Because of this it takes CO2 quite a long time to break down. It’s temperature curve is interesting when comparing it with water vapour. Notice that H2O goes from critical point to triple point (on the right above) between 4.4 kPa and -0.4 kPa. Where as CO2 (below) only starts to get to critical point at close to 80 kpa and retains stability even in colder temperatures.

Finally we can actually look at the bond itself. H2O looks like this

and CO2 like this

Water has one bond between the hydrogen atom and the oxygen atom per hydrogen atom.

CO2 has duel oxygen atoms. Further the connection between the carbon and each oxygen is double bonded. And on top of that, each bond is almost 20% stronger than the one bond link in the water molecule.

We also know that sustained pressure or heat will break down the bonds. Ice becoming water becoming steam is basically all water, but the bonds of a solid are stronger then that of a liquid and that of a gas.

But we also know that CO2  can withstand much much more pressure then H2O and sustain it under that pressure for longer. It is why we can make compressed CO2, but struggle to try and keep steam under compression.

Water vapour in the atmosphere tends to reform into water over a small amount of time. Whereas CO2 takes much much longer to breakdown.

Average reservoir residence times
Reservoir	Average residence time
Antarctica		        20,000 years
Oceans 			        3,200 years
Glaciers		 	20 to 100 years
Seasonal snow cover		2 to 6 months
Soil moisture			1 to 2 months
Groundwater: shallow		100 to 200 years
Groundwater: deep		10,000 years
Lakes (see lake retention time)	50 to 100 years
Rivers				2 to 6 months
Atmosphere			9 days

The next thing to do then is find out how long CO2 stays in the atmosphere. We know its going to be much longer then water vapour as we already looked at the volatility of the different bonds at a molecule level.If you talk to Jo Nova, she will go on about how its only 4 years. According to her. She makes her case here.

Her argument is since CO2 is moved around the atmosphere on a regular cycle, and given the planet has several carbon sink systems that capture a certain percentage of that CO2 (she talks 25%), then surely, according to her, after four years of cycling, then 4 lots of 25% is 100% and then in four years it is all gone.

Simplistic really. And wrong.

She does not at any stage introduce the emissions that are added by man into the cycle that is over and beyond the ability for the planet to capture. It also does not in any way explain why then would the parts per million concentration of CO2 be increasing at all.

She does not deny that the concentration has increased from 280ppm to 390 ppm in the last one hundred years or so. Why would this occur if it did not mean that the excess CO2 could not be captured by the natural carbon cycle and the excess therefore RETURNS to the atmosphere once again.

This time it joins the new cycle of carbon in the atmosphere including more of the stuff we make. Remember, the contents that were returned did so because they could not be captured. If this repeats, then you have more and more CO2 returning to the atmosphere increasing the concentration as it is joined with yet another yearly cycle and so on.

She also makes the terribly foolish mistake of not checking the sinks. You see, like water vapour, CO2 has a much slower break down life in other parts of the system.

The water vapour has a 20,000 year life trapped as ice. Likewise, while a tree for example can capture CO2, it can only process so much. While the sea can absorb CO2, it takes a while to do so and as such, until that chemical process completes, it has less and less carbon sink capability.

If you like you can imagine a train pulling into the same station on a circular route. The train leaves the atmosphere station packed with visitors of form CO2 land.

As the train pulls into the carbon sink stations, some CO2 is able to get out. However some can not (no more sink capacity) and remain on the train. The train pulls back into the atmosphere where it fills up again. But this time not all passengers can get in to the train as there are still some passengers on it so they have to stay there and are soon joined by new passengers.

The train now pulls up to the carbon sink stations only some stations are still processing the passengers from the last trip and so less passengers can actually get out.

That means more and more passengers are left on the train as it returns to the atmosphere station where less and less passengers can now fit on the train and more and more are left at the station and so on.

FACT: The oceans sink capability is hitting a wall. They are becoming more and more less effective as they are already capturing at capacity. And it is the Oceans that are the biggest carbon sink we have. If they are capturing less and less, then that explains why the concentration of CO2 has moved from 280 ppm  to 390ppm.

FACT: The carbon sink cycle takes time. It is a chemical process that is severely impacted by pressure and temperature. To separate carbon from CO2, the bonds that hold the carbon and oxygen together need to be softened. If the oceans warm up, and they are, then that means that chemical process is slowed down. Further at the warmer temperatures, the CO2 is likely to not sink to the colder water and therefore the entire process is slowed.

This is the thing with climate change people. It is not just about what we put up there, but if putting it up there decreases the ability of the planet to clean it up.

And that is what we are seeing. A steady increase in concentration as the carbon cycle train has less room for passengers.

As to Ms Nova’s comments re Tim Flannery, I am afraid he is right. Even if we were to stop emissions tomorrow, the chemical process in the oceans will still need to finish before it can take any of the new passengers. And on land, it takes a really long time to make a mountain.

While it does not take long to burn a log of wood, you are releasing carbon in a few minutes that has taken years to capture and store. That is basically the problem. The carbon released in coal has taken MILLIONS of years to get to that state. And in an instant it is released. The same with oil.

While it is cheap to use the energy from fossil fuels, it takes a long long time to put the genie back into the bottle.

PS. Good non biased series on climate science in these videos. Implore you to watch them all.

Feel free to make a comment Jo.

A. Ghebranious   2011   (All Rights Reserved)

From → Science

  1. Jennifer Baratta permalink

    Well Said Ash.

  2. Thanks for this wonderfull blog.

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