By Sam Clamons
Editor’s Note: This article contains numerical estimates compiled from various research articles. It was reviewed by three climate experts: Casey Handmer, Paul Reginato, and Jonathan Burbaum. Their notes are recorded in the footnotes. Our full dataset and calculations are available for download. We hope this will be a useful starting point for much deeper discussions.
Modern biotechnology is powered by sunlight. Light-gobbling algae, both natural and engineered, are harvested and squeezed for biofuels or dried and pressed to make shoes. We use sugarcane and sugar beets — essentially autonomous, self-replicating, solar-powered biofactories — to mass-produce sugar. That sugar, along with hydrolyzed yeast, form the basic media used to grow genetically engineered E. coli, yeast, and other microbes that make various medicines and foods. At some point in the supply chain, nearly every bioengineered product either is a solar-powered plant or derives its energy from one.
Sunlight is an exquisite energy source. It’s abundant, renewable, and cheap to access. Its consumption produces no greenhouse gasses or toxic byproducts. However, “renewable and abundant” is not the same as “infinitely abundant.” Wildlife, agriculture, and solar electricity generation all use sunlight, too. In principle, all of these processes compete for photons. A joule absorbed to synthesize plastic precursors in an algae can’t also be used to feed a tree or charge a cell phone.
Fortunately, there is a lot of sunlight to go around. While humanity might be forced to make trade-offs between wild plant life and food for humans in the future, it won’t be because we run out of sunlight.
To get a sense of where sunlight goes, we can compare the sunlight inputs to our most sunlight-intensive industries — solar power and agriculture — to the sunlight used by wild organisms, while also accounting for the photons that do nothing but heat the ground or bounce right back into space. These numbers are important enough that scientists around the globe have put effort into estimating them. Just note that these are estimates rather than direct measurements, and they vary between sources. Collectively, these estimates give us an order-of-magnitude picture of Earth’s sunlight budget.
Solar power generation is relatively easy to account for. The Energy Institute’s 2025 Statistical Review of World Energy estimates that humanity’s solar industry currently soaks up sunlight at a rate of approximately 1,200 gigawatts (GW), of which about 240 GW is usefully converted to electricity. If we were to expand our solar capacity to match current global electricity demand at the same conversion efficiency, that absorption would balloon to about 18,000 GW.
What about human agriculture? Figuring out the total amount of sunlight used by humans worldwide is no easy feat, but several groups have attempted it. One somewhat dated but thorough example from 2007 comes from Helmut Haberl and his six co-authors at Klagenfurt University, who use a combination of global crop yields published by the Food and Agriculture Organization and land-use estimates based on satellite imagery. They estimate that human-managed crops, wood, and grazing land together fix about 8 billion metric tons of carbon each year. We can convert carbon mass to stored energy using the energy content of glucose, which comes out to about 1,700 GW. We must also adjust this number upward by 30 percent to account for the increase in agricultural production since 2007.
Different plants convert sunlight to sugar at different efficiencies. A theoretical-best-case estimate comes out to about 10 percent efficiency. This estimate begins with successfully-absorbed photons and ends with final accumulated biomass (for C3 plants, those that use the Calvin cycle for carbon fixation and make up the bulk of plant productivity worldwide) to arrive at 22,000 GW of sunlight absorbed — pretty close to the amount of sunlight required for current global electricity needs.
Determining nature's use of sunlight is more complicated. Natural ecosystems are massively more complex and diverse than agricultural fields, so any estimate of natural photosynthesis will come with large error bars. Nevertheless, there have been rigorous attempts.
Haberl et al. lean on a modified version of a complex model called LPJ-DGVM, which describes flows of carbon and water between plant life (broken down into ten subtypes, such as “tropical broadleaved evergreen tree” and “C3 perennial grass”), the atmosphere, and multiple different kinds of soil, all as a function of climatic variables like sunlight, temperature, snowmelt, and more. Feeding the model detailed satellite data, they estimate total global, wild plant productivity at about 50 billion metric tons of carbon per year, which, using the same methods as before, equates to about 120,000 GW of sunlight absorbed by terrestrial plants.
That estimate doesn’t even include photosynthesis in the ocean, which is itself substantial. Plants, it turns out, don’t account for very much of total marine productivity. Instead, almost all photosynthesis in the ocean is done by a combination of cyanobacteria and single-celled eukaryotic algae like diatoms and dinoflagellates. We don’t have the same kind of census-based models, like LPJ-DGVM, for oceanic plankton that we do for terrestrial plants. Instead, oceanographers measure the sum total of photosynthesis in any particular spot of ocean using one of the following methods: by measuring changes in dissolved oxygen content as a proxy for photosynthesis; by tracing carbon isotopes, using principles similar to carbon dating to estimate carbon absorption rates; and by using fluorescence to estimate concentrations of various kinds of chlorophyll, and by proxy, amounts of photosynthesis.
Marine researchers have taken numerous such measurements all over the world and across seasonal cycles. Several groups have combined all that data, arriving at different estimates of total marine productivity, but they trend around 50 billion metric tons of carbon per year — almost exactly the same as the total productivity of land plants.
By putting all of this together, it is possible to estimate human sunlight usage. On the scale of all productively-harvested sunlight, all human solar use (most of which goes into cultivated crops) adds up to about 11 percent of the energy input to wild photosynthesis. We’re more than a rounding error next to natural photosynthesis, but we’re hardly a competitive rival. Almost all anthropogenic sunlight capture is through plants, though this would change if we substituted solar for most of our existing power sources.
Flow of energy through the major consumers of sunlight. Flow width is measured in gigawatts (GW). “Biomass” is energy put into building new plant (or protist) matter — what can be eaten or harvested. “Respiration” is energy burned to keep the plant (or protist) alive, estimated from biomass measurements.
To keep this number in perspective, though, all of the solar panels, lumber forests, grazing lands, crop fields, wild plants, and oceanic phytoplankton combined only account for about 0.5 percent of all the sunlight absorbed at the Earth’s surface. Both nature and humanity have only just begun tapping into the awesome potential of sunlight.
The vast majority of solar energy is either never taken up by any of Earth’s systems or is simply absorbed as heat. To be clear, “absorbed as heat” is the eventual fate of nearly all sunlight captured anywhere on Earth by anything. Energy captured by a solar panel might charge a battery, but every time that battery is discharged in the course of doing something useful, some (usually most) of that energy is dissipated as heat.
The same is true for a plant — approximately three-quarters of the chemical energy that goes into synthesizing a molecule of glucose is released as heat. That glucose might be burned to power other reactions or might be consumed by another organism, but most of its energy will be dissipated every time it changes hands. The heat energy will slowly bleed out as new photons, mostly in infrared wavelengths, which may be reabsorbed into heat again or may fly off into space, leaving the Earth a little bit colder. Ultimately, every photon absorbed by the Earth is transient, here just long enough to be split into several weaker, higher-entropy ones. A few do something useful for something alive on this journey, but most don’t.
So just how much sunlight is “wasted” in conversion to heat without either feeding an organism or providing utility as electricity?
To begin, roughly half of all sunlight — enough to power all of human civilization sixteen thousand times over — is either absorbed or reflected by the atmosphere. Another 9,000,000 GW (~15 percent of the light reaching the ground) is reflected straight back into space. And, of course, all of this waste pales utterly compared to the 10 billion Earths’ worth of sunlight that misses the planet entirely!
Now, before we begin dreaming up ways to hold on to more of the sun’s energy, we should remember that holding onto more sunlight is exactly how we got global warming. We may do better to figure out how to get more sunlight to reflect back into space, not less. It wouldn’t take much, proportionally speaking — Earth is currently absorbing a net of 0.6 W/m2 out of 340 W/m2 of incoming sunlight, suggesting that we would only need to reflect an extra 0.6/340 = 0.2 percent of sunlight to approximately offset current levels of greenhouse gas warming.
Even with this added consideration, there is a lot of room to improve how we use the light we get. Wildlife may consume 10 times as much sunlight as humanity, but even nature only uses about 0.5 percent of the sunlight absorbed at ground level! That means the vast majority of sunlight — roughly 200 times the total captured by all ecosystems — is absorbed without contributing to photosynthesis. This includes light falling on deserts and snowfields, light absorbed by the ocean before it can reach plankton, and light that slips through forest canopies or grassland cover without being used.
But if there’s so much wasted sunlight bouncing around, why hasn’t life already evolved to take advantage of it? Wild ecosystems are packed full of millions of species of photosynthesizers, each constantly adapting to more efficiently consume the resources at their disposal. Why are they leaving so much energy on the table?
Of all the sunlight that reaches Earth (122.4 million GW), the vast majority is reflected or absorbed by the atmosphere, or reflected back into space. Only about 200,000 GW total is absorbed by terrestrial plants and marine plankton, who use the energy to respire and make biomass.
This is largely because energy isn’t actually what limits most of Earth’s primary producers. Photosynthesis doesn’t just take energy — it also takes water and carbon dioxide. Water limits growth more than sunlight in, for example, deserts. Meanwhile, carbon dioxide is generally plentiful on land — but so is oxygen, which inhibits photosynthesis at high enough concentrations. And even if organisms had infinite access to photosynthesis, life can’t be built from glucose alone. Proteins require elemental nitrogen in a bioavailable form, and DNA and RNA require both nitrogen and phosphorus, both of which are rare relative to CO 2 and water. Iron and silica salts are similarly limiting for marine phytoplankton.
This is how agriculture can be deeply disruptive to natural ecosystems, despite using only a small fraction of the sunlight — light simply isn’t what we’re competing with nature for. Human agriculture currently takes up an estimated 45 percent of the world’s arable land., It needs that land for its water, soil, and nutrients; not for access to limited sunlight.
Solar farms can also compete with nature for land. The most cost-effective forms of solar power have always been large, contiguous utility-scale farms, typically built by leveling large areas and paneling them with silicon collectors, mirrors, or some equivalent light-harvesting machinery. It’s not as disruptive to wildlife as covering the landscape in parking lots and houses, but it’s close.
Fortunately, solar power doesn’t have to be this destructive. For one thing, we have a lot of already-degraded land that we can build solar panels on, including houses, office buildings, malls, roads, and parking lots. Alternatively, solar panels can be used to support plant growth instead of replacing it, in a practice known as agrivoltaics. Many crop plants actually grow better with some shade, and solar panels create cool, moist microclimates that are healthier for many animals — including sheep — than an open field.
We don’t even need to use solar panels to better capture underutilized solar power. Once light has been absorbed as heat, it can still be used. Wind, for example, is largely powered by the temperature gradients produced by mass absorption of sunlight, ultimately making it solar power with extra steps. Wind also supplies most of the energy in ocean waves, so wave power, too, is solar power. Even fossil fuels are just compressed and stockpiled plant matter — hundreds of millions of years’ worth of collected solar energy now being burned off as heat in a geologic instant.
Solar farms already face protests over land use. We should expect to see these intensify as the industry scales up. Agricultural land used for large-scale engineering products (or to grow feedstock for those products) will be in even more direct competition with existing wildlife and with food production. But this won’t happen because we “run out of sunlight.” Instead, competition will be over water, land, fertilizer, or other resources — or because we simply haven’t been clever enough about how we use the ultra-abundant energy at our disposal.
Samuel Clamons is a bioinformatics scientist at Illumina, Inc. with a PhD in Bioengineering and training in applied mathematics and computer science. Outside of his day job, he writes science fiction and researches theoretical questions in biology at Asimov Press.
Cite: Clamons, S. “The Sunlight Budget of Earth.” Asimov Press (2025). https://doi.org/10.62211/39qp-47hf
Header image by Ella Watkins-Dulaney.