2-2: Climate Models

Unit Objectives

• To identify the basic principles used to construct climate models.
• To understand the strengths and weaknesses of climate models.

Preparation for Online and Class Discussion

1. Read the summary information.

   Class images

   Related unit images

2. Print for offline review.

3. Take the summary information quiz. Quizzes are available from your portfolio.

4. Additional Related Reading. Although you are not ultimately responsible for content listed in this section, we encourage you to fully explore the links in order to provide a better scope on the upcoming class discussion.

   	How climate models work. From UNEP.
   	Houghton, J. T., L. G. Meira Filho, B. A. Callander N. Harris, A. Kattenberg, and K. Maskell, 1996: Climate Change 1995. The Science of Climate Change. Cambridge University Press, pp 236-245.
   	Houghton, J. T., G. J. Jenkins, and J. J. Ephraums, 1990: Climate Change, The IPCC Scientific Assessment. Cambridge University Press. p. 80-89.

Online and Class Discussion

• Social Comments
• Knowledge-building Comments
• Ethical Comments

• Unit Problem to Ponder
• Unit exam questions from previous years
• Unit discussions from previous years

Unit Wrap-up Materials

• Group Discussion Summary:

  Discussion Summary Unit 2-2 Prepared by: Karen Bandhauer, Deborah Frundle, Krystina Schwartz and Sue Vansice

  Learning Unit:

  In order to understand climate change, we must understand the interactions, which occur in the climate system. Feedback occurs when a change in one climate parameter changes another, which, in turn, causes changes in the initial variable. There is both positive and negative feedback in relation to climate change depending on if they magnify or minimize the parameters effects on climate change. Feedback is incorporated in climate models. The following are examples of feedback:

  • Temperature-radiation feedback: Employs the Stefan-Boltzmann equation. Says that an initial increase in energy leads to an eventual decrease in temperature. This is an example of negative feedback.
  • Water-vapor/greenhouse feedback: As temperature increases the rate of evaporation increases, which increases the amount of water vapor in the air creating an increased absorption of infrared radiation by the atmosphere, contributing further to the greenhouse effect. The increased greenhouse effect in turn increases the surface temperature causing more evaporation. This cyclic effect is a positive feedback.
  • Snow and ice cover / albedo feedback: As the temperature increases, the amount of snow and ice cover decreases, which decreases the albedo effect. This increases the amount of radiation that is absorbed, further raising the temperature and melting more ice, creating a positive feedback.
  • Radiative-dynamic coupling: This relationship has the potential for positive or negative feedback.

  Climate models use the same laws of physics as weather forecasting over longer periods of time; conservation of energy, conservation of momentum, conservation of mass, and an equation of state that describes the relationship of temperature to pressure and density. When using these laws, three separate equations are needed for water, one for water vapor, liquid water, and cloud-ice particles. Physical processes that occur on too small of a scale or are too complicated are approximated by parameterizations in the climate model. Global climate models must determine values for temperature, wind speed, pressure, density, and concentration of water vapor, liquid water, and ice crystals at each the grid points over the earth (around 50,000 to 150,000) at each time step (typically an hour) for the length of the simulation (30-year period, 262,800 time steps). Typically global climate models have horizontal grid spacing of about 500 km (300 miles), they often preclude Florida, and a single surface grid point will represent all of the surface processes in an area larger than Iowa, which can produce limitations for the models on a local scale. Comparisons of models using parameters like surface pressure are useful to gage their accuracy.

  Class Lecture:

  Weather is predicted by taking measurements all over the world. Those measurements are fed into a computer, which uses non linear equations to predict the forecast for the next few days. Weather cannot be predicted more then a few days in advance because the uncertainty would add up and the forecast would no longer be accurate. (Pinball model) When using models you need to respect the uncertainties involved. Models show, for instance, what would happen if Co2 were to double? There are two types of models, equilibrium and transient models. With equilibrium models you set the controls and let the model run until equilibrium is reached. For a transient model the changes are more gradual. You start with one setting and gradually change to what you want to see. The equilibrium model is cheaper and faster to run, but is not very realistic do to the world is never in equilibrium. The transient model is more accurate but takes much more time and money. The IPCC (Intergovernmental Panel and Climate Change) uses weather models and in 1995 released the statement that humans contribute to climate change.

  Online Dialog:

  In the public dialog the question of how greenhouse gases and clouds differ in IR absorption. Throughout the dialog it is determined that greenhouse gases are closer to the Earth than clouds, and that greenhouse gases absorb more energy that higher clouds. This energy absorbed by the greenhouse gases is then held close to the Earth and increases the Earth's temperature. Whereas clouds that are high up in the atmosphere reflect more energy than it absorbs. Since a good portion of the solar energy is reflected back into space, the energy never hits the Earth. Thus, clouds cause a cooling effect. A useful website about this material:

  Climate Change and Greenhouse Gases

  Another topic briefly discussed in the public dialog was the mean sea-level pressure at 30 degrees south and 30 degrees north during specific months of the year. A student suggest that the difference between the latitudes of 30 degrees north and 30 degrees south during the months of December-February may be caused by the Hardy cells at the 30 degrees north and 30 degrees south deserts. But, is still unanswered why there is a difference in the mean sea-level pressure between the 30 degrees north and 30 degrees south latitudes between the months of June-August.