Does our breathing contribute to global warming?

Sometimes, those who decide to seriously think about CO2 and global warming end up with an interesting question: Hey, don’t we humans breathe out CO2 as well? So aren’t we, just by being alive, contributing to global warming?

Perceptive question indeed.

Before I give a detailed answer, here’s a tip: it has a happy ending.

Of course we breathe out CO2, the very same CO2 that is the villain-in-chief. A human being, on average, gives out about 1 Kg of CO2 a day.

The 1 Kg CO2/human/day estimate is of course indicative, because how much CO2 you give out depends on how much work you do. A guy spending most of this time at the gym will be giving out a lot more CO2 than a lazy couch potato, but an average of 1 Kg per human per day seems to be an accepted number. (Some estimates are a bit lower than 1 Kg, but as you will realize after you finish reading this post, it really does not matter what the actual number is!).

At 1 Kg per day, and taking 7.5 billion as the current world population ( we are close to that, see here), breathing from the entire world’s population releases 7.5 billion Kg of CO2 a day (7.5 million tons), or about 2.75 billion tons of CO2 per year.

Now, how large is this number 2.75 billion tons/year?

The total CO2 emissions from all our industrial and domestic processes amounted to about 35 billion tons in 2014 (IEA gives slightly lower estimates at about 32 billion tons) – China, USA, EU and India alone contributed 21 of the 35 billion tons! The 35 billion ton estimate includes CO2 emissions from all human activities – power plants, transportation, industrial and domestic heating/cooling etc., but does not include the CO2 emitted by our breathing.

So! The CO2 from 7.5 billion sods breathing their lungs out is less than 10% (2.75/35) of the total CO2 emitted by all other human-related processes.

That alone should make us breathe easy.

But there’s more to put us at ease – Our breathing does not add to the amount of CO2 in the atmosphere.

Hello. How is this possible?

You see, it’s like this. Think back on how firstly we are able to breathe out CO2. It is because of our body’s metabolism which turns the carbon present in our body into CO2 before it gets breathed out. (Here’s a nice Quora thread discussing how the carbon is turned into CO2)

How did the carbon get in our body in the first place? Because of the foods we eat.

And what are these foods? Our foods comprise plants and animals.

And as we will all agree, the animals themselves got there because of the plants, which are at the bottom of the food chain.

So, it is fair to say that the CO2 humans breathe out is directly or indirectly because of the plants around us.

How did these plants grow? By actually sucking in CO2 and giving out oxygen.

Voila!

The carbon in the CO2 we breathe out is nothing more than the carbon captured by plants when they took in CO2 during their growth.

Thus, we are only letting out CO2 that was captured earlier, and not adding anything extra.

So, it’s like this:

respiration_carbon_cycle

Pic credit: Skeptical Science

In fact, it is not difficult to see that it is mathematically impossible for us to add any CO2 that was not already captured by plants – I mean, how else did you get the carbon into your body in the first place? (Well, the air we inhale in each breath contains CO2 as well, but it is very small compared to the amount we exhale – about one hundredth).

For those who are curious, if at all, through our metabolism we are only subtracting and not adding to CO2 in the atmosphere. How? Some part of the carbon we have drawn from plants stays as part of our body mass, and this goes into the soil post death if your tradition just buries a dead body instead of cremating it! Thus, in this case, the carbon that was originally in the atmosphere (and through plants or animals had entered your body) has now been buried under the soil!

Sleep well. Breathe really easy.

Some after-thoughts, and interesting links

The same argument made for humans in the post above goes for animals as well. And thank your stars it does, because every year, 70-100 billion land animals are killed for human consumption alone – just imagine if all the CO2 from their breaths actually added up in the atmosphere! (not surprisingly, there’s a wide variation on the number of animals killed every year for human consumption – see here, here and here)

In another post, I had made a similar argument on why burning biomass for energy does not add to global warming.

Not surprisingly, such an intriguing question (“does human breathing result in global warming?” has been discussed at many other places on the Internet. I am providing some of the more interesting links on this topic:

Does our breathing contribute to global warming? - Cleantech Guide

9 comments

  1. To what measnureable extent humans contribute to an altered ratio of OC and CO2 in the atmosphere is impossible to measure but I’m sure that it’s dwarfed by CO2 borne from fossil fuels, but nonethless I don’t think we can or should claim that human respiration from 7 billion people does not increase atmospheric CO2 to some degree.

    1. Dear Barry – sorry abt the late response, had missed your note.

      Which portion of the blog post is yours? I had of course referenced many expert opinions and to the extent possible provided links and references, but to a large extent I had done the research and written this post.

      All the same, would like to know which section you feel is yours and do whatever best is possible to attend to your complaint.

  2. “The carbon in the CO2 we breathe out is nothing more than the carbon captured by plants when they took in CO2 during their growth.”

    If that’s true, can you comment on this:

    “The carbon in the CO2 we release by burning fossil fuel is nothing more than the carbon captured by plants when they took in CO2 during their growth, before decaying to fossil fuel.”

    1. Well, I could comment on it instead. Such carbon was locked out of the cycle for millions of years and is being reintroduced faster than it can be absorbed. That’s a large part why atmospheric CO2 increase in the first place: If output (emissions) roughly equaled “uptake” (into plants), the concentration should be largely static.

      A better question is:
      “The carbon in the CO2 we release by burning plants is nothing more than the carbon captured by said plants when they took in CO2 during their growth”

      Because that carbon is the same carbon as we would have emitted if we had eaten said plant, and then breathed it out. Logically, if us breathing out that carbon is no problem, then burning said plant instead would not be a problem either. But it is – thus the whole “breathing doesn’t matter” argument can be shown to be false.

      The source of emissions do not matter much, what matters is size of output vs the ability of the cycle to absorb carbon. (especially if you also decrease said ability at the same time by burning plants) That’s it. It’s a bit like dieting, if you put in way more stuff than you can burn, you’ll become fat, no matter what (actual) food you eat.

      If you could chop down a forest and eat it all (of course, humans cannot proccess cellulose, as well as some other issues making this pretty impossible) it would have the same CO2 output as burning it, and only take a relatively neglible time longer. (We don’t burn all C input right away, but eventually “all that comes in will come out”)

      This “humans don’t contribute CO2” is clearly political more than scientific. Because if one affirms that human population in itself can contribute to climate change (either directly through breathing or all the “second effects”, after all – it’s humans who also burn fossil fuel for their energy needs etc) this leads to ideas of overpopulation and how such an issue would be tackled. Which carry implications of regulating human breeding, genocide and eugenics in the minds of most people, making the ill effects of a large human population a sensitive issue that few are willing to talk about or even affirm as real.

  3. >> So, it is fair to say that the CO2 humans breathe out is directly or indirectly because of the plants around us. How did these plants grow? By actually sucking in CO2 and giving out oxygen.

    This is a false statement. Plants consume a specific amount of CO2 regardless of how much we breathe out. The author makes a leap of “cause and effect”. If we as humans breath out more, the plants automagically consume more. That is false.

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