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.
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:
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:
- How much CO2 is emitted by human breathing? – many, many interesting discussions in the comments section.
- A nice video on respiration – how little we know about the most common thing we do every day!
- Does breathing contribute to CO2 buildup in the atmosphere? – another lively discussion at ClimateSight.
- Trends in global CO2 emissions – for those interesting in knowing where the 35 billion tons of CO2 are coming from