How Does Earth Maintain an Energy Balance?
SIXTY-NINE PERCENT OF INSOLATION received at the outside of the Earth's atmosphere is available for sensible, ground, and latent heating. Ultimately all of this energy must be returned to space as longwave radiation in order to attain a balance between incoming and outgoing radiation. A greater loss to space would cool the global system, and a smaller loss would increase global temperatures. Just as there is a shortwave radiation budget, there is a budget of global outgoing longwave radiation. By interacting with outgoing longwave radiation (OLR), greenhouse gases help maintain Earth's hospitable temperature. Earth would be a very different planet if the greenhouse effect operated in a different manner or if the amount of greenhouse gases was different.
Sensible and Latent Heat Flux from Earth's Surface
1. There are various ways that Earth's surface and atmosphere transfer longwave energy. This page examines losses via the transfer of sensible and latent heat, and the facing page deals with losses through emission of longwave radiation. Read both pages in a counterclockwise order.
2. Of the 100 units of insolation (shortwave) that enter the top of Earth's atmosphere, 31 units are reflected or scattered by the atmosphere directly to space. That leaves 69 units within the surface and atmosphere.
3. Gas molecules, clouds, and various particles absorb 20 units, or 20% of insolation. For now, remember that this energy must be somehow released back into space.
4. The other 49 units of insolation are transmitted through the atmosphere. These units are then absorbed by Earth's surface, including land, oceans, and other bodies of water. These 49 units must somehow also escape the surface, or else the surface would keep heating up indefinitely as the Sun continued to transmit shortwave energy.
5. If 49 units of insolation reach Earth's surface, this is more than twice the amount (20 units) that is absorbed in the atmosphere. An implication of this is that the Sun heats Earth's surface more than it does the atmosphere, and in turn, the surface heats the atmosphere (by 29 units). Warming of the atmosphere from below in this way is one reason why air temperatures generally decrease upward with increasing altitude.
6. As the atmosphere is heated from below, the warmer air near the surface starts to rise upward, inducing convection in the troposphere. Approximately 7 units of energy are transmitted, mostly by convection, to the adjacent air as sensible heat. This flow of energy is called the sensible heat flux.
7. Most of the Earth is covered by ocean, and many land areas include lakes, wetlands, and heavily vegetated regions, so much of the energy reaching the surface goes to latent heat flux (melting ice, evaporating water, and transpiration from plants). Melting of ice only transfers energy between different parts of the surface (ice sheet to sea, for example), so it does not directly impact the atmospheric energy budget — but evaporation does. As the warm air rises convectively, it carries aloft the recently evaporated water vapor into the ever cooler air at higher altitudes. Eventually the moist air cools sufficiently to condense into water drops and form clouds, which then release the latent heat into the atmosphere. Almost half of all the energy reaching the surface of Earth (23 of 49 units) is returned to the atmosphere in this way.
8. The combined contributions of sensible and latent heat carry about 30 of the 49 units of the shortwave radiation stored at the surface into the atmosphere. Note that the numbers on these two pages add up to slightly more than 100 units (100%) because values have been rounded to whole numbers. When carried out with more precise numbers, it all adds up to 100 units.
Longwave Energy Flux From Surface and Atmosphere
9. Of the 49 units of shortwave radiation that reach Earth's surface, 12 units are emitted directly from the surface (water and land) to space as longwave radiation, without significant interactions with the atmosphere. This is possible because of the atmospheric window that allows certain IR wavelengths to radiate upward through the atmosphere with only minimal losses to absorption, reflection, and scattering.
10. At this point, the atmosphere has 50 units of energy — 20 units from insolation initially absorbed by the atmosphere and 30 units it received from the surface via sensible and latent heat flux. This heat energy cannot be transmitted to space directly via conduction or convection because space is essentially a vacuum. Instead the atmosphere releases this energy by emitting longwave radiation in all directions: upward to space, sideways to other parts of the atmosphere, and downward to the surface. There is a back-and-forth exchange of radiant energy between the atmosphere and surface, largely controlled by the greenhouse effect. As a result of these interactions, there is a net flux of 7 units upward. These 7 units join the 50 units already in the atmosphere for a total of 57 units.
11. The 57 units in the atmosphere are eventually emitted to space, in the form of longwave radiation.
12. A total of 69 units of longwave energy go back into space: 57 units emitted by the atmosphere and the 12 units emitted directly from the surface.
13. We started with 100 units of insolation, so these 69 units plus the 31 units of shortwave insolation reflected (the planetary albedo) provide a perfect balance of input and output of energy to and from the Earth's land-ocean-atmosphere system. Keep in mind that these values are average annual values for the globe. Any individual place is unlikely to experience such a balance. Circulation of the atmosphere and oceans transfers energy from places that have an excess relative to the global average to those areas that have a relative deficit.
A World without the Greenhouse Effect
Imagine two Earths, identical in all other ways (solar constant and planetary albedo) except the presence of a greenhouse effect. In the figure shown here, the lower globe has greenhouse gases in its atmosphere, whereas the upper globe does not. From surface and satellite observations, average global surface temperatures on Earth currently are about +15°C (+59°F).
We can estimate the average surface temperature of the imaginary planet using the Stefan-Boltzmann Law, relating the energy emission and temperature. Using this law, the surface temperature of the imaginary world is predicted to be ?18°C (?9°F), significantly colder than the currently observed temperature of Earth.
Some of the difference between these two estimates must be caused by greenhouse gases, which are keeping Earth's current temperatures warmer, thereby allowing water to exist in a liquid state, a key factor in supporting life as we know it. Calculating the actual contribution (in degrees of warming) of greenhouse gases is too complex to pursue here, because such calculations involve many other factors.
From these rough calculations, we can see why there is concern over the impacts of increased concentrations of greenhouse gases due to humanrelated emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). We address the changing concentrations of greenhouse gases and the broader topic of climate change in a later chapter on climate.