Measuring life on Earth

According to Atlas Obscura, it’s hard to conduct a census of all of the living stuff scattered across Earth. The planet’s citizens—in the forms of tiny ocean bacteria, galloping mammals, or tangles of terrestrial plants—are found all over the place, from submarine fissures to craggy mountaintops. Many of these residents don’t answer their doorbells or respond to surveys, and they can be difficult to track down: Some spend their lives in realms most humans don’t see up close, and others are far too minuscule to be seen with the naked eye. You’ve got scores of prokaryotic neighbors you’ve never met.

Gauging exactly how much living stuff there is across the planet, and where, is an even trickier business. Most previous work in this vein has been limited to a single taxon—say, for instance, the distribution of microbes in ocean sediment. But researchers from Israel’s Weizmann Institute of Science and the California Institute of Technology recently took a stab at it. Their goal was to calculate the mass and broad-strokes distribution of every living thing across plants, animals, bacteria, and archaea.

To do it, they measured each taxon’s biomass—that is, the weight of all the carbon they contain. Getting there required a bit of contortion. First, they collated hundreds of studies, ranging from field observations to remote data collected by sensors or satellites. (Speaking to The Guardian, the lead author Ron Milo described the process as a “meta-meta-analysis.”) Where this wasn’t available, they estimated—first puzzling out the general population estimate, average weight, and then converting that into suspected biomass, McdVoice survey.

This biomass tactic, the researchers explain, allows them to compare taxa whose members are vastly different sizes (making it possible, for instance, to pit termites against elephants). It doesn’t account for numbers of members, or for the number of species within a particular taxon.