Why aren't plants getting bigger with increased atmospheric CO2?

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Offline Lethalwolf

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Oliver asked the Naked Scientists:
Hi Chris and the gang,
Great show... how many times you must have heard it but still l have to congratulate you guys.
My name is Olivier  and l have a question regarding the rise of carbon dioxide in our atmsphere.  Since plant use CO2 to make their food, why aren't plants getting bigger since the source of food is more abundant now in terms of CO2.

Where is most the co2 we are producing as a civilisation ending up? upper or lower atmosphere? l mean in all tv shows and articles l've encountered the level mentioned is at ground level, for eg. in traffic, in a house, on a street.  But again in most description of the green house effect you can see that it mentioned that the CO2 in the upper atmosphere.  Now which is true and how come if the with regards to ground level of CO2 rising as well are our plants absorbing more of it and getting bigger?
Keep up the good work.

from Adelaide, South Australia

What do you think?
« Last Edit: 01/03/2011 03:30:03 by _system »


Offline CliffordK

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Most studies seem to indicate an increase in tree growth and forest growth with the increased Carbon Dioxide.


However, at least one study indicated an early growth spurt, followed by a levelling off, or decline in growth of trees in Canada. 


Presumably the trees are responding to other stressing agents such as changes in water, or perhaps soil nutrient depletion (from the prior growth spurt).  Or a change in temperatures/climate might force a change in tree species.

About half of our excess annual CO2 is being absorbed by the oceans.  And, according to the study above, perhaps 1/5 of the excess (probably part of the half above) is being absorbed by forests.  That means that there is an increasing atmospheric burden.

Finally found the vertical distribution of CO2 in the atmosphere.


So, that puts the greatest concentration in ppm in the lower troposphere, although the total variation throughout the troposphere is only about 20ppm. 

However, the atmospheric density also decreases exponentially with altitude. 

Concentration slightly favors CO2 in the lower atmosphere.

However, by density, the total amount of air/CO2 decreases very fast.  So, most of the CO2 is actually near the ground.

What you hear about the stratosphere, in particular, is the theory that the CO2 acts as a blanket over the earth, thus decreasing outgoing IR emissions.  And, thus, less outgoing heat gets to the stratosphere, and the stratosphere gets cooler while the lower atmosphere, the troposphere, gets warmer.


Offline yor_on

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Interesting stuff Clifford, and nicely done.

I had a study (some year ago?) which was done on very old trees in USA, checking their CO2 'respiration' that I tried to find reading you, but I can't find it :( . Still, I have this one, although slightly contra dictionary as a collection of studies. Respiration (Response to CO2 - Woody Plants: Multiple Tree Studies). with links to the respective studies.

The conclusion of the author though (of the linked page) is that. "Perhaps the most reasonable conclusion would be that atmospheric CO2 enrichment may either increase or decrease woody-plant respiration, but not to any great degree, and that in the mean, the net result for the conglomerate of earth's trees would likely be something of little impact, one way or the other."
« Last Edit: 24/03/2011 23:11:40 by yor_on »
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