The input of energy from the sun and the release of energy (in the form of infrared radiation) by the Earth dominate the temperature dynamics of the planet. At the simplest level, then, understanding how the temperature of the Earth changes over time is just a matter of balancing an energy budget: if the Earth absorbs more energy than it emits, it will warm until it reaches thermal equilibrium. The simplest energy balance models, so-called “zero-dimensional energy balance models,” (ZDEBM) model the Earth and the Sun as point-like objects with particular temperatures, absorption characteristics, and emission characteristics. We can quantify the amount of energy actually reaching any particular region of the Earth (e.g. a piece of land, a layer of the atmosphere, or just the Earth simpliciter for the most basic ZDEBM) in terms of Watts per square meter (Wm-2). The amount of energy reaching a particular point at a given time is called the radiative forcing active on that point. Assuming that the Earth is in equilibrium—that is, assuming that the radiated energy and the absorbed energy are in balance—the simplest possible ZDEBM would look like this:
Here, represents the amount of solar energy input to the system (i.e. absorbed by the Earth), and represents the amount of energy radiated by the Earth. How much solar energy does the
- A very simple model of this sort treats the Earth as an “ideal black body,” and assumes that it reflects no energy. Thus, the model only needs to account for the energy that’s radiated by the Earth, so we can work only in terms of temperature changes. This is an obvious simplification, and the addition of reflection to our model changes things (perhaps even more significantly than we might expect). We’ll discuss this point more in a moment.
- The Intergovernmental Panel on Climate Change (IPCC) uses the term “radiative forcing” somewhat idiosyncratically. Since they are concerned only with possible anthropogenic influences on the climate system, they express radiative forcing values in terms of their deviation from pre-Industrial levels. In other words, their values for the amount of energy reaching certain points on the Earth “subtract out” the influence of factors that they have good reason to think are unrelated to human intervention on the climate. These radiative forcing values might be more properly called net anthropogenic radiative forcing; an IPCC value of (say) .2 Wm-2 represents a net increase of .2 Watts per square meter, over and above the radiative forcing that was already present prior to significant human impacts. Unless otherwise specified, I will use ‘radiative forcing’ in the standard (non-IPCC) sense.