Module 04: How Climate Changes


In Module 3, we learned about equilibrium climates in which the rate of energy input from the Sun is balanced by the rate of energy emitted back to space by the Earth. Now we consider what happens when these flows of energy become unbalanced. 

Radiative forcing (measured in Watts per square meter) of climate is defined as the instantaneous net difference between the rate of energy inflow and the rate of energy outflow before the temperature has time to adjust to the imbalance. The response to radiative forcing is the  resulting change in global mean surface temperature (in Kelvin) that is required to restore radiation balance. The strength of this response is called Climate Sensitivity: it’s the ratio of climate response to radiative forcing measured in Kelvin per (Watt per square meter). 

Climate Sensitivity is determined by feedback processes in the Earth’s climate system. Positive feedbacks increase the response to forcing and make the climate more sensitive. Negative feedbacks reduce the response to forcing and make the climate less sensitive. Without any feedback the Earth’s climate would warm about 0.27 Kelvin for each Watt/square meter of radiative forcing. Including feedbacks associated with water vapor, albedo, clouds, and vertical mixing the Earth’s overall climate sensitivity is about 0.8 Kelvin for each Watt/square meter. The radiative forcing of atmospheric CO2 is about 3.7 Watts per square meter per doubling of CO2, so the climate sensitivity can be written as 3 Celsius per doubling of CO2.

Learning Objectives

Upon completion of this module you should be able to: 

  1. Define climate forcing, response, and sensitivity.
  2. Enumerate several kinds of radiative forcing of climate and compare their magnitudes in contemporary climate.
  3. Explain the concept of climate feedback, and give examples of prominent feedback systems in the Earth system.
  4. Compare and contrast the strengths of various climate feedback processes.
  5. Describe two independent methods for estimating Earth’s overall climate sensitivity.

Slide Deck / Lecture

Review Questions

  1. Define the following terms and give their physical units as used in this module:
    • Radiative forcing
    • Climate feedback
    • Climate sensitivity
  2. Give three examples of processes that cause radiative forcing of climate 
  3. For each doubling of CO2, how much radiative forcing of climate do we expect?
  4. Without any feedback, we can calculate that the Earth’s equilibrium climate sensitivity is about how much?
  5. Explain the difference between positive and negative climate feedback and give two examples of each.
  6. Positive feedback involves amplification of climate forcing. Explain how forcing can be amplified by feedback and yet not “run away” to an unsurvivable state.
  7. Explain two independent and commentary ways Earth’s climate sensitivity can be estimated.
  8. Using geologic data on past temperatures, gases, and ice extent, how much radiative forcing of climate during deglaciation was due to changes in albedo? How much was due to changes in the strength of the greenhouse effect?
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