Climate Change Lesson 5: Climate Forcing and Uncertainty

1. What are the major factors which can influence climate change?

Distribute Changes in the Earth’s Energy Balance student resource. Read together and have students discuss and define radiative forcing and natural and manmade forcings. Reinforce the main points with these questions:

What are three ways to change the radiation balance of the Earth?

1. Change the incoming solar radiation (e.g., by changes in Earth’s orbit or in the Sun.);

2. Change the fraction of solar radiation that is reflected; and

3. Change the long-wave (heat) radiated from Earth back towards space

(e.g., by changing greenhouse gas concentrations).

These destabilizing influences are called climate forcings. Climate, in turn, responds directly and indirectly through a variety of feedback mechanisms, over a range of time spans.

What is radiative forcing? (the change in the balance between solar radiation entering the atmosphere and the Earth’s radiation going out.)

What do we call radiative forcings that tend to cool the surface of the Earth? (negative forcing)

What are some examples of manmade forcings? (particle pollution; deforestation; rising concentration of atmospheric carbon dioxide and other greenhouse gases)

What are some natural climate forcings? (changes in the Sun’s brightness; large volcanic eruptions)

Give students the Radiative Forcing student resource and read the text below the graph. Ask the following questions:

What is the title of the graph? (Radiative Forcing of Climate 1750-2005)

Where are the forcings identified? (The left hand column)

What are they? (Long-lived greenhouse gases; ozone; stratospheric water vapor; surface albedo; total aerosol; linear contrails) 

What are the long-lived greenhouse gases? (Carbon dioxide (CO2), Methane (CH4), Nitrous Oxide (N2O) and Halocarbons)

What does the term albedo mean? (The percentage of solar energy reflected)

What is the difference between stratospheric ozone and tropospheric ozone? (The troposphere is the lower layer of the atmosphere, up to a maximum of about 65,000 feet, and the layer in which weather occurs. The stratosphere is above that.)

What is shown on the horizontal or x axis? (The amount of change measured in watts per square meter in 2005 compared to 1750)

Why is a comparison with 1750 used? (This represents the start of the industrial revolution)

What is the estimate of the impact the change in cloud albedo has had on temperature? (A reduction of about .75 Watts/square meter in the energy impacting climate in 2005 when compared with 1750)

The forcings are divided into two groups, Human Activities and Natural Processes. What is the natural process that is identified? (Change in solar irradiance)

Use the Radiative Forcing graph and the reading to complete the worksheet activity. Review answers as a class.

2. Why is making climate prediction so hard?

The bar graph on forcings also includes estimates of the range of uncertainty. Scientists may be wrong, natural events like volcanoes may disrupt predictions, and there may be mechanisms which people don’t understand, or even know about. In addition, humans make a difference. Policies will change, technology will continue to develop, economic activity will have an impact on fuel consumption, and individual preferences may change.

In addition feedback loops also add to uncertainty.

What forcing is associated with the greatest level of uncertainty? (Albedo, or the reflectivity, largely of clouds)

Why might this factor be hardest to predict? (This factor is hardest to predict because it is a consequences of many other factors. The more earth warms, the more water will be evaporated, creating clouds to reflect sunlight and have a cooling effect.)

Even though there is uncertainty associated with each of the major climate forcings, that uncertainty can be estimated. The forcings diagram indicated that even if cooling is assumed to be at a maximum of the uncertainty range, and warming at the minimum, the earth will still be heating up.

Show Feedback Cycles (transparency master) which illustrates how combinations of factors add complexity through feedback. The schematic shows two possible impacts of a temperature increase.

In the positive feedback cycle, what will happen to the surface temperature? (It will continue to increase)

In the negative feedback cycle, what will happen to the surface temperature? (It will decrease)

What makes the difference between the positive and the negative cycles? (whether increased evaporation from the oceans cause more water vapor in the atmosphere or form more low clouds)

Examples: With an increase in temperature, melting of glaciers and polar ice caps expose darker surfaces underneath that may absorb more solar radiation and contribute to further temperature increase. If there were less snowfall and ice cover in Michigan, there could be a similar effect.

Have students write a response to this question: Why is making climate prediction so hard?

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