In an impressive effort to synthesise a data-set, researchers from Israel’s Weizmann Institute of Science and the California Institute of Technology have calculated the mass distribution of every living thing across the kingdoms of life, i.e. a census of the biomass on Earth(1).
Biomass is a useful metric for quantifying stocks of elements “sequestered” in living organisms, allowing for the comparison of taxa whose members are of very different sizes. They report biomass using the mass of carbon. This measure is independent of water content and extensively used in the literature.
Here, we assemble the overall biomass composition of the biosphere, establishing a census of the ≈550 gigatons of carbon (Gt C) of biomass distributed among all of the kingdoms of life. We find that the kingdoms of life concentrate at different locations on the planet; plants (≈450 Gt C, the dominant kingdom) are primarily terrestrial, whereas animals (≈2 Gt C) are mainly marine, and bacteria (≈70 Gt C) and archaea (≈7 Gt C) are predominantly located in deep subsurface environments. We show that terrestrial biomass is about two orders of magnitude higher than marine biomass and estimate a total of ≈6 Gt C of marine biota, doubling the previous estimated quantity.
Interestingly, Earth’s biosphere concentrates on land. While the ocean covers 71% of the Earth’s surface, land biomass, at ≈470 Gt C, is about two orders of magnitude higher than the ≈6 Gt C in marine biomass.
Now, what’s the impact of humanity on the biosphere?
The biomass of humans (≈0.06 Gt C) and the biomass of livestock (≈0.1 Gt C dominated by cattle and pigs) far surpass that of wild mammals, which has a mass of ≈0.007 Gt C. This is also true for wild and domesticated birds, for which the biomass of domesticated poultry (≈0.005 Gt C, dominated by chickens) is about threefold higher than that of wild birds (≈0.002 Gt C). Even though humans and livestock dominate mammalian biomass, they are a small fraction of the ≈2 Gt C of animal biomass, which primarily comprises arthropods (≈1 Gt C), followed by fish
Human activity contributed to the Quaternary Megafauna Extinction between ≈50,000 and ≈3,000 years ago. However, the impact of human civilisation on global biomass has not been limited to mammals but has also profoundly reshaped the total quantity of carbon sequestered by plants. A comparison of actual and potential plant biomass, suggests that the total plant biomass (and, by proxy, the total biomass on Earth) has declined approximately twofold relative to its value before the start of human civilisation. The total biomass of crops cultivated by humans is estimated at ≈10 Gt C, which accounts for only ≈2% of the extant total plant biomass.
My take on this numbers, paraphrasing Feynman, is that there is plenty of room in the carbon space on Earth… I am afraid that the optimistic pro-growth anti-environmentalist faction have a solid case here. If we were able to develop the technology necessary to re-engineer the food chain and carbon cycle so as to get rid of those stupid plants and bacteria, and humans were able to grab and directly control let’s say 25% of current biomass, we would be able to continue multiplying ourselves up to more than 3 Trillion humans on Earth!!!
Let’s be fruitful and multiply! (…I don’t want to imagine football stadiums)
(1) Bar-On, Yinon M., et al. ‘The Biomass Distribution on Earth’. Proceedings of the National Academy of Sciences, 2018, p. 201711842.
Featured Image: NASA The Earth’s Biosphere