As everyone knows, Earth receives energy radiating from the sun which warms the planet. But if that were the whole energy story, Earth would get increasingly hotter and hotter. Since Earth has been around for billions of years and is still cool enough for liquid water and life to exist, Earth must have some way of getting rid of excess energy; it does so by also radiating energy into space. Since we call radiation from the sun “sunshine”, I will call energy radiated by Earth “earthshine”. Understanding earthshine is the key to understanding global warming.
Sunshine and earthshine are the same stuff—electromagnetic radiation in the form of waves of varying wavelengths. The average wavelength in sunshine is much shorter than the average wavelength in earthshine. In the scientific literature it is common to refer to sunshine as “short wave radiation” and earthshine as “long wave radiation”.
When Earth’s temperature is stable it is said to be in thermal equilibrium, which occurs when energy absorbed from sunshine equals the energy radiated as earthshine. When less sunshine is absorbed than radiated as earthshine, Earth’s surface temperature decreases. When the reverse is true, Earth’s temperature increases. Notice that temperature increases can occur in two ways—either the energy from sunshine has increased, and/or the energy radiated as earthshine has decreased.
Since the year 1960 or so the amount of energy radiated as earthshine has been decreasing while the energy absorbed from sunshine has remained fairly constant. As a result, Earth’s average surface temperature has been increasing. If one thinks of sunshine as Earth’s heating system, earthshine is Earth’s cooling system. Global warming is occurring because we humans have been messing with the cooling system.
Phenomena which influence Earth’s energy absorption and/or radiation are referred to as “climate forcings”. The science behind global warming seeks to describe these forcings quantitatively, how and why they change, and how they influence Earth’s surface temperature. Climate forcings are discussed in detail later, but first we need a measurement process.
The flow of energy is measured in Watts—an old-fashioned 100 Watt incandescent light bulb needed electric energy to flow through it at a rate of 100 Watts in order to shine at its rated brightness. Unfortunately, when measured in Watts, the rate at which energy is absorbed from sunshine or radiated as earthshine is a large unwieldy number, about 120,000,000,000,000,000 Watts. In order to make this number more user-friendly, climate scientists routinely divide it by the total surface area of Earth measured in square meters. After doing so, the rate at which energy is transferred to and from the Earth is about 240 Watts per square meter of Earth’s surface (Watts/m2). [By multiplying this by Earth’s area, you can arrive back the energy totals, the unwieldy number above with lots of zeros.]
Using this convention, earthshine has decreased by about 2 Watts/m2 since 1960, less than 1%. Nevertheless, Earth’s surface temperature has increased by about 1 °Celsius or nearly 2 °Fahrenheit as our temperature chart shows. [This is the last time I’ll refer to the goofy Fahrenheit temperature scale. If you insist on having temperature changes in °F, multiply the °C number by 1.8.]
Understanding the fact of earthshine is the key to understanding global warming. The amount of energy radiated as earthshine is not small—it is nearly identical to the huge amount of energy we get from the sun. In fact, when Earth cools as it has in the past and will again some day, the energy in earthshine is greater than all the energy we get from the sun.
