# Earth's energy flows: sun, wind, water, heat and life: Air and water in motion

Source page: https://www.materialinsights.org/learn/energy/earths-energy-flows/p2/

[Understanding Energy](https://www.materialinsights.org/learn/energy/) / [Module 3: Earth's energy flows: sun, wind, water, heat and life](https://www.materialinsights.org/learn/energy/earths-energy-flows/) Part 2 of 5

## Air and water in motion

Sunlight does not heat the planet evenly. The tropics take in more than they radiate and the poles radiate more than they take in. Air and water carry the surplus towards the poles, and in doing so they create winds, rain, rivers, ocean currents and waves. These are the flows that the Energy table lists as Wind, Water cycle, Ocean currents and Ocean waves, and they are very large next to human demand.

### 2.1 Uneven heating turns the atmosphere into a heat engine, and wind is its small mechanical output

Near the equator the Sun is high all year, so each square metre receives far more energy than it does near the poles. Without transport, the tropics would grow ever hotter and the high latitudes ever colder. Instead, warm air rises over the tropics, flows poleward at height, cools and sinks, and the returning surface flow, bent by Earth's rotation, becomes the trade winds. Further from the equator, great swirling weather systems and the jet streams carry heat the rest of the way. Together the atmosphere and the oceans move roughly 5 to 6 PW of heat poleward across the mid-latitudes, with the atmosphere carrying the larger share.

Wind is the mechanical work that this heat engine produces. Like any heat engine it is inefficient: only around 1% of the sunlight the planet absorbs ends up as the kinetic energy of moving air. Yet that small share is still large: the standard estimate is that winds generate, and lose again to friction and turbulence, kinetic energy at about 900 TW, with climate models giving roughly 840 to 1,060 TW. About half of that is dissipated in the lowest kilometre or so of air. One modelling study put the most that turbines could ever extract at about 400 TW from near-surface winds and about 1,800 TW if every altitude could be tapped, though taking that much would itself change the climate.

Even so, the near-surface wind resource is many times human electricity demand. By the end of 2024 the world had about 1,133 GW of installed wind capacity, the Onshore wind and Offshore wind tiles of the Energy table. The constraint is not the size of the flow but its low power density, its variability from hour to hour and season to season, and the land, steel and grid connections needed to harvest it.

Data Natural energy flows compared with human energy use

| Flow | Approximate power | Multiple of human use (18.8 TW, calculated) |
|---|---|---|
| Sunlight intercepted by Earth | 174,000 TW | about 9,000 |
| Sunlight absorbed at the surface | 86,000 TW | about 4,600 |
| Latent heat of evaporation | 40,000 TW | about 2,000 |
| Poleward heat transport, mid-latitudes | 5,000-6,000 TW | about 300 |
| Kinetic energy of winds (generated and dissipated) | about 900 TW | about 50 |
| Photosynthesis (net primary production) | 100-150 TW (calculated from NPP) | about 5-8 |
| Heat from Earth's interior | 44-47 TW | about 2.5 |
| Tidal energy dissipated | about 3.7 TW | about 0.2 |

Rounded orders of magnitude. Wind from Miller, Gans and Kleidon (2011), after Lorenz and Peixoto and Oort; tides from Egbert and Ray (2000); surface absorption taken as about 168 W/m2 over 5.1 x 10^14 m2. Estimates for photosynthesis vary widely between studies.
 Source: Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008), via Smil Mastery Course, M2 'Air and water in motion'  Worked example

#### The monsoon as a seasonal heat engine

India's monsoon is a vivid case of uneven heating. In late spring the land of the subcontinent and the Tibetan plateau heat up much faster than the Indian Ocean, because rock and soil store heat only near the surface while the ocean mixes heat downwards. Hot air rises over land and draws moist ocean air inland. As that air rises and cools, its vapour condenses and releases latent heat, which strengthens the circulation. In winter the gradient reverses and the winds blow from land to sea.

- Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Air and water in motion'
- Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'Air and water in motion'
- 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
- 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

### 2.2 The water cycle moves more energy than anything except radiation

Evaporation is the single largest use of the solar energy that reaches the surface. About half a million cubic kilometres of water evaporate each year, mostly from the oceans. Turning each kilogram of liquid water into vapour takes about 2.45 MJ, so the latent heat carried upward amounts to about 40,000 TW, roughly half of all the solar energy absorbed by surfaces.

That heat is not lost. When the vapour rises, cools and condenses into cloud droplets, it releases exactly the same energy into the surrounding air. This is the fuel of thunderstorms and tropical cyclones, and it is a major way in which the tropics export heat. A mature hurricane releases latent heat at a rate of hundreds of terawatts, many times all human energy use, which is why storms can do damage on a scale no human machine approaches.

Only a tiny fraction of this flow becomes something we can use as mechanical energy. Rain that falls on high ground has potential energy, and as it runs downhill to the sea it can turn turbines. Global river runoff is about 40,000 cubic kilometres a year. One detailed global assessment puts the gross theoretical potential of that water, if all of it could fall from where it lands to the sea, at about 128 PWh a year, an average flow of roughly 15 TW (calculated). Only about a fifth of that, some 26 PWh or 3 TW, is technically usable, because most rivers drop gently or run where dams would be impractical. The world's hydropower capacity was about 1,283 GW at the end of 2024, which places the Hydropower dams and Run-of-river and small hydro tiles near the top of the renewable list after solar.

So the water cycle is a vast heat-transport system first, a source of mechanical energy only incidentally.
 In the data

#### The heat in a millimetre of rain

One millimetre of rain over one square kilometre is 1,000 cubic metres, or a million kilograms, of water. Condensing that much vapour released about 2.45 TJ of heat into the atmosphere, roughly 680 MWh. A heavy monsoon day with 50 mm of rain over a 10,000 km2 district therefore corresponds to latent heat of about 1.2 million TJ released in the clouds above it, roughly what India's power stations generate in two months.

- Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Air and water in motion'
- Smil, Energies: An Illustrated Guide to the Biosphere and Civilization (1999) · via Smil Mastery Course, M2 'Air and water in motion'
- Smil, Cycles of Life: Civilization and the Biosphere (1997) · via Smil Mastery Course, M2 'Air and water in motion'
- 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
- 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

### 2.3 Oceans store most of the heat, keep it moving, and add waves and tides

Water's capacity to store heat is unusually high, about 4.2 kJ per kilogram per degree, four times that of dry air. Because the oceans are also deep and vast, the top few metres alone hold as much heat as the entire atmosphere. That is why the oceans take up more than 90% of the extra heat trapped by greenhouse gases, and why coastal climates are milder than continental ones.

Ocean circulation comes in two kinds. Surface currents such as the Gulf Stream and the Kuroshio are driven mainly by winds and steered by Earth's rotation and the shape of the basins. Beneath them runs the slow thermohaline circulation, driven by differences in density: water that is cold and salty, for example in the North Atlantic after sea ice forms, sinks and spreads along the ocean floor, to rise again elsewhere centuries later. Both carry heat poleward. The Energy table's Ocean currents tile refers to the kinetic energy of the faster surface flows, which some designs try to tap.

Waves are wind energy handed on to the water. They can travel thousands of kilometres with little loss, so exposed western coasts in high latitudes receive powerful swells. Tides are the odd one out: they are driven by the gravitational pull of the Moon and Sun on a rotating Earth, not by solar heat. Satellite measurements of the sea surface show the tides dissipating about 3.7 TW, roughly three quarters of it by friction in shallow seas and about 1 TW where tidal currents drag over rough deep-ocean floors. That is a modest flow, and only a few places with large tidal ranges, such as the Bay of Fundy, the Severn estuary or the Gulf of Khambhat, concentrate it enough to be worth harvesting.

The Ocean thermal gradient tile rests on yet another stock: in the tropics, surface water can be over 20 C warmer than water a kilometre down, a temperature difference that a heat engine can in principle exploit. Total marine power capacity worldwide, though, was only about 0.5 GW at the end of 2024.
 Case

#### Tides are slowing Earth's spin

The energy dissipated by tides does not come from the Sun. It comes from the rotation of the Earth and the motion of the Moon. Tidal friction acts as a brake: the day lengthens by about two milliseconds per century, and the Moon slowly moves away from Earth by a few centimetres a year. A tidal power station is, in the end, drawing a tiny amount of energy from Earth's rotation. This makes Tides one of the few energy sources on the Energy table that is not ultimately solar or nuclear.

- Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Air and water in motion'
- Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'Air and water in motion'
- Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M2 'Sunlight and Earth's energy balance'
- 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
- 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

### See it in the table
[H₂O Water](https://www.materialinsights.org/commodities/water/)[St Steel](https://www.materialinsights.org/commodities/steel/)[H2O Water cycle](https://www.materialinsights.org/energy/e_hydro/)[Oc Ocean currents](https://www.materialinsights.org/energy/e_currents/)[OT Ocean thermal gradient](https://www.materialinsights.org/energy/e_ocean_thermal/)[WOn Onshore wind](https://www.materialinsights.org/energy/e_onshore_wind/)[WOf Offshore wind](https://www.materialinsights.org/energy/e_offshore_wind/)[HyD Hydropower dams](https://www.materialinsights.org/energy/e_hydro_dams/)[HyR Run-of-river and small hydro](https://www.materialinsights.org/energy/e_small_hydro/)

### Check your understanding

Which process carries the most energy within the climate system after radiation itself?
 Evaporation takes about half the solar energy absorbed at the surface, roughly 40,000 TW, and releases it when vapour condenses.

Why do the oceans absorb most of the extra heat from global warming?
 At about 4.2 kJ per kg per degree and with enormous mass, the oceans can store far more heat than air or land for the same temperature rise.

Which ocean energy flow is not driven, directly or indirectly, by sunlight?
 Tides come from the gravitational pull of the Moon and Sun on a rotating Earth; their energy comes from Earth's rotation and the Moon's orbit.

[← Part 1: Sunlight and the planet's energy budget](https://www.materialinsights.org/learn/energy/earths-energy-flows/p1/)[Mark read · Part 3: The restless Earth: heat from below →](https://www.materialinsights.org/learn/energy/earths-energy-flows/p3/)

## Other sections

- [Module overview](https://www.materialinsights.org/learn/energy/earths-energy-flows/index.md)
- [Sunlight and the planet's energy budget](https://www.materialinsights.org/learn/energy/earths-energy-flows/p1/index.md)
- [The restless Earth: heat from below](https://www.materialinsights.org/learn/energy/earths-energy-flows/p3/index.md)
- [Photosynthesis and the productivity of life](https://www.materialinsights.org/learn/energy/earths-energy-flows/p4/index.md)
- [How animals use energy, and the biosphere as one system](https://www.materialinsights.org/learn/energy/earths-energy-flows/p5/index.md)
- [Key terms, quiz and sources](https://www.materialinsights.org/learn/energy/earths-energy-flows/review/index.md)