Climate Science - The Energy Balance

Weather and climate are ubiquitous. Our constant contact with the atmosphere results in a familiarity that can make the science of the climate seem untenable, obscure, or just plain wrong. The fact is, climate science operates by the same rules as all the other branches of science. Scientific endeavour of all disciplines involves uncertainty, theory and error. These are not unique to climate science but underpin the critical requirements of the scientific method; a testable hypothesis, an measurable experiment and a thorough examination of the results, including details of errors and limitations in the research method chosen.

This series of articles attempts to explain what we currently understand about what drives the Earth's climate and how these processes interrelate, resulting in an incredibly dynamic and complex system. It is not the intention of this information to advocate one way or another on the issue of anthropogenic climate change, rather, by providing knowledge on how the climate system works, we invite the reader to make up their own mind.

The sun's energy is the fundamental driver of our climate. It emits a range of wavelengths from the electromagnetic spectrum, from high energy, short wavelength ultra violet radiation, to lower energy, longer wavelength infra red radiation. We will refer to these as shortwave and longwave radiation respectively.

When an object is hit by radiation, a portion of this radiation is reflected away, and the rest is absorbed. As an object absorbs this energy, it heats up and begins to emit some of its own longwave radiation. In the case of the Earth, as the sun's radiation reaches the Earth, some of this energy is reflected away by the atmosphere, some is absorbed by the atmosphere (heating it, causing it to emit its own longwave radiation), and a little over half of this energy passes through the atmosphere unimpeded to reach the Earth's surface. At the surface, some more radiation is reflected but most is absorbed, resulting in heating and the emission of longwave radiation from the Earth itself.

While the composition of the atmosphere allows for the transmission of a large portion of the shortwave radiation it is exposed to, most longwave radiation tends to be absorbed by the atmosphere. There is a small amount of transmission of longwave radiation through the atmosphere and also the emission of longwave radiation at the atmosphere space boundary, however most of the longwave radiation emitted by the Earth is reflected back to the surface by the atmosphere, a process known as back radiation. This system represents the Earth's energy balance. It moderates the climate relative to the extreme temperature fluctuations outside the atmosphere, and keeps the Earth habitable.