All life needs energy. And some organisms on Earth take up far more energy than what is warranted by their quantity: You and I, and the rest of humanity, are gorging on energy if we include our domestic animals and technology.

This is one of the conclusions of a new Danish-American study, in which the researchers have analysed the flows of energy – first and foremost from the sun – that organisms on Earth absorb and pass on through the food chains.

"Our objective is to calculate the planet’s biological carrying capacity, both for the biosphere as a whole and for the most important marine and terrestrial ecosystems. This could indicate how we can best manage our planet," says Bo Barker Jørgensen, professor of biology at Aarhus University.

He participated in the study together with researchers from NASA and the American universities of Boston, Harvard and Colorado.

3,000 species spread over the tree of life

More specifically, the researchers have studied almost 3,000 species of organisms spread over the entire tree of life and mapped how quickly they each convert nutrients into energy –  known as the metabolic rate – in relation to their weight.

The vast majority of the energy flow comes from the sun. Green plants (including algae) capture 2,800 terawatts of energy from the sun (see fact box). Only one-tenth of this energy is bound in organic matter through photosynthesis, constituting almost all the energy that organisms on Earth have to drive their metabolism.

Facts:
1 terawatt is 1 trillion (1,000,000,000,000) watts.
280 terawatts correspond to hundred times the world's total electricity consumption.

On top of that (or perhaps rather: underneath that), geochemical processes contribute 0.035 terawatts to the metabolism of microorganisms deep underground and on the seabed, where there is neither oxygen nor light.

Our total energy utilisation is enormous

The researchers focus on the 280 terawatts that come from the sun. Of these, plants use 170 terawatts for respiration, and they convert the rest into organic matter, which constitutes the bottom of marine and terrestrial food chains.

For comparison, the researchers converted the data into a common unit of power which they call mass-specific power (MSP): the organism's energy utilisation per time unit (watts) per gram of carbon.

The researchers calculated the average MSP for the almost 3,000 species to be 0.0054 watts per gram. This figure spans huge differences. For example, a hovering hummingbird has a metabolic rate that is a thousand times higher than a microbe on the seabed.

For humans, this figure is 0.012 watts per gram. But when the researchers include the human technological energy utilisation, which is largely based on the carbon cycle, the figure is 18 times higher. And 50 times higher for the US.

For our domestic animals, the figure is 0.039.

... and so is our weight

Those are just the figures for energy utilisation per gram of carbon. When including the total weight of all species, the balance in energy flows is put into perspective: 

• Humans plus domestic animals: 0.2 gigatonnes (1 gigatonne is 1 billion tonnes)
• All wild mammals: 0.003 gigatonnes
• All wild birds: 0.002 gigatonnes

In other words, humans and our domestic animals have a combined biomass that is 40 times higher than all wild mammals and birds, and we use approximately the same share of the energy available to us from the photosynthesis of plants.

"One thing that has shocked me is how the continents are dominated by humans and domestic animals, while wild animals make up a tiny share of the total biomass. For example, our study has shown how we humans interact with the rest of Earth’s biosphere. With our plant and domestic animal production, we have doubled the production of biomass on Earth, while simultaneously reducing the total biomass," says Bo Barker Jørgensen.

The scientific article "The metabolic rate of the biosphere and its components" has been published in the prominent scientific journal PNAS. This is Bo Barker Jørgensen's inauguration article on the occasion of his membership of the prestigious National Academy of Sciences, USA, in 2020. You can read more about that here: https://nat.au.dk/om-fakultetet/nyheder/nyhed/artikel/au-professor-i-eksklusivt-selskab-verdenskendt-akademi-vil-have-bo-barker-joergensen

… and PNAS' own portrait of him here: https://www.pnas.org/doi/10.1073/pnas.2309067120