Global Energy Balance: the Greenhouse Effect Learning Objectives

U N I T 2

Global Energy Balance: The Greenhouse Effect

Learning Objectives

 Know how Earth is warmed and cooled.  Know what happens to the infrared radiation emitted by Earth’s surface.  Know what greenhouse gases are.  Know what the most abundant gases in Earth’s atmosphere are.  Know how much greenhouse gases warm the Earth’s surface.  Understand how the greenhouse effect is important to support life on the Earth.  Know which of the atmospheric gases contribute to the greenhouse effect and how they make this contribution.  Know what the characteristics of greenhouse gases are.  Know the different layers of Earth’s lower atmosphere.  Know the different mechanisms by which heat energy can be transferred.

Review Questions

1.) How are the wavelength and frequency of an electromagnetic wave related?

An electromagnetic wave can be described by: ln = c . Thus, the longer the wavelength of an electromagnetic wave, the lower must be its frequency, and vice versa.

2.) What is a photon?

A single “particle” or pulse, of electromagnetic radiation is referred to as a photon.

3.) What is the major contributor to Earth’s albedo?

Clouds

4.) What are the three most abundant gases in Earth’s atmosphere?

Nitrogen, N2 Oxygen, O2 Argon, Ar

16 5.) List the two lower layers of Earth’s atmosphere. How are they defined?

Troposphere, stratosphere. They are defined by the inflection points (maximum and minimum) of the vertical temperature profile. The troposphere extends from the surface up to 10 to 15 km (higher in the tropics, lower near the poles). The stratosphere, which is located from about 10 to 15 km to 50 km above the surface, where temperature increases with altitude. Above the stratosphere, temperature decreases with altitude in the mesosphere (from about 50km to 90 km) and then increases once again in the uppermost layer, the thermosphere (above about 90 km).

6.) Name three mechanisms by which heat energy can be transferred. Which two are important in Earth’s global energy budget?

Heat energy can be transferred by three different mechanisms: convection, conduction and radiation. Convection and radiation are important in Earth’s global energy budget.

7.) Identify two physical processes by which gases can absorb infrared radiation. Give examples of each process.

1- One way is by changing the rate at which the molecules rotate. If molecules absorb the photon, the molecule’s rotation rate increases. The H2O pure rotation band extends all the way from 12 m into the microwave region of the electromagnetic spectrum.

2- A second way in which molecules can absorb or emit IR radiation is by changing the amplitude with which they vibrate. If the frequency at which the molecule vibrates matches the frequency of the wave, the molecules can

absorb a photon and begin to vibrate more vigorously. The CO2 15-m band is one of the fundamental vibration modes of the CO2 molecule.

8.) Why are O2 and N2 not greenhouse gases?

O2 and N2 are poor absorbers of IR radiation and, thus, are not considered greenhouse gases. As O2 and N2 are totally symmetric molecules, electromagnetic waves pass by such molecules without being absorbed.

9.) Describe the different ways in which climate is affected by high and low clouds.

Low, thick clouds, such as stratus clouds, generally cool the surface because their primary influence is to reflect incoming solar radiation. High, thin clouds, such as cirrus clouds, tend to warm the surface because they contribute more to the greenhouse effect than to the planetary albedo.

17 10.) Identify two positive feedback loops in Earth’s climates system. Why is Earth’s climate stable despite these destabilizing, positive feedbacks?

The water vapor feedback is a positive feedback that tends to amplify small temperature perturbations. The ice albedo feedback is another positive feedback that tends to amplify changes to the Earth’s surface temperature. Earth’s climate is stable because of the negative feedback between the surface temperature and the flux of outgoing infrared radiation.

Websites: http://yosemite.epa.gov/oar/globalwarming.nsf/content/Emissions.html http://gaw.kishou.go.jp/wdcgg.html http://www.ncdc.noaa.gov/oa/climate/globalwarming.html http://icp.giss.nasa.gov/research/methane/greenhouse.html http://www.seeds2lrn.com/greenIndex.html http://www.rmi.org/sitepages/pid340.php http://www.agu.org/eos_elec/99148e.html (Vol. 80, No. 39, September 28, 1999, p. 453)