Much of our discussion about climate change begins with temperature. The reason for this is rather simple, namely that surface air temperature is a consequence of the amount of heat energy in the lower atmosphere where we live. To understand what this means and how it relates to the climate changes we now seeing, it is useful to look at the earth’s atmospheric energy budget.
Simply put this budget is an accounting of how the energy that drives winds, ocean currents, the hydrological cycle, and other parts of earth’s climate system flows through the system. In its simplest form, shortwave electromagnetic radiation (visible light) from the sun strikes the earth at the top of the atmosphere. Approximately 30% of this energy is reflected back out into space, while nearly 70% is of this incoming radiation is absorbed by the atmosphere and converted into thermal energy (heat). Eventually this heat energy is radiated out into space in the form of long wave electromagnetic energy (infrared). The trick about the latter step is that some of this long wave radiation is absorbed by greenhouse gasses such as water vapor and carbon dioxide on its way out of the atmosphere. This means that these gasses heat the lower atmosphere to temperatures that are higher than they would be if greenhouse gasses were not present (see Figure 4.1.1).
As long as the earth’s outgoing energy is roughly equal the incoming energy, average global surface temperature remains constant. However, if there is an imbalance then surface temperatures either fall (incoming is less than outgoing) or rise (incoming is greater than outgoing). The latter case describes our current predicament.
RE is radiant or electromagnetic energy which includes visible light, infrared, x-rays, microwaves, gamma rays, and radio waves. The incoming solar on the right is largely visible light – radiant energy having a short wavelength. Approximately 30% of this is reflected from clouds, particulates in the air, and light-colored land surfaces such as snow-covered slopes. The degree of reflectivity of a surface is referred to as its albedo. High albedo (highly reflective) surfaces include snow and cloud tops, while low albedo (low reflectivity) surface include dark ocean, asphalt roadways, and forests.
TE is thermal energy (heat). In the case of the atmospheric energy budget, the incoming RE that is absorbed converted into heat. It is important to note that while temperature and heat are related, they are not the same thing. This is best understood by noting that different substances can possess the same thermal energy but have very different temperatures. The temperature response of a substance to the amount of thermal energy it possesses is called heat capacity. Water having a higher heat capacity than rock and soil is one of the reasons continents tend to be colder in the winter and warmer in the summer than ocean at the same latitude.
ME is mechanical energy which in this case is moving air (winds) and water (ocean currents) ultimately driven by difference in surface temperature. More about this in the Atmosphere / Ocean Circulation activity later in this manual.
CE is chemical energy produced by photosynthesis. Though only a small portion of the absorbed incoming RE is converted into CE by this process, photosynthesis is important in regulating carbon dioxide concentrations. A major determinant in surface air temperature.
The outgoing terrestrial on the left is principally infrared radiation – radiant energy having a longer wavelength. In general, the higher the surface temperature of the earth, the “brighter it glows” in the infrared part of the electromagnetic spectrum.