Global Warmer – A Primer 3 – The Carbon Cycle

You know, I may have jumped the gun a bit.  But no worries, let’s get some more information about carbon and such down right now.  And here’s the big question:

Why are fossil fuels bad for the environment (in terms of carbon dioxide), but burning wood, natural wax, and breathing not bad for the environment?

Well, it has to do with the carbon cycle.  Most of us learned about the carbon cycle when we where 10-12 and then promptly forgot all about it.  And I don’t mean this carbon cycle

The basics are pretty simple.  Two formulas dominate the carbon cycle.  You do one, plants do the other.

Photosynthesis (plants)  6CO2 + 6H2O –> 6O2 + C6H12O6

Respiration (plants and animals)  6O2 + C6H12O6 –> 6CO2 + 6H2O

The details don’t too much matter, but notice that the amount of carbon dioxide, water, oxygen, and sugar are exactly the same in both equations.  In a balanced ecosystem, the amount of carbon is a constant.  It just gets changed from carbon dioxide to sugar and back again.

For people, even things like wood fires, we just using the carbon that is already in the system.  When we burn a tree, then we’re converting the carbon in the tree to carbon dioxide.  Much of that carbon would be released eventually anyway (especially after the tree dies), we’re just speeding it up a bit.

The problem with fossil fuels that we are adding carbon to the system.  We’re digging up carbon that’s over a hundred million years old and adding it to the air (by burning coal, oil, natural gas, and producing carbon dioxide).

Carbon Cycle with Storage and Flux amounts (2004)

OK, take a look at this chart.  The arrows show the carbon cycle.  The blue numbers are the amount of carbon (in billions of tonnes) that are in flux, that is actually a part of the carbon cycle.  The black numbers show how much carbon is stored in that area (also in billions of tonnes).

Now take a look at that monstrosity on the right.  That represents all the fossil fuels and chemical processes that generate carbon dioxide.  That’s 5,500,000,000 tonnes of carbon entering the atmosphere per year.  That’s a lot.  But if you look at all the blue numbers other than that one, you will see that they are pretty close.  That means that the carbon cycle is very near equilibrium.

The 5.5 billion tonnes humans are pumping into the atmosphere are extra, over and above what the carbon cycle is prepared to deal with. That doesn’t sound like much (especially compared to the 750,000,000,000 tonnes of carbon already in the atmosphere), but that’s how much we’re pumping in the atmosphere every year.  An actually, this is considerably less than the 2009 value of about 7 gigatonnes per year.

Now we get to crux of the matter.  Adding all that carbon dioxide is not good.

First, some of that extra carbon is absorbed by the ocean.  Unfortunately, carbon dioxide reacts with water to form carbonic acid, which lowers the pH of the ocean, which destroys reefs, kills fish, etc.

Second, carbon dioxide is a greenhouse gas (previously talked about).

Finally, the effect on plants…

Oh sure, you’ll hear global warming deniers talk about how higher carbon dioxide is good for plants and they’ll just grow faster.  Unfortunately, that’s not really the case.

Current evidence suggests that that the concentrations of atmospheric CO2 predicted for the year 2100 will have major implications for plant physiology and growth. Under elevated CO2 most plant species show higher rates of photosynthesis, increased growth, decreased water use and lowered tissue concentrations of nitrogen and protein. Rising CO2 over the next century is likely to affect both agricultural production and food quality. The effects of elevated CO2 are not uniform; some species, particularly those that utilize the C4 variant of photosynthesis, show less of a response to elevated CO2 than do other types of plants. Rising CO2 is therefore likely to have complex effects on the growth and composition of natural plant communities.

Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants  By: Daniel R. Taub (Biology Department, Southwestern University) © 2010 Nature Education

So some plants don’t react well to higher carbon dioxide.  Interestingly some grasses don’t change their response.

Here’s a good one.  Plants in Florida are responding to increased carbon dioxide badly and even causing (yes, causing) changes in the climate.

Here’s part of the conclusion:

Further, our simulations predict that doubling today’s CO2 will decrease the annual transpiration flux of subtropical vegetation in Florida by ≈60 W·m−2. We conclude that plant adaptation to rising CO2 is altering the freshwater cycle and climate and will continue to do so throughout this century.

And we can’t forget the classic 1993 Poorter paper that global warming deniers love to quotemine.  Here’s the part they always quote:

Within the group of C3 species, herbaceous crop plants responded more strongly than herbaceous wild species (58% vs. 35%) and potentially fast-growing wild species increased more in weight than slow-growing species (54% vs. 23%). C3 species capable of symbiosis with N2-fixing organisms had higher growth stimulations compared to other C3 species. A common denominator in these 3 groups of more responsive C3 plants might be their large sink strength. Finally, there was some
tendency for herbaceous dicots to show a larger response than monocots.

What they don’t mention is this part at the end.

A doubling in the ambient CO2 concentration leads to only a small increase in growth, much smaller than could be expected on the basis of short-term measurements of photosynthesis.

It seems that plants are as used to the rise in carbon dioxide as we thought.  In fact, that seems to be normal (evolutionarily speaking).  The current levels of carbon dioxide have massively increased in (evolutionarily and geologically speaking) a very short time (150 years).

Now do you begin to see?  Excess carbon dioxide in the atmosphere is bad and we’re causing it.


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