The California judge in the climate change liability trial wants to know if human respiration is a problem for climate change. Humans are breathing out carbon dioxide (CO2), and human population has increased dramatically in just the past century. Is this part of the climate problem? (I wrote about this previously but am now revisiting it.)
Here’s the question that the judge asked:
“In grade school, many of us were taught that humans exhale CO2 but plants absorb CO2 and return oxygen to the air (keeping the carbon for fiber). Is this still valid? If so, why hasn’t plant life turned the higher levels of CO2 back into oxygen? Given the increase in human population on Earth (four billion), is human respiration a contributing factor to the buildup of CO2?”
Well, yes, the grade school analysis IS still correct. Further, given the increase in human population, human respiration IS a contributing factor to the buildup of CO2. But it is not nearly as significant as livestock respiration. The biomass of large animals — mostly livestock — has dramatically increased, to levels not seen in the past 100,000 years or so. Livestock biomass far exceeds that of humans and wild animals combined. And the reason why plant life hasn’t “turned the higher levels of CO2 back into oxygen” is that humans and livestock are devastating plant life on earth. Plant phytomass has declined by about half in the last 2000 years and by about 17% just in the 20th century.
Unfortunately, this is not the answer that some otherwise well-informed climate scientists are giving. The Real Climate site gives this answer (as of March 16):
“The carbon in the exhaled CO2 comes from the food that the animals have eaten, which comes (ultimately) from carbon that plants have taken from the atmosphere during photosynthesis. So respiration is basically carbon neutral (it releases CO2 to the atmosphere that came from the atmosphere very recently).”
This answer is similar, though not identical, to explanations I have seen other respected people making. Al Gore (who presumably is being well advised by some scientists) simply says vaguely that “we get carbon from plants, breathe it out, and they take it in again.” Doug Boucher puts forward a similar argument, saying that “the CO2 that plants take out of the atmosphere, goes back into the atmosphere [through decomposition by microorganisms], whether or not they are eaten by animals.”
All of these explanations fail in approximately the same way. They do not compare the rate of photosynthesis and the rate of respiration. Respiration MAY be carbon-neutral, if the rate at which animals (and microorganisms) exhale CO2 is balanced by the rate at which plants take up CO2. Boucher’s, Gore’s, and Real Climate’s explanations seem to want to say that by definition respiration and photosynthesis will be matched. Whether they are actually matched, though, is an empirical, scientific question, not a question of logic or definitions. The fact that these three answers are all slightly different but that none of them address the question of balance, suggests that these climate scientists have not really thought through this whole issue.
A key part of the carbon cycle is on display here: plants take up CO2, animals give it off. If animals are exhaling CO2 faster than plants are taking it up, then CO2 will start to build up in the atmosphere, resulting in global warming. (It’s also possible that plants would take up CO2 faster than the animals could exhale it, in which case CO2 levels would fall and we would have global cooling.)
We need to be investigating whether this part of the carbon cycle is out of balance. We have dramatically increased the number of cows (and people) during the past 500 years. All of these cows will all consume plant matter and “[release] CO2 to the atmosphere that came from the atmosphere very recently,” but if we keep adding cows, the rate at which CO2 is put back into the atmosphere will go up. To be in balance, we would need to see a corresponding increase in the rate at which CO2 is taken out of the atmosphere by plants.
Common sense would suggest, though, that a different result is entirely possible. As more and more cows are added, we might see a decline in plant matter — a consequence, perhaps, of soil erosion, overgrazing, and cutting down the rainforest to create pasture land. And that is exactly what we observe during the past few centuries: animal life increasing, plant matter decreasing.
The central issue is whether this respiration-photosynthesis process is in balance, so that respiration is balanced by an equal amount of photosynthesis. Will the extra CO2 from billions of extra people and extra cows be taken up by plants? Well, maybe, unless we’ve destroyed the plants. Given the evidence of mass destruction of the natural world, the burden of proof should be on those who say that respiration is “carbon neutral,” not on those who are saying that the cycle is currently tipped against photosynthesis. Tipping this cycle against photosynthesis is what we are doing right now with livestock agriculture.