Living Planet. Blue Planet. Blue Planet 2: Deservedly popular, beautiful to watch, natural history series these days promise a view of the big picture. But planetary thinking is harder to do than the film-makers suggest - and that’s true, too, when it comes to considering life as a whole.
Take the basic question: can we quantify life? That could be taken as an invitation to probe biodiversity, an urgent task when we know we are ensuring there will soon be less of it. But there’s a still more basic question. How much living stuff is the Earth supporting? In a word: biomass. How much?
Not easy to say. We can count some big creatures, survey greenery from satellites, sample insects. But piecing together a global view is a complex, painstaking job, full of assumptions and guesstimates. And then there is the other world of the oceans, which is harder to get at, and the many types of microbial life, both on the surface and buried deep.
A new go at all this delivered results the other week that have got a lot of attention. An Israeli-US team published estimates of the biomass on Earth in all the main categories of life. Well, not biomass precisely, but a proxy - carbon content. A defensible choice, but not the only possible one. Most life is mainly water. So we’re not talking about actual tonnage of, say, cows, here.
But their numbers do allow some eye-opening comparisons (take a look at the open access paper) and the findings have got a lot of attention. Expressed as gigatons of carbon, the mass of plants dominates everything else. Then bacteria. Other creatures, especially humans are much less important in these crude terms. True, much of plant matter (stems, tree trunks) is barely alive, but even correcting for that, plants are still the most important form of life.
This is an essential contribution to understanding the planet. But how to represent it? Assessing all life involves an unusual combination of trying to get to grips with things that are not readily visualisable because they are too large - life as a whole on the planetary scale - as well as too small - bacteria, viruses. Viruses, incidentally, aren’t included in the sums, although their total mass is presumably pretty small.
The key summary diagram from the paper looks like this:
This is what’s known as a Voroni diagram, where the area of each section is proportional to the biomass of the category labelled. As the lines indicate, the right-hand square is an expansion of the small corner of the left hand diagram. This both emphasises how relatively unimportant (on this measure) animals are, and draws attention to the relations between humans, wild mammals and livestock.
Much media comment has focussed on the last point. Folk who didn’t have time to get a graphic together mainly used pics of cows, or chickens (farmed fowls account for most bird biomass just as livestock are dominant for mammals.) The Guardian went one better with a diagram of a giant cow (standing for the 60% of mammals that are livestock), towering over a man (36 % of mammalian biomass is human) and a teen-tiny rhino, for the remaining 4%.
Quite effective, that. But how can we relate better to the big picture? Vox went to town on the numbers, with an elaborate but clear diagram using cubes of carbon stacked up - a “tower of life”. Each of their cubes represents a gigaton of carbon, and the plants do indeed tower impressively over everything else - including humans. Take a look here:
To our way of thinking, though, that visual - it’s an elaboration of a bar chart really - lacks something. As we often point out, there are many important quantities that people can’t be expected to have a intuitive feel for. A gigaton of carbon is almost certainly one of them. So while the relative proportions come across clearly in Vox’s diagram, you don’t get any real sense of how much living stuff we are actually talking about.
That’s a comment, not a criticism. Visuals that do one thing well often achieve it at the expense of some other plus point. And we’re not sure how one would depict biomass in more intelligible terms. Our past work gives us lots of ways of indicating what a gigaton of carbon might look like. Below we have visualised a cube of CO2 over New York, with another ten alongside rendered as a heap of coal.
These images show one gigatonne of carbon (one billion tonnes) in the form of carbon dioxide gas (3.6642 GtCO2) and alongside in the form of coal. Real World Visuals (2018)
Not much help? Maybe not. Still it at least gives a start in imagining what 450 gigatons might look like if you smooshed all life together. And we could bring in other, larger contexts for scaling if it turned out Manhattan is too small to be helpful - as it probably would - though they in turn would quickly become less intuitive because unfamiliar.
Then we need to ask: how one could combine this approach with a depiction of the various different kinds of life, and their relative abundance? We’re not sure. Perhaps a single human could contemplate their personal share of the planetary biomass of plants, animals or insects. We’ve done this for trees and grassland in one country, so it seems a logical extension of that approach. But the actual graphic might fall down on the bacteria. A few tons of bacteria is not something one ever sees, but if you did it would just look like a vat of sludge to the unaided eye. Hmm, work still to do here!
Bar-On, Y. Philips, R and Milo, R. (2018) The biomass distribution on Earth. Proceedings of the National Academy of Science [online] 21st May Available from: www.pnas.org/cgi/doi/10.1073/pnas.1711842115
NB; the paper is open access and creative commons licensed, so we may use the diagram. This open access article is distributed under Creative Commons Attribution-NonCommercial- NoDerivatives License 4.0 CC BY Bar-On, Y. Philips, R and Milo, R. (2018) YNC-ND.
Guardian (2018) Humans just 0.01% of all life but have destroyed 83% of wild mammals – study [online] 21st May. Available from: https://www.theguardian.com/environment/2018/may/21/human-race-just-001-of-all-life-but-has-destroyed-over-80-of-wild-mammals-study
Resnick, B & Zarracina, J (2018) All life on Earth, in one staggering chart. Vox [online] 29th May. Available from: https://www.vox.com/science-and-health/2018/5/29/17386112/all-life-on-earth-chart-weight-plants-animals-pnas