Arguments
Does breathing contribute to CO2 buildup in the atmosphere?
The skeptic argument...
Breathing contributes to CO2 buildup
"Pollution; none of us are supporting putting substances into the atmosphere or the waterways that might be pollutants, but carbon dioxide is not a pollutant. If Senator Wong was really serious about her science she would stop breathing because you inhale air that's got 385 parts per million carbon dioxide in it and you exhale air with about ten times as much, and that extra carbon comes from what you eat. So that is absolute nonsense." (Ian Plimer)
What the science says...
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| By breathing out, we are simply returning to the air the same CO2 that was there to begin with. | |||||
The very first time you learned about carbon dioxide was probably in grade school: We breathe in oxygen and breathe out carbon dioxide. Any eight-year-old can rattle off this fact.
It should come as no surprise that, when confronted with the challenge of reducing our carbon emissions from the burning of fossil fuels, some people angrily proclaim, "Why should we bother? Even breathing out creates carbon emissions!"
This statement fails to take into account the other half of the carbon cycle. As you also learned in grade school, plants are the opposite to animals in this respect: Through photosynthesis, they take in carbon dioxide and release oxygen, in a chemical equation opposite to the one above. (They also perform some respiration, because they need to eat as well, but it is outweighed by the photosynthesis.) The carbon they collect from the CO2 in the air forms their tissues - roots, stems, leaves, and fruit.
These tissues form the base of the food chain, as they are eaten by animals, which are eaten by other animals, and so on. As humans, we are part of this food chain. All the carbon in our body comes either directly or indirectly from plants, which took it out of the air only recently.
Therefore, when we breathe out, all the carbon dioxide we exhale has already been accounted for. We are simply returning to the air the same carbon that was there to begin with. Remember, it's a carbon cycle, not a straight line - and a good thing, too!
Last updated on 26 September 2010 by climatesight.
I was questioned the other day about this exact issue but the argument went... If we have gone from 1 billion to nearly 7 billion in 200 years and in that time have cut down massive quantities of vegetation without planting more then surely some of the rise is attributable to the rise in co2 - ie we have messed with the carbon cycle?
Well yes, population is the elephant in the room said I. Then I pointed out all the evidence showing that we can attribute the rise in co2 to the combustion process (o2 levels dropping, c12/c13 ratio falling and of course how much we have burned compared to co2 rise).
However it does seem to many an obvious equation...Less plants - less o2 is produced - more people combining carbon and oxygen - co2 has to increase surely?
I suppose it comes down to how much GHG's humans bodily functions produce annually compared to how much is recycled by plants annually. I can feel myself beginning to answer my own question here!
Does anyone have any figures relating to this topic?
All the CO2 we exhale comes from burning food. An expanding human population means more crops - otherwise we'd starve. So there is no nett contribution to GHG from human respiration.
I would imagine most deforestation is for agriculture i.e. replacing one sort of vegetation with another. This is not harmless - forests are good "carbon sinks" (all that wood is "locking up" carbon) and they photosythesize all year round unlike crops. Forests may be more efficient at absorbing CO2 because of their height too.
I have no figures - sorry, but to do this calculation you would need to take into account the changes in other animal populations (down for wild animals, up for domesticated) as well. I would doubt whether such comprehensive figures are available
The figures shown below suggest that the CO2 flux into the atmposphere from terrestrial plants is about 60Gt C per year (which is vast)
if you divide that 60Gt by the human population it would give the number of tons of carbon we would need to consume each year to balance the flux from land plants, I doubt any of us eats quite that much! ;o)
I'm a lecturer at a construction college of further education in the uk.
I think the construction industry has a hugely important role in reducing CO2 emissions, and I for one (and I'm sure many more here) are very happy to see people like you making use of sites like Skeptical Science.
From a USEIA report regarding the drop in US CO2 emissions in 2009:
In 2009, energy-related CO2 emissions in the US saw their largest absolute and percentage decline (405 million metric tons or 7.0 percent) since the start of EIA’s comprehensive record of annual energy data that begins in 1949 ...
Changes in CO2 emissions can be decomposed into changes in four major contributing factors: population, per capita GDP, energy intensity of the economy, and carbon intensity of the energy supply. All of these fell in 2009 except for population. Population grew 0.9 percent.
So it would appear that what we do and how much of it we do are far more significant than what we eat.
I wonder whether this piece could be improved with some more figures on the exact amount of CO2 that human respiration contributes. I get the idea that we're only exhaling the carbon that has first been photosynthesised out of the atmosphere, but from one point of view, the origin of the CO2 is irrelevant, what matters is the total amount. So I wonder whether this argument could be supplemented with a consideration of the total contribution of human respiration to CO2 emissions (for completeness, perhaps we would also need to consider human CH4 emissions...).
P.S. "net" to take into account plants respiration.
Thank you very much.
This is not a comment about the carbon intensity of agriculture (a separate discussion) but rather the source of the carbon in the carbohydrate content of our diet. There are many chemicals (polyethylene glycol, for example) in our diet that are synthesized from petroleum. I would be interested to know what portion of the CO2 breathed out by humans has fossil C.
I think this would rather strengthen the "real food" argument more than anything else.
For example, CO2 emmissions from burning coal or oil are also part of this cycle.. The plants that absorb the CO2 grow, die, fall to the ground, are pressurised for millions of years and then turn into Coal or Oil.
So isn't burning fossil fuels just an extension of this carbon cycle?
"For example, CO2 emmissions from burning coal or oil are also part of this cycle."
That carbon had been locked out of the carbon cycle for hundreds of millions of years. What we are doing is reintroducing it to the carbon cycle, at a very great speed, far faster than natural carbon sinks can absorb it. Therefore, atmospheric CO2 is steadily rising.
"The plants that absorb the CO2 grow, die, fall to the ground, are pressurised for millions of years and then turn into Coal or Oil."
Almost all of the plants that die now decay and ultimately release CO2 back into the cycle. They do not become coal or oil.
Humans then came along, extracted, refined and burnt those fossil fuels, releasing all that 'additional' carbon back into the atmosphere. Hence we have a problem of humungous proportions.
We also now grow more food to feed the extra people. Any CO2 we breathe out was carbon we locked up in plants before eating them.
1) what we exhale, and how we acquired that CO2 - fixed from the atmosphere by plants, so not a net contribution to atmospheric CO2
2) don't try to estimate the fluxes in and out, and just look at the change in storage. On that basis, I'm fairly sure (99-44/100ths % pure) that the 7 billion people we have now store more carbon (i.e., weigh more in total) than the 4 billion in the 1970s, so humans represent a net sink of carbon, not a source.
Once climate change is bad enough that we see large decreases in the human population, we'll become yet another source of positive feedback as the stored carbon is released back to the atmosphere.