4.5: The Greenhouse Effect

Greenhouse Gases

So, how do greenhouse gases affect Earth's temperatures? Greenhouse gases absorb some of the outgoing longwave radiation emitted by the Earth. This increases their temperature, and in turn they emit longwave radiation back towards the Earth, keeping us warm. However, as greenhouse gas concentrations in the atmosphere increase, that effect is amplified. For a more precise estimate, we can consult simplified climate models to investigate this effect. A simplified model is available from the University of Colorado, which you can find on PHET interactive simulations page. This allows us to modify the model to observe how greenhouse gas concentrations in our atmosphere affect our temperature. This includes examining Earth's temperatures prior to the Industrial Revolution! For the rest of this investigation, we'll examine this model and answer the following questions.

• Go to the PHET interactive simulations page.
• On the home screen, select “Waves”. If you select “Photons,” you’ll still get the same results. But don’t select “Layer Model”, since that will be used in a future activity.
• To answer the next three questions, we'll adjust the "Greenhouse Gas Concentration" slider on the right-hand side of the page.

18. Let’s get rid of greenhouse gases altogether. If it’s not already selected, click the “by concentration” radio button in the “Greenhouse Gas Concentration” area to activate the slider. Move it all the way down to “None”. Then select the “Start Sunlight” button. As the model plays, it shows changes in the surface temperature (in Celsius). A Thermometer is shown at the bottom-left of the animation. After about a minute, the temperature will stabilize. This represents the Earth's estimated surface temperature. In a world without any greenhouse gases, the estimated surface temperature of Earth would be approximately:
    1. -18°C
    2. -2°C
    3. 0°C
    4. 4°C

19. Now select the “Reset All” button (at the bottom right of the screen) to refresh the model. Let’s set the greenhouse gas concentrations to the maximum level (“Lots”), select the “Start Sunlight” button, and give the model a minute or two to adjust. The estimated surface temperature of Earth would be approximately:
    1. 25.8°C
    2. 15.1°C
    3. 17.4°C
    4. 31.2°C

20. Now, let’s examine Earth's temperature during the pre-industrial period. First, select the “Reset All” button to refresh the model. There are two buttons at the bottom of the Greenhouse Gas Concentrations area. We have been using “By Concentration,” which has a slider. Now we will select the “Time Period” button, which looks like a calendar. This lets you select greenhouse gas levels for several time periods. Select the button to activate this mode, then choose the year “1750” (if it is not already selected). Select the “Start Sunlight” button to run the model. After a minute of the model playing, the temperature stabilizes at approximately:
    1. 17.4°C
    2. 13.6°C
    3. 14.9°C
    4. 11.2°C

21. Finally, select 2020 and run the model again. As the model is running, the temperature ________ from the 1750 level.
    1. increased
    2. decreased

As you can see in the model, greenhouse gases have a significant impact on the climate. You can play around with the model a little bit to see how changes in cloud cover would affect Earth’s temperature. We will revisit this model in future investigations and delve deeper into climate models.