Investigating the Greenhouse Effect and Passive Solar Heating

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We all know a car sitting outside in the sun all day can get dangerously hot inside, but why? What does this have to do with the greenhouse effect?

Big Ideas: 
  • Electromagnetic radiation has a broad spectrum, and different parts of the spectrum behave in different ways.
  • Incoming radiation from the Sun is mostly visible, while outgoing radiation from the Earth is not. When visible light is absorbed, its energy is re-emitted in the form of invisible longwave radiation.
  • Different surfaces absorb and reflect light in different ways. Some are more reflective, while others absorb more light.
  • Molecules like carbon dioxide and water in the Earth's atmosphere block outgoing thermal radiation. Altering the fraction of these compounds in the atmosphere alters the balance between incoming and outgoing radiation.
  • The stability of the Earth's mean temperature depends on the balance between incoming and outgoing radiation.

Purpose

The goal of these experiments is to explore the way materials and surfaces interact with solar radiation. We introduce the idea of the greenhouse effect, and build a small model to illustrate how it works.

Introduction

Our atmosphere is a combination of gases surrounding the Earth that keeps us, plants, and other animals alive on this planet. The Sun, 150 million kilometres away and with a surface temperature of nearly 6000 degrees Celsius, radiates electromagnetic energy that enters our atmosphere.  This energy is mostly in the form of visible and near-infrared light ("shortwave" radiation), which passes through the atmosphere.

When light hits reflective surfaces such as the tops of clouds, arctic ice caps, and white desert sand, it bounces back up into space, and this is what happens to about a third of the incoming radiation. The remainder lands on surfaces that absorb the incoming energy and warm up. These surfaces then re-emit the energy back to space, but in the form of invisible thermal infrared waves ("long-wave" radiation).

The rate at which the Earth loses energy depends on its temperature and the composition of its atmosphere. If the rate of loss equals the rate of absorption, our mean surface temperature (averaged over the year) will remain constant.

This is where the greenhouse effect comes in. Unlike visible light, this re-emitted infrared radiation cannot just pass straight through the atmosphere. Instead, it gets blocked by molecules like carbon dioxide, water, or methane which absorb and then re-emit this radiation in all directions. This causes about half of the infrared energy escaping the earth is sent back down to warm the earth still further. We call the molecules that absorb and re-emit infrared while allowing visible light to pass, "greenhouse gases", because they trap energy inside the atmosphere, like the glass of a garden greenhouse.

This trapping of heat keeps Earth's temperatures warm enough for us to live. However, when we add extra greenhouse gas into the atmosphere, as we have been doing since the industrial revolution, the temperature rises inexorably. In addition, there are "positive feedbacks" which intensify the process. For example, when white glaciers melt because of this heat and are replaced by dark rocks, they are no longer there to reflect the Sun's rays away, and temperature rises further.

The following video by the Pacific Institute for Climate Solutions gives a good visual of the greenhouse effect:

In one of the following experiments, temperatures inside cubes of different materials are measured in the presence of direct sunlight to explore how different surfaces reflect and absorb light. In a related experiment, temperatures are measured inside partially transparent containers with bases made of different surfaces, as a simplified model of the greenhouse function of the Earth's atmosphere;

Materials

  • Pencil/ Pen
  • Ruler
  • Scissors
  • Glue
  • Wet Erase/Transparency Marker
  • Plastic Cups
  • Cardstock or Poster Paper in various brightnesses and/or finishes e.g. white, black; glossy, smooth, matte)
    (Several sheets of construction paper glued together would also work)
  • Transparency paper
  • Screw, Nail, or X-acto Knife (for puncturing small hole in card)
  • Digital Probe Thermometers (We purchased ours from DX, shownhere)
  • Infrared Thermometer (optional) (We purchased ours from DX, shownhere)

Teachers' Toolbox and Tips

  • The experiment is best done in bright, direct sun exposure. Choose a clear, sunny day, and do the experiment around noon. Avoid shaded areas.
  • A wide variety of grade levels can benefit from this experiment. For younger students, simple observation of temperatures prior to and after a certain amount of time in sun exposure may be sufficient, comparing the different types of surfaces and conditions. For older grades, groups can record temperatures over time (e.g. every minute) and plot on a graph. For intermediate grades, teachers can do the plotting to show to the class.
  • Younger students can opt to make only the cups, which require less fine-motor skill work.
  • The coloured cubes can spark a discussion on the concept of radiation (vs. conduction and convection), and especially on how different surfaces absorb, reflect, and transmit light (and other forms of electromagnetic radiation) in different amounts. 
  • This is a great opportunity for students to practice making predictions and testing them- ask, what do they expect to happen? After the experiment, discuss what went as predicted and what was unexpected. What were the factors that could have caused the unexpected results, if any? Older students could think about what they could do to find this out, and what changes to the experiment set-up (material choices, etc.) they would make if they were to do it again.
  • For older students, this can be structured as a competition: challenge them to obtain the highest possible temperature from a probe using only the paper and transparencies provided.
  • NASA's Atmospheric Science Educator Guide (Updated 2005) provides a detailed lesson plan specifically on the effect of greenhouse gases. This is similar to the cup model (II) (vs sheets of paper (III)) experiments below.

 

Steps

  1. Construct the different environments. Options include-
    1. Cubes: Construct cubes with a side length of about 1.5 inches from the different types of cardstock. Measuring the temperature inside while the cube is exposed to sunlight will explore how the different materials of construction reflect and absorb light.
      1. First trace the cube outline on the cardstock or transparency paper, using a ruler as necessary. For transparency paper, a transparency marker is useful and can be erased afterward. Printable templates on the Internet may also work.

      2. Cut out the cube, and fold along edges. Lightly scoring the edges with a blade before folding will help for a cleaner fold on cardstock.

      3. Cut a small slit/ puncture a small hole through one of the cube sides and insert the probe of the thermometer. Glue the tabs together to complete the cube. Repeat for each type of cardstock/ paper used.

    2. Cup model: Attach transparent, plastic cups to a circular base cut out of the different cardstock. The plastic cup acts as a simplified model of the greenhouse function of the Earth's atmosphere.
      1. Trace the base of the cup onto the cardstock and cut out the circle. Cut a small slit/ puncture a small hole in the centre of the circle, and glue the cup onto the circle. (White glue will work.)

      2. When dry, poke the probe of the thermometer through the hole, making sure that the probe can stay upright without touching any of the sides (sits in the air inside the cup).
    3. Sheet: Simply lay the temperature probe onto the different types of cardstock so that the probe is just slightly above the paper (not touching). This will measure the temperature above each of the surfaces in the absence of the greenhouse effect that is modelled by the plastic cup in (II).

  2. Check the accuracy of the probe thermometers: let the probes sit indoors for a few minutes, and check that they have consistent room temperature readings, within 1 degree.
  3. Outside, place the cubes/ cup models/ sheets on a flat surface, preferably through which air can circulate (e.g., a milk crate). Make sure that the temperature probes, cubes, and cup models, stay out of direct sunlight (covered or shaded) until the start of temperature measurements.

  4. Expose the cubes/ cup models/ sheets to even, direct sunlight, and record the temperature readings at regular intervals.* Ensure that the temperature probes are sitting in the air inside the cube/ cup, not touching any surface. Keep a record of the times at which the readings are taken.

    *As a control (to keep track of possible air temperature fluctuations from wind currents, etc.), record the temperature of a probe sitting in the shade at the same time.

    Infrared thermometers can also be used to measure and keep track of the surface temperature (infrared radiation emitted from) of the surface that the cubes/ cup models/ sheets are sitting on, as well as the materials being tested; it may be interesting to record this data as well.

  5. Plot temperature as a function of time (temperature on the y-axis, as the dependent variable, and time elapsed on the x-axis) for each of the different materials and conditions tested. Discuss how fast the temperature rose, any interesting features of the graph (dips in temperatures, irregularities). What kinds of things caused an increase in temperature?

    If the cubes were tested:
    What does a lower temperatures inside the cube mean? A hotter temperature? What does this tell you about how the material reflects or absorbs light?

    If the cups were tested:
    The cup model illustrates the greenhouse function of the Earth's atmosphere.* What does the paper base represent? What does the plastic cup represent? If the plastic cup represents the Earth's atmosphere, in what ways are they similar? How are they different?

    [*This particular model is a small experiment to illustrate a principle (i.e. greenhouse effect). Models can also be more sophisticated, like models that use mathematics to describe the climate system.]

    Compare the results between the cups and the sheets of paper experiments- can you see the difference in temperature readings? If so, what is this difference from? If not, what are some possible reasons why a difference was not found?

    If both the cubes and cup model experiments were carried out, compare the results. For each, what did you find out about each of the materials tested? Are the results consistent? If not, what are some possible reasons why?

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