Earth’s Energy Balance: The Facts

A simple explanation of Earth’s energy balance - the balance between incoming solar energy from the Sun and outgoing infrared energy from the surface of the Earth. We look at how the Stefan-Boltzmann Law works, as well as the impact of clouds. The energy balance - the so-called radiative equilibrium - is the critical mechanism that raises or lowers Earth's temperature.
Earth's Energy Balance Diagram
Earth’s energy balance: solar energy received must equal heat energy emitted, to keep temperature stable.

Earth’s energy balance refers to the equilibrium between incoming solar radiation and outgoing infrared radiation from the surface of the Earth. The energy balance has a critical influence on the greenhouse effect and global warming, and thus on the health and well-being of the planet as a whole.

Please note: radiation is simply energy that moves through space from one object to another. Also, in the context of this article, shortwave radiation refers to solar energy, while longwave radiation means energy from Earth. Finally, climate science has not yet established exact values for energy flows in Earth’s climate system, thus all figures cited should be treated as approximations only. Image credit (above): © Rhcastilhos

Earth’s Energy Balance and the First Law of Thermodynamics

The balance between incoming energy from the sun and outgoing energy from the Earth is the key factor that drives our climate system, and keeps the planet habitable. This energy balance is regulated by the “First Law of Thermodynamics”, also called the “Law of Conservation of Energy”, which states: energy may be transferred from one system to another in many forms, but it cannot be created or destroyed. Thus, any energy which is “lost” during one process will be equivalent to the energy “gained” during another.

The energy balance is influenced by Earth’s atmosphere, held in place by gravity.

The Greenhouse Effect

Just under half the energy from the sun (solar energy) passes through the atmosphere and reaches Earth’s surface, where it is absorbed by land, vegetation and water. In accordance with the Stefan-Boltzmann Law and Wien’s Law, some of this energy is re-radiated back into the atmosphere from the Earth’s surface in the form of infrared radiation (longwave radiation).

As this infrared energy rises into the troposphere, most of it is absorbed by certain ‘greenhouse gases‘, such as water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and others, who then re-radiate much of it back down to the surface.

This mechanism, known as the greenhouse effect reduces the amount of heat energy that would normally be lost to space. By doing so, it keeps the average surface temperature of Planet Earth at a cosy around 59°F (15°C), making the planet perfectly habitable for humans. By comparison, the moon has no atmosphere, and its average surface temperature is a chilly 0°F (-18°C). 1

One key point: In general, the more greenhouse gas there is in the atmosphere, the more heat is trapped and the warmer the Earth’s temperature. Unfortunately, current concentrations of atmospheric CO2 are at record levels (around 410 ppm), which is why we’re seeing rising temperatures, enormous bushfires and serious melting of ice sheets in the Arctic and Antarctic.

Scientists believe that the last time CO2 levels reached 400 ppm, was about three million years ago, during the Pliocene Epoch. Then, Arctic surface temperatures were 15-20 degrees Celsius warmer than today’s and the Arctic ocean was ice free. 2 3

How Does Earth’s Energy Balance Work?

Earth’s energy balance describes the balance between the amount of incoming energy from the sun and outgoing energy from the Earth. Over a 12-month period, these flows of radiation to and from Earth must be equal, or else the temperature of Earth will change. This state of balance is known radiative equilibrium.

To make things simple, let’s assume that incoming solar energy amounts to 100 units. This means, for example, that Earth must emit 100 units back into space to maintain a stable temperature.

In order to appreciate how Earth’s climate system balances the overall energy budget, we need to consider how incoming energy equals outgoing energy at three different levels: (a) the surface of the Earth, which receives most of the solar energy; (b) the atmosphere, probably the most complex zone; and (c) Earth’s overall energy budget (surface and atmosphere). At each level, the amount of incoming and outgoing energy must be equal, to maintain a stable temperature.

Energy Balance and Temperature

That said, rising temperature is not always a definitive signal of an energy imbalance. When temperature is static, for instance, it doesn’t necessarily mean that there isn’t an energy imbalance. Both observational studies and computer models show that there is only a weak relationship between Earth’s energy balance and surface temperature, at least over a decade or so. 4

The reason for this is because the climate system can re-arrange ocean heat content to offset the long-term rate of global surface temperature rise over a decade or so. Bear in mind that 93 percent of the excess heat generated by climate change is being absorbed and ‘hidden’ by the ocean. See also: How Do Oceans Influence Climate Change?

Diagram of Earth's energy balance.
Earth’s energy balance, using 100 units of solar energy as a baseline. Essentially 100 percent of the energy that fuels Earth, comes from the sun. The absorbed sunlight drives photosynthesis, fuels transpiration and evaporation, warms the Earth system and melts snow and ice. Image: © National Oceanic and Atmospheric Administration (NOAA). National Weather Service. 5

Incoming Energy from the Sun

When 100 units of shortwave solar energy reach Earth’s atmosphere, 30 units are immediately reflected back into space by clouds (17 units), aerosols (6 units) and the albedo (7 units) of Earth’s surface.

The remaining 70 units enter Earth’s atmosphere, where greenhouse gasses absorb 19 units; clouds absorb 4 units; and 51 units reach the Earth’s surface, where they are absorbed by water, land and vegetation, and converted into heat energy.

Outgoing Energy from Earth

In keeping with the Stefan-Boltzmann Law (which states that energy released by a body is proportional to its temperature), the Earth releases all the energy it receives back into the atmosphere. Some is lost by evaporation, some by convection. In addition, over the course of a full day, the Earth emits 116 units of longwave radiation. (This is because it only receives incoming energy during daylight hours, but radiates energy day and night.) Of these 116 units, 104 units are trapped by greenhouse gases, of which 98 units are re-radiated back to Earth, while 12 units escape into space.

Outgoing Energy from the Atmosphere

The atmosphere, too, maintains an energy balance. As well as re-radiating longwave radiation back to Earth, its greenhouse gases (GHGs) also re-radiate longwave energy from Earth out into space, while its clouds re-radiate 9 units into space.

Earth’s Overall Energy Balance

INCOMING RADIATION (TOTAL = 100 UNITS)
• 100 units of shortwave radiation from the sun.

OUTGOING RADIATION (TOTAL = 100 UNITS)
• 23 units of shortwave radiation reflected back into space by clouds.
• 7 units of shortwave radiation reflected back into space by earth’s surface
• 49 units of longwave radiation from Earth re-radiated by GHGs into space.
• 9 units of longwave radiation from Earth, re-radiated by clouds into space.
• 12 units of longwave radiation emitted from Earth into space.

The Atmosphere’s Energy Balance

INCOMING RADIATION (TOTAL = 156 UNITS)
• 19 units of energy from sun absorbed by GHGs.
• 4 units of energy from sun absorbed by clouds.
• 24 units from Earth via evaporation of water vapor.
• 5 units from Earth via warm air convection.
• 104 units of longwave energy received from Earth.

OUTGOING RADIATION (156 = UNITS)
• 9 units of longwave radiation from Earth re-radiated to space by clouds
• 49 units of longwave radiation from Earth re-radiated to space by GHGs.
• 98 units of longwave radiation re-radiated back to earth by GHGs.

The Earth’s Surface Energy Balance

INCOMING RADIATION (145 = UNITS)
• 47 units of shortwave radiation from the sun.
• 98 units of longwave radiation from GHGs in atmosphere.

OUTGOING RADIATION (145 = UNITS)
• 24 units lost to atmosphere by evaporation.
• 5 units lost to atmosphere by convection.
• 116 units of longwave radiation emitted by the surface.

Global Warming: The Result of an Energy Imbalance

Over the past 120 years, average mean global temperature has risen by 1 degree Celsius. This shows that the global energy balance has been upset and that the planet has been forced to raise its temperature to try and stabilize the situation. Unfortunately, global temperature projections are forecasting a likely gain of 3 degrees Celsius over pre-industrial levels, by the end of the century. The impact of this could be catastrophic for the health and stability of biomes and ecosystems around the globe.

No ecosystem is likely to remain unaffected by this degree of climate change, and the resulting loss of biodiversity could be horrendous. A one degree Celsius rise in temperature caused the loss of 3 billion animals in the recent Australian bushfires. Think what three degrees will do. See also: Why Does Half A Degree Rise in Temperature Make Such a Difference to The Planet?

The effects of global warming on humans are already extremely serious, especially in Asia. The effects on the Antarctic Ice Sheet are also extremely worrying.

A lot depends upon how successful we are in reducing our greenhouse gas emissions, and conserving our main carbon reservoirs such as the Amazon Rainforest and the circumpolar permafrost biome.

References

  1. “Earth’s Energy Budget.” Climate Science Investigations (CSI). []
  2. “Atmospheric CO2 decline during the Pliocene intensification of Northern Hemisphere glaciations.” Gretta Bartoli, et al. Paleoceanography and Paleoclimatology. Volume 26, Issue 4. December 2011. []
  3. “The amplification of Arctic terrestrial surface temperatures by reduced sea-ice extent during the Pliocene.” Ashley P. Ballantyne, et al. Palaeogeography, Palaeoclimatology, Palaeoecology, 2013 []
  4. “Earth’s energy imbalance.” []
  5. “The Earth-Atmosphere Energy Balance.” []
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