From Meteorology to Mitigation: Understanding Global Warming

Overview of the Climate System (part 2)


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.)

"Short Wave" Components of the Energy Budget
© 2003 Prentice Hall, Inc., A Pearson Company
"Long Wave" Components of the Energy Budget
[Please note that there is a slight error in the spoken part of the "Long Wave" video above, beginning around 3:17. While adding up the components of the long wave energy emitted by the atmosphere to space, which total 66 units, the narrator neglects to add in 8 units of direct heat loss to space.]
© 2003 Prentice Hall, Inc., A Pearson Company

Now explore these animations by 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 by clicking the button at the ned of the first animation.

Short wave Energy Budget
© 2003 Prentice Hall, Inc., A Pearson Company

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 shortwave 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:

Latitudinal Distribution of Various Sources of Incomoing and Outgoing Radiation see long description in caption
Figure 1.6: Latitudinal Distribution of Various Sources of Incoming and Outgoing Radiation
Click here to expand a text description
Latitudinal radiation at different latitudes
Radiation (W/m 2) 90ºn 90ºs
rELECTED BY cLOUDS 95 100 95
Credit: Ruddiman, Earth's Climate: Past and Future (W.H. Freeman, 2001)

Moreover, the distribution of outgoing longwave radiation also varies substantially with latitude:

Incoming & outgoing radiation intensity W/m2 between 0-80*N Outgoing starts@ 200 W/m2(0N) & ends @ 175(80N) Incoming starts@ 260 & ends @ 50
Figure 1.7: Net Incoming vs. Outgoing Radiation as a Function of Latitude
Credit: Ruddiman, Earth's Climate: Past and Future (W.H. Freeman, 2001)

More terrestrial radiation is emitted from the warmer tropical regions and less emitted from the cold polar regions:

Annual Mean Temperature shown on world map. Red (hot) at equator, gets colder further away till white (cold) on poles
Figure 1.8: Annual Mean Temperature
Credit: Wikimedia Commons

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.

If you guessed that certain components of the climate system, namely the atmosphere and ocean, help to transport heat from where there is a surplus (low latitudes) to where there is a deficit (higher latitudes), you guessed right!
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:

Earth's Revolution
© 2003 Prentice Hall, Inc., A Pearson Company

The consequence of all of this, is that amount of shortwave radiation received from the Sun at the top of the Earth's atmosphere varies as a function of both time of day and season:

Solar Radiation by latitude & month. North latitudes get more radiation during April-Aug but lower latitudes get more during Sept-Mar
Figure 1.9: Seasonal Distribution of Net Solar Radiation Received at Earth's Surface With Latitude
Credit: Ruddiman, Earth's Climate: Past and Future (W.H. Freeman, 2001)

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.