Basics of Energy Balance and the Greenhouse Effect
An interactive animation provided below allows you to explore the balance of incoming and outgoing sources of energy within the climate system. A brief tutorial is provided below, first with the short wave component and then the long wave component of the energy budget. (Click image or link below to open the animation in a new window.)
Now explore this tool yourself, at your own pace. It takes some time to absorb all of the information that is contained here. Start with the short wave energy budget. Once you are satisfied that you have got that down, go on to the somewhat more complex long wave energy budget.
Consider how incoming and outcoming energy sources of shortwave and longwave radiation achieve a net balance:
- at the surface
- within the atmosphere
- at the top of the atmosphere
In future lessons, we will examine the greenhouse effect in a more quantitative manner. Note here how the greenhouse effect works qualitatively. It involves the ability of greenhouse gases within the atmosphere to absorb longwave radiation, impeding the escape of the longwave radiation emitted from the surface to outer space.
In our first discussion session at the end of this lesson, you will be asked to speculate on certain aspects of this schematic, and to pose some questions of your own for your classmates to attempt to answer.
Seasonal and Latitudinal Dependence of Energy Balance
Next, let us note that the above picture represents average climate conditions, that is, averaged over the entire Earth's surface, and averaged over time. However, in reality, the incoming distribution of radiation varies in both space and time. We measure the radiation in terms of power (energy per unit time) per unit area, a quantity we term intensity or energy flux, which can be measured in watts per square meter (W/m2).
The dominant spatial variation occurs with latitude. On average, there is roughly 343 W/m2 of incoming short wave solar radiation that is incident on the Earth, averaged over time, and over the Earth surface area. Obviously, there is more incoming solar radiation arriving at the surface near the equator than near the poles. On average, roughly 30%, or about 100 W/m2 of this incident radiation is reflected out to space by clouds and reflective surfaces of the Earth, such as ice and desert sand, leaving roughly 70% of the incoming solar radiation to be absorbed by the Earth's surface. The portion that is reflected by clouds and by the surface also varies substantially with latitude, owing to the latitudinal variations in cloud and ice cover:
Moreover, the distribution of outgoing long wave radiation also varies substantially with latitude:
More terrestrial radiation is emitted from the warmer tropical regions and less emitted from the cold polar regions:
The disparity shown above (Figure 1.8) between the incoming solar radiation that is absorbed at the surface and the outgoing terrestrial radiation emitted from the surface poses a conundrum. As we can see in Figure 1.8, outgoing radiation exceeds incoming radiation near the poles, i.e., there is a deficit of radiation at the surface. Conversely, there is a surplus of incoming radiation near the equator. Should the poles, therefore, continue to cool down and the tropics continue to warm up over time?
Think About It!
Any idea what the solution to this conundrum might be?
Click for answer.
We will explore the details of how this is accomplished a bit later...
It is also worth noting that the incoming solar radiation is not constant in time. As we will see in later lessons, the output of the Sun, the so-called solar constant, can vary by small amounts on timescales of decades and longer. During the Earth's early evolution, billions of year ago, the Sun was probably about 30% less bright than it is today--indeed, explaining how the Earth's climate could have been warm enough to support life back then remains remains somewhat of a challenge, known as the "Faint Young Sun" paradox.
Even more dramatic changes in solar insolation take place on shorter timescales—the diurnal and annual timescale. These changes, however, do not have to do with the net output of the Sun, but rather the distribution of solar insolation over the Earth's surface. This distribution is influenced by the Earth's daily rotation about its axis, which of course leads to night and day, and the annual orbit of the Earth about the Sun, which leads to our seasons. While there is a small component of the seasonality associated with changes in the the Earth-Sun distance during the course of the Earth's annual orbit about the Sun (because of the slightly elliptical nature of the orbit), the primary reason for the seasons is the tilt of Earth's rotation axis relative to the plane defined by the Earth and the Sun, which causes the Northern Hemisphere and Southern Hemisphere to be preferentially oriented either towards or away from the Sun, depending on the time of year.
Check it out for yourself with this animation:
The consequence of all of this, is that amount of short wave radiation received from the Sun at the top of the Earth's atmosphere varies as a function of both time of day and season:
Subtle changes in the Earth's orbital geometry (i.e., changes in the tilt of the axis, the degree of ellipticality of the orbit, and the slow precession of the orbit) are responsible for the coming and going of the ice ages over tens of thousands of years. We will revisit this topic later in the course.