Australia: The Land Where Time Began

A biography of the Australian continent 

Climate Change Science - Radiation Laws Affecting the Earth

Solar radiation comprises by far the majority of the energy that drives the systems of the Earth, with almost every known physical, chemical, and biological cycle on the surface of the Earth being driven by energy from the Sun. The other source of energy that drives geological phenomena such as volcanoes, fumaroles, and geysers, and powers plate movements across the surface of the Earth is derived from the interior of the planet, though with regard to energy affecting systems at and near the surface of the Earth it is solar energy that drives them.

A simple set of rules, the laws of radiation, govern the average properties of the solar radiation, electromagnetic radiation from the Sun, which interact with matter.  These laws apply when the radiating body is what is known as a blackbody radiator.

To determine the effects the solar radiation has on the Earth it is first necessary to determine the amount of solar radiation that impinges on the atmosphere and the surface of the Earth. When atmospheric and climate change scientists are dealing with solar radiation impacting on the Earth there are 3 terms they often use:

·         Irradiance – This is defined as the amount of electromagnetic energy incident on a surface per unit time per unit area. The quantity was previously often referred to as the “flux.” When solar irradiance is measured (by satellite) scientists are measuring the amount of electromagnetic energy that is incident on a surface perpendicular to the incoming radiation at the top of the atmosphere, not the output at the surface of the Sun. Total solar irradiance (TSI) is the amount of solar radiative energy incident on the upper atmosphere of the Earth.

·         Solar Constant – This is the amount of energy that is received at the top of the Earth’s atmosphere on a surface that is perpendicular to the Sun’s rays (at the mean distance of the Earth from the Sun). The generally accepted solar constant of 1,368 W/m² is a yearly average measured by satellites.

·         Insolation – In general, radiation received at the surface of the Earth. The rate of direct solar radiation incident upon a unit horizontal surface at any point on or above the surface of the Earth.

The main problem with the determination of the temperature of the surface of the Earth with calculations made from the top of the atmosphere (TOA) is that clouds and gases in the atmosphere can block up to 70 % of the incoming radiation. The amount of energy actually reaching the surface is calculated and plugged into climate models.

It is also necessary to assume that the surface receiving the radiation is perpendicular to the incoming radiation. This problem results from a number of factors, the rotation of the Earth, the obliquity (the axial tilt), the latitude and surface orientation relative to the solar radiation, and the season.

The angle of the surface that is receiving the radiation is changed by all these factors, which changes the intensity of the received energy.

The assumption that the radiation emitted from the Sun is constant is also a problem, as this value fluctuates with the solar activity cycles. As of June 2012 a long period of a solar minimum, a period of reduced activity, has ended. Since 1978 NASA satellites have measured the incoming radiation and changes have been recorded in the solar irradiance. This data can be accessed from the Goddard Space Flight Center (GSFC):

http://www.nasa.gov/centers/goddard/home/index.html

Solar energy arrives at the Earth as electromagnetic waves, or as an electromagnetic spectrum. Electromagnetic waves travel at the speed of light and consist of the entire range of frequencies and wavelengths at which electromagnetic waves can travel from the smallest, a single atom, to the largest, theoretical infinity.

Solar energy is emitted as electromagnetic energy. The amount of energy can be expressed by the units of Planck’s Law, which describes the electromagnetic radiation that is emitted from a blackbody at absolute temperature T.

The wavelength emitted from an object is dependent on the temperature of that object. This wavelength can be calculated according to Wien’s Law when the temperature of the body is known. This is also known as Wien’s Displacement Law.

The distribution of wavelength of thermal radiation from a blackbody at any given temperature is essentially of the same shape as the distribution at any other temperature, with the exception that each wavelength is displaced on the graph, thus the description as Wien’s Displacement Law.

It is possible to determine the wavelength of maximum radiation emission from the Sun using Wien’s Law by differentiating Planck’s Law.

The peak wavelength of radiation emitted from an object is inversely proportional to the temperature of that object which is determined by the use of Wien’s Law. The Stefan-Boltzmann Law can be used to calculate the radiation output of an object when the temperature is known. The Stefan-Boltzmann Law states that the total energy that is radiated per unit surface area of a blackbody per unit of time is directly proportional to the 4^th power of the thermodynamic temperature of the blackbody, T, also called the absolute temperature.

The decrease in radiation intensity with increasing distance from the radiation source is calculated by use of the Inverse Square Law.

Solar declinations can be found for each hemisphere on NASA’s websites such as the following for the Northern Hemisphere:

http://edmall.gsfc.nasa.gov/inv99Project.Site/Pages/science-briefs/ed-stickler/ed-irradiance.html

The public can access and analyse solar irradiance data that is collected by orbiting satellites and pyranometers that are ground-based, an instrument for measuring solar radiation, as well as making calculations for solar radiation based on physics concepts. Irradiance data from satellites is available from 1978 to the present on Internet.

The NASA satellites that have collected the irradiance data are:

·         Nimbus 7 (Earth Radiation Budget) 1978-1993;

·         Solar Maximum Mission: Active Cavity Radiometer Irradiance Monitor I (ACRIM I) 1980-1989;

·         Earth Radiation Budget Satellite (ERBS) Solar Monitor Measurements 1984-1996;

·         Upper Atmosphere Research Satellite (UARS) ACRIM II Measurements 1991-1997.

The NASA Space Flight Center Data Archive Center and further information related to these satellites and others is made available to the public at:

http://daac.gsfc.nasa.gov/

With an atmosphere the Earth has an average surface temperature of 13^oC. It can be calculated by use of the Stefan-Boltzmann Law the absorptivity required to increase the global average surface temperature by 1^oC, which gives an absorptivity of about 0.02.

Climate change can be seen to be happening, and that global climate is very sensitive to change, whether by the use of experiments and observations, or through calculations. The sooner this is accepted the soon agreements can be reached and emissions reduced and adaptations can begin to be made to live with the changes that are already locked in.

Many still believe the changes are all part of the natural cycle. Even if this were true the link between the climate and the levels of carbon dioxide in the atmosphere can be clearly seen in the climates of the past millions of years. When temperatures were high so were the atmospheric levels of carbon dioxide, and when the global temperatures were low, so were the concentrations of atmospheric carbon dioxide. Lowering the emissions will eventually lead to a halt in the increase of CO₂ in the atmosphere, and eventually lower these levels.

Sources & Further reading

1. Farmer, G. Thomas & Cook, John, 2013, Climate Change Science: A modern Synthesis, The Physical Climate Vol.1, Springer Dordrecht