Earth's energy flows: sun, wind, water, heat and life: Key terms, quiz and sources

Understanding Energy / Module 3: Earth's energy flows: sun, wind, water, heat and life Wrap-up

Earth's energy flows: sun, wind, water, heat and life: review

Key terms

Solar constant: The sunlight crossing a square metre facing the Sun just outside the atmosphere, about 1,361 W/m2.
Albedo: The share of incoming sunlight a surface or planet reflects; about 30% for Earth as a whole.
Greenhouse effect: Warming of the surface because certain gases absorb outgoing infrared and re-emit part of it downwards; it adds about 33 C to Earth's average temperature.
Earth's energy imbalance: The small excess of absorbed sunlight over outgoing infrared caused by added greenhouse gases, about 1 W/m2 in recent years.
Latent heat: Energy absorbed when water evaporates and released when vapour condenses, about 2.45 MJ per kilogram.
Thermohaline circulation: The slow, deep ocean circulation driven by differences in water temperature and salinity, and so density.
Geothermal heat flow: The heat escaping from Earth's interior, about 44-47 TW in total or roughly 0.09 W/m2 on average.
Plate tectonics: The slow movement of Earth's rigid outer plates, driven by internal heat, which builds mountains and basins.
Photosynthetically active radiation: Light between about 400 and 700 nm that plants can use, about 45% of the energy in sunlight.
Gross primary production: All the carbon and energy fixed by photosynthesis before plants use any for their own respiration.
Net primary production (NPP): Gross photosynthesis minus plant respiration: the new plant matter available to the rest of the biosphere.
C4 photosynthesis: A pathway that concentrates CO2 before fixing it, giving maize, sugarcane and sorghum an advantage in heat and strong light.
Phytomass: The mass of plants, usually expressed as dry matter or as carbon.
Power density: Energy flow per unit of area, in W/m2; for most biomass harvests it is below 1 W/m2.
Basal metabolic rate (BMR): The minimum power an animal needs at rest to stay alive, about 80 W for an adult human.
Kleiber's rule: The observation that basal metabolism scales roughly with body mass to the power of 0.75.
Trophic level: A step in a food chain, such as plants, herbivores or carnivores; roughly 10% of energy passes from one to the next.
Biosphere: The thin planetary layer of life and the cycles of energy and elements it runs, framed as a geological force by Vernadsky.

Try it in the table

Open the Sunlight tile and find the figures for solar energy reaching the surface. Check them against the 340 W/m2 top-of-atmosphere average and the roughly 165-170 W/m2 absorbed at the surface, then compare with the conversion efficiency shown on the Solar photovoltaics tile.
Compare the Wind tile with the Onshore wind and Offshore wind tiles. Note how the natural flow of about 900 TW is described, and how much of it the capture tiles say is practically reachable near the surface.
Go from the Water cycle tile to Hydropower dams and Run-of-river and small hydro. Ask why only about 15 TW of the 40,000 TW moved by evaporation is left as river potential energy, and only about 3 TW of that is technically usable.
Open Earth's heat, then Geothermal power plants, Advanced geothermal and Geothermal direct use and ground-source heat pumps. For each, identify whether it uses concentrated deep heat or shallow ground warmed by the Sun.
Look at the Biomass tile and then Liquid biofuels and Solar fuels and photosynthesis. Find the stated photosynthetic efficiency or power density and compare it with the 1 W/m2 maize example.
Compare Tides with Ocean waves and Ocean thermal gradient. Which of the three is not ultimately powered by the Sun? Then open Tidal and ocean-current power and OTEC and salinity power and see how a global tidal flow of only about 3.7 TW limits what the first can deliver.

Final quiz

About what share of incoming sunlight does Earth reflect straight back to space?

Earth's albedo is about 0.3, mostly from clouds, with ice, snow, deserts and particles contributing.

Without any greenhouse gases, Earth's average surface temperature would be about:

A planet radiating 240 W/m2 without greenhouse trapping sits at about -18 C; the natural greenhouse effect adds about 33 C.

Which of these natural flows is the largest?

Evaporation moves about 40,000 TW, far more than winds (about 900 TW), NPP (100-150 TW) or geothermal heat (about 45 TW).

Global human waste heat is about 0.04 W/m2 and Earth's energy imbalance about 1 W/m2. What follows?

The imbalance from greenhouse gases is about 25 times larger than all heat released by human energy use.

Where does the energy used by a tidal power station ultimately come from?

Tidal friction slows Earth's spin; tides are gravitational, not solar.

What is the best net efficiency with which crops store sunlight as chemical energy, and only briefly?

Peak values approach 4%; season-long crop values are 1-3% and the global average is a few tenths of a percent.

Net primary production is best described as:

NPP is the new plant matter left after plants have paid their own energy costs, available to grow and to feed other organisms.

A 70 kg person's basal metabolism of about 7 MJ a day corresponds to a continuous power of about:

7,000,000 J divided by 86,400 seconds is about 81 W.

Next module: Muscles, wood, water and wind: energy before fossil fuels →

Sources for this module

Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2: all seven units
Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance', 'Photosynthesis', 'How animals use energy'
Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance', 'Air and water in motion', 'The restless Earth: internal heat', 'Photosynthesis', 'The biosphere as a system'
Smil, Energies: An Illustrated Guide to the Biosphere and Civilization (1999) · via Smil Mastery Course, M2 'Air and water in motion', 'How animals use energy'
Smil, Cycles of Life: Civilization and the Biosphere (1997) · via Smil Mastery Course, M2 'Air and water in motion', 'The biosphere as a system'
Smil, Global Catastrophes and Trends: The Next Fifty Years (2008) · via Smil Mastery Course, M2 'The restless Earth: internal heat'
Smil, Biomass Energies: Resources, Links, Constraints (1983) · via Smil Mastery Course, M2 'Photosynthesis'
Smil, Harvesting the Biosphere: What We Have Taken from Nature (2013) · via Smil Mastery Course, M2 'Productivity and the mass of life', 'The biosphere as a system'
Smil, Size: How It Explains the World (2023) · via Smil Mastery Course, M2 'How animals use energy'
Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
Energy Institute, Statistical Review of World Energy 2025 web ↗ · 2024 world primary energy of 592 EJ (about 18.8 TW averaged over the year); https://www.energyinst.org/statistical-review
IPCC, Climate Change 2021: The Physical Science Basis, Working Group I contribution to AR6, Chapter 7 · Earth's energy imbalance of about 0.8 W/m2 for 2006-2018, most of it stored in the ocean; https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-7/
Loeb et al., 'Satellite and ocean data reveal marked increase in Earth's heating rate', Geophysical Research Letters (2021) · imbalance roughly doubled 2005-2019; reported by NOAA Climate.gov (2021), https://www.climate.gov/news-features/feed/joint-nasa-noaa-study-finds-earths-energy-imbalance-has-doubled
Davies and Davies, 'Earth's surface heat flux', Solid Earth 1, 5-24 (2010) · preferred estimate 47 +/- 2 TW; https://se.copernicus.org/articles/1/5/2010/
IRENA, Renewable Capacity Statistics 2025 (Highlights), International Renewable Energy Agency (2025) · end-2024 capacity: solar 1,865 GW, hydropower 1,283 GW, wind 1,133 GW, bioenergy 151 GW, geothermal 15 GW, marine 0.5 GW; https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2025/Mar/IRENA_DAT_RE_Capacity_Highlights_2025.pdf
Bar-On, Phillips and Milo, 'The biomass distribution on Earth', PNAS 115 (2018) · total biomass about 550 Gt C, plants about 450 Gt C, humans about 0.06 Gt C; summary by the Weizmann Institute, https://www.weizmann.ac.il/WeizmannCompass/node/1200
Miller, Gans and Kleidon, 'Estimating maximum global land surface wind power extractability and associated climatic consequences', Earth System Dynamics 2, 1-12 (2011) · about 900 TW of wind kinetic energy generated and dissipated (after Lorenz 1955; Peixoto and Oort 1992), model runs 838-1,064 TW; about half dissipated in the boundary layer (ERA-40: 513 TW); https://esd.copernicus.org/articles/2/1/2011/esd-2-1-2011.pdf
Marvel, Kravitz and Caldeira, 'Geophysical limits to global wind power', Nature Climate Change 3, 118-121 (2013) · maximum extractable: at least 400 TW from near-surface winds, over 1,800 TW from winds at all altitudes; summarised by Lawrence Livermore National Laboratory, https://www.llnl.gov/news/sufficient-wind-energy-available-meet-global-demands-without-damaging-climate
Egbert and Ray, 'Significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data', Nature 405, 775-778 (2000) · total tidal dissipation about 3.7 TW, of which about 1 TW (25-30%) in the deep ocean and the rest in shallow seas; NASA summary, https://svs.gsfc.nasa.gov/stories/topex/press.html
Zhou et al., 'A comprehensive view of global potential for hydro-generated electricity', Energy and Environmental Science 8, 2622-2633 (2015) · gross theoretical potential about 128 PWh/yr, technical about 26 PWh/yr, economic about 21 PWh/yr; https://doi.org/10.1039/C5EE00888C
Whittaker and Likens, 'The biosphere and man', in Lieth and Whittaker (eds), Primary Productivity of the Biosphere, Springer (1975) · table of net primary production by ecosystem type, g dry matter per m2 per year (ranges and means, e.g. tropical rain forest 1,000-3,500, mean 2,200; temperate deciduous forest mean 1,200; cultivated land mean 650; open ocean mean 125; extreme desert mean 3); means also reproduced at https://www.batrachos.com/en/produktyvnist/