2.7: Surface Energy Balance

Radiative Equilibrium Temperature

The average temperature of a planet depends on the balance between incoming radiation and outgoing radiation. If incoming radiation exceeds outgoing radiation, a planet will continuously warm. If outgoing radiation exceeds incoming radiation, a planet will constantly cool. Eventually, a planet tends toward radiative equilibrium, in which the energy of outgoing radiation equals that of incoming radiation. The average temperature at which this occurs is called the radiative equilibrium temperature.

Figure \(\PageIndex{1}\): Incoming solar energy warms the Earth's surface via shortwave radiation, and the Earth emits all the energy back to space via Infrared (IR) or longwave radiation. Without greenhouse gases (GHGs) in the atmosphere, the Earth's radiative equilibrium temperature would have been -18 °C. However, GHGs in the atmosphere absorb outgoing IR radiation and radiate some of it back toward the surface, increasing the surface temperature to a more comfortable 15 °C. (CC BY 4.0; Neel Desai)

If the Earth didn't have an atmosphere, it would be just a rock in space at approximately 150 million kilometers (93 million miles) from the Sun, as illustrated in Figure \(\PageIndex{1}\). At this distance, half of the Earth will always receive Solar radiation and the other half will be in darkness. Since Earth's temperature is above 0 K, it continuously emits radiation out to space. During the day, the incoming solar radiation is much larger than the outgoing terrestrial radiation, resulting in warming the Earth. However, at night, there is no incoming solar radiation, and outgoing radiation cools the Earth. The combined effect of incoming and outgoing radiation would result in the Earth's radiative equilibrium temperature being about 255 K (-18°C, 0°F). This temperature is much lower than the Earth's observed average surface temperature. This is because the Earth isn't just a rock floating in space. It has an atmosphere and a surface with varying degrees of reflectivity. When considering the Earth's atmosphere, including albedo and the greenhouse gases that are transparent to incoming shortwave radiation but absorb most outgoing longwave radiation, the Earth's Radiative equilibrium temperature increases to 288 K (15°C, 59°F).

Earth's Annual Energy Balance

When solar radiation interacts with the Earth, it is partially absorbed by the Earth’s surface and partially reflected, depending on the albedo of the surface. The interaction of atmospheric gases in Earth’s atmosphere is an important part of its energy balance. Figure \(\PageIndex{2}\) illustrates that clouds reflect some of the incoming solar radiation, the Earth’s surface reflects some, but most is absorbed by the Earth’s surface or the atmosphere.

Figure \(\PageIndex{2}\): On average, 340 watts per square meter of solar energy arrives at the top of the atmosphere. Earth returns an equal amount of energy to space by reflecting some incoming light and by radiating heat. Most solar energy is absorbed at the surface, while most heat is radiated back to space by the atmosphere. (Public Domain; Climate and Earth's Energy Budget via NASA).

The details and percentages from Figure 2.7.2 are provided in Table 2.7.1

Table 2.7.1: A breakdown of Earth's energy budget.

Process / Component	Percentage (%)	Notes
Incoming solar radiation	100	340 W/m² total input
Reflected solar radiation (total)	29	Returned to space
– Reflected by clouds & atmosphere	23	Major reflection component
– Reflected by surface	7	Surface albedo
Absorbed solar radiation (total)	71	Energy retained in Earth system
– Absorbed by atmosphere	23	Includes gases and clouds
– Absorbed by surface	48	Land and oceans
Surface radiation (emitted upward)	117	Infrared emission from surface
Back radiation (toward surface)	100	Re-emitted by greenhouse gases
Emitted by atmosphere	50	Outgoing to space
Emitted by clouds	9	Outgoing to space
Atmospheric window (direct escape)	12	Surface radiation escaping directly
Outgoing heat radiation (total)	71	Balances absorbed solar energy
Evaporation (latent heat transfer)	25	Surface to atmosphere
Convection (sensible heat transfer)	5	Surface to atmosphere

Aerosols, water vapor, and ozone in our atmosphere directly absorb 23% of incoming solar energy. Nearly 7% of the sunlight is reflected from the Earth's surface, and clouds reflect an additional 23%. The surface absorbs the rest. To maintain a constant radiative equilibrium temperature, the Earth must radiate away all the energy it receives. Evaporation and convection account for 25% and 5% of the incoming solar energy transferred from the surface to the atmosphere, respectively. However, if you calculate the energy emitted by the Earth using the Stefan-Boltzmann Law, with an average surface temperature of 288 K, you will find that the Earth's surface emits nearly 117% of the incoming solar energy, of which only 48% is actually absorbed by the surface. Where did this additional energy come from?

The Greenhouse Effect

The greenhouse effect is the result of greenhouse gases in the atmosphere absorbing outgoing infrared radiation from Earth’s surface and re-emitting it. Because Earth’s outgoing longwave radiation is partially absorbed by the atmosphere, this has a warming effect on Earth’s surface, making it warmer than it would be otherwise. While the greenhouse effect has a bad reputation, it has, in fact, made Earth habitable. In Figure \(\PageIndex{2}\), you'll notice that the greenhouse gases in our atmosphere radiate a full 100% of solar energy in the form of back radiation to the surface. This additional energy raises Earth's radiative equilibrium temperature to 288 K, and the Earth's surface must re-emit more than 48% of the absorbed solar radiation back into space.

Over the past 100 to 150 years, atmospheric (\(\ce{CO2}\)) concentration has increased rapidly from 313 parts per million (ppm) in 1960 to 424 ppm in 2024. Human activities such as burning fossil fuels for energy and transportation, cement production, and tropical deforestation are the primary drivers of this rapid increase in atmospheric (\(\ce{CO2}\)) concentrations. This has led to increased absorption of outgoing terrestrial longwave radiation by the atmosphere, resulting in an average warming of 2.62 °F (1.35 °C) since the pre-industrial era (1850-1900). This strengthening of the greenhouse effect due to additional greenhouse gases from human activities is known as the enhanced greenhouse effect. According to the 2017 U.S. Climate Science Special Report, if yearly emissions continue to increase rapidly, as they have since 2000, models project that by the end of this century, global temperature will be at least 5 °F warmer than the 1901-1960 average, and possibly as much as 10.2 °F warmer.