Climate Forcing and Radiation Balance of Earth

Earth’s climate system includes Oceans, Land surface, Cryosphere (polar ice), Biosphere and Atmosphere. This system evolves in time as per influences of its own internal dynamics. However, external factors also may affect the climate. Any external force / factor that can become a cause of climate change is called Climate Forcing. Such forcing originates from outside the climate system itself. They key examples of Climate Forcing are Albedo (surface reflectivity), Human induced changes in Green House Gases, Atmospheric aerosols due to human activity or volcanic eruption etc.

The peculiar feature of all climate forcing is that they influence the balance of the energy entering and leaving the Earth system.

Radiation Balance of Earth

Earth’s radiation balance is the equation of the incoming and outgoing thermal radiation. The Radiation Balance is shown by the following equation:

Rb= G-R-AE

In the above equation, G is the total incoming solar radiation, while R is the reflected portion of the incoming radiation which is generally known as Albedo. AE is known as Effective Radiation.

Effective Radiation

The radiation which is incoming is generally shortwave, while the surface and atmosphere of earth radiate back the short waves as well as long waves in the infrared spectrum (long wave radiation). This implies that part of the solar radiation is reflected back while part of it is absorbed by earth’s surface which reflects infrared radiation in turn. Out of the total long wave reflected by earth, part is trapped as Green House Effect. Thus,

AE = AO – AG, where

AO = Radiation of the Earth’s surface

AG = Radiation trapped as Green House Effect.

Factors influencing the radiation balance

Internal factors include all mechanisms affecting atmospheric composition (volcanism, biological activity, land use change, human activities etc.).

The main external factor is solar radiation. The sun’s average luminosity changes little over time.

External and internal factors are also closely interconnected. Increased solar radiation for example results in higher average temperatures and higher water vapour content of the atmosphere. Water vapour, a heat trapping gas absorbing infrared radiation emitted by the Earth’s surface, can lead to either higher temperatures through radiation forces or lower temperatures as a result of increased cloud formation and hence increased albedo.

Thus, the radiation balance of the Earth can change by

• By change in the incoming solar radiation (e.g., by changes in earth’s orbit or in the sun itself)
• By change in the fraction of solar radiation that is reflected (called albedo’; e.g., by changes in cloud cover, atmospheric particles or vegetation)
• By alternation in the long wave radiation from earth back towards space (e.g., by changing greenhouse gas concentrations).

Implications of Radiation Balance

The Climate responds directly to changes in the Radiation Balance, as well as indirectly, through a variety of feedback mechanisms.

Climate Feedback mechanisms

There are many feedback mechanisms in the climate system that can either amplify (‘positive feedback’) or diminish (‘negative feedback’) the effects of a change in climate forcing.

For example, as rising concentrations of greenhouse gases warm Earth’s climate, snow and ice begin to melt. This melting reveals darker land and water surfaces that were beneath the snow and ice, and these darker surfaces absorb more of the Sun’s heat, causing more warming, which causes more melting, and so on, in a self reinforcing cycle.

Ice-albedo feedback

This feedback loop, known as the ‘ice-albedo feedback’, amplifies the initial warming caused by rising levels of greenhouse gases. Detecting, understanding and accurately quantifying climate feedbacks have been the focus of a great deal of research by scientists unraveling the complexities of Earth’s climate.