Future Food #1: Trophic levels
Food is ultra-processed sunlight
Food is ultra-processed sunlight
At a first principles level, all of the food we eat is stored sunlight, assembled from air and water. Using photons as an energy source, plants absorb CO₂ from the air, strip the carbon atom off and combine it with water to produce stored carbohydrates.
The carbohydrates could be a mix of glucose, cellulose, etc, and there are also proteins1 and fats that we’re ignoring, but the details don’t matter yet.
Virtually all life on Earth is downstream of light energy stored in chemical bonds.
Photosynthesis isn’t very efficient
How efficient are plants at turning light energy into chemical energy? Not very. While the theoretical efficiency of photosynthesis (chemical energy stored per unit of solar energy received) is about 12%, plants have a practical efficiency of less than 1% on an annual, areawide basis2. Some of the most light-efficient crops we farm are maize and sugar beets, which can capture about 0.3% and 0.4% of the light energy that falls on a given patch of land, respectively3. So by the time you eat a plant, more than 99% of the original solar energy has already been lost!
Not all of Earth's surface is suitable for growing biomass, and most biomass is not growing as efficiently as our food crops. Zooming out to the global scale, the Earth's surface receives 3x10^24 J/yr of solar energy, while the net primary production of Earth's plant biomass is 4.2x10^21 J/yr – a mere 0.14% conversion of solar energy to chemical energy (Source).
Trophic levels
Energy efficiency losses start to become a real problem when we move further up the food chain.
I like to think about the food system in terms of trophic levels, which are tiers that indicate how many steps removed a food source is from sunlight. If the sun is L0, primary producers (plants) have a trophic level of L1, herbivores L2, carnivores that eat herbivores L3, carnivores that eat other carnivores L4, and so on4.
Omnivores (e.g., most humans) might eat from many different trophic levels on a daily basis, so a weighted average would put us somewhere between L2 and L3.
As you move to higher trophic levels, energy is inevitably lost as heat due to the fact that organisms do a lot more than just eat and grow. An animal who eats 1 kilocalorie of vegetation doesn't actually gain 1 kilocalorie of edible body mass, since their metabolism is inefficient, and they do lots of other energy-consuming things for survival (or just for fun). The feed conversion efficiency of animals might be 10% under “ideal” (read: highly confined) conditions.
Putting it all together, the fraction of solar energy available at level L is given by:
If we assume an upper bound of 1% for the photosynthetic efficiency of food crops, and a convenient upper bound of 10% for subsequent conversions:
Herbivores have less than 1% of the sun’s energy available to eat
L2 carnivores have less than 0.1%
L3 carnivores have less than 0.01%
Another way to think about this is that each additional trophic level has an order of magnitude less carrying capacity to sustain life5.
The land footprint of food
Because virtually all food energy originates from sunlight hitting a leaf (or microorganism), energy requirements and land requirements are effectively interchangeable. Producing a calorie at one trophic level higher generally requires 10x more land. The “food pyramid” we grew up with is actually an apt metaphor, as you need a very wide area of the base (photosynthesis) to support the increasingly narrow levels above.
We are already running into the practical limitations of inefficient food production, simply because there are a lot of people and finite arable land. Flying over the midwestern United States actually provides a pretty accurate picture of how Earth's surface has been terraformed to capture sunlight and turn it into food. About half of Earth's land is used for agriculture, and that 80% of agricultural land is dedicated to livestock farming.
As the world's population grows, we need to expand the pool of food energy on Earth. If we continue business-as-usual, we'll need more land to produce more calories. That land comes from somewhere, and historically it has cost us valuable carbon storing ecosystems, like grasslands and forests.
Historically, deforestation would have been even worse if not for massive improvements in crop yields. One breakthrough was the invention the Haber-Bosch process for producing nitrogen fertilizer. It is estimated that 5.5 billion people owe their existence to synthetic fertilizers!
The carbon footprint of food
You're probably aware that animal products, like meat, dairy, and eggs, tend to have high carbon footprints. That's not a coincidence, and can be largely explained by trophic levels.
Chickens are one of the most energy-efficient farmed animals, but are only produce about 1 edible calorie for every 10 calories they're fed. Each calorie of chicken inherits the emissions from producing 10 calories of feed, as well as farming the chicken. Cows and other ruminants are even less efficient feed converters, suggesting that they should require a lot of land, which is indeed the case6.
Thermodynamics dictates that livestock must be an order of magnitude more carbon-and land-intensive than the food they eat. Indeed, animal-based foods currently have twice the total GHG emissions as plant-based foods, despite providing 17% of global calories. This implies that the average animal calorie is about an order of magnitude (14x) more carbon-intensive than the average plant calorie.
In 2018, Poore & Nemecek published an important paper which showed that shifting to plant-based diets would cut emission in half, and reduce land use by three-quarters. Reforesting that land, by the way, would remove an estimated 800 gigatons of CO2 from the atmosphere! This is about half of anthropogenic emissions since the industrial revolution.
Another study by Eisen & Brown found that phasing out animal agriculture would effectively pause global warming for the next 30 years.
Eating closer to the sun
What I'm building towards is an amoral, first-principles argument for why food choices matter for climate change.
Because of the thermodynamics of trophic levels, our planet can support roughly an order of magnitude more herbivores (L1) than carnivores (L2). Equivalently, we could support the same number of herbivores using an order of magnitude less land, emissions, and other resources. Luckily, us omnivores get to choose which trophic levels we eat from.
The math of trophic levels implies a few broad strategies for reducing emissions while feeding more people.
Eating lower the food pyramid, where energy is exponentially more abundant
Improving the efficiency of photosynthesis, effectively widening the food pyramid
Reducing food waste, which is responsible for about one quarter of food emissions
Reducing food waste gets a disproportionate amount of attention, because it’s straightforward and not a political third rail like food choices. We should reduce food waste. But let’s not lose sight of the fact that livestock are the largest source of food waste by far. At least 90% of the calories they eat are lost from the human food supply as heat. Counterintuitively, reducing the number of farmed animals leads to a net increase in food, since more plant calories become available for direct human consumption.
We should be very skeptical of any climate solution that promises decarbonization without any material change to how food energy is produced. Examples abound, like carbon neutral milk, regenerative grazing7, and feed supplements for cows. These are incremental improvements on a fundamentally inefficient technology (livestock) that partially help and partially confuse consumers.
Where do alternative proteins fit in?
Eating lower on the food pyramid doesn't mean eating grass, as the straw-man argument often goes. Some of us might voluntarily change our diets, but I imagine most people probably won't have to compromise much at all.
From a thermodynamics perspective, plant-based alternatives for meat, dairy, eggs, etc are an elegant solution that allow us to do more with less. They recombine plant ingredients from L1 to recreate foods from L2, L3, and so on – collapsing all trophic levels down to L1. I’m not under any illusions that we’ve accomplished the price-taste-convenience trifecta needed for widespread adoption, but I’m optimistic for reasons that I’ll cover in future posts.
Outperforming photosynthesis
A plant-based diet is more energy- and land-efficient, but it still runs up against the poor efficiency of photosynthesis (below 1%). Which leads us to the question: can photosynthetic efficiency be improved? Short answer: yes!
In the next post, I'll explore synthetic food, which could be an order of magnitude more resource-efficient than plants.
To build things besides carbohydrates, plants also need nitrogen and some minerals. Some plants sometimes exchange sugar with symbiotic bacteria to get these!
See the supplemental data from this paper. For comparison, a modern solar panel has a theoretical efficiency of ~32%, a ~20% efficiency under ideal conditions, and more like a 5-10% efficiency if you factor in cloudy days, suboptimal sun angles, dust, etc. Even at 5-10% efficiency, solar panels are still about 10x more efficient than most staple crops.
Decomposers like mushrooms make the picture slightly more complicated, since they can recycle energy from higher trophic levels down to lower levels.
Carrying capacity is why you'd probably find far fewer carnivores in a given area of land, and maybe why they tend to be more solitary than herbivores?
Cows and other ruminant animals are infamous for their methane emissions, which is more due to the idiosyncrasies of their digestive biology than a consequence of trophic levels.
Regenerative grazing may promote soil carbon, but proponents usually gloss over the limitations: it requires way more land, and the cows grow more slowly, which means they'll produce more methane in their lifetime. The push for regenerative grazing also implies a false choice between factory-farmed beef and sustainable beef. There is a third option, where we allow intensively grazed pastureland to return to its natural state and eat something else.