Penn State NASA

Module Summary and Final Tasks


Module Summary and Final Tasks

End of Module Recap: Please go down this list carefully and make sure you understand all of the points below.

  • Earth's climate system is essentially an energy balance system, where the energy in (from sunlight or insolation) is balanced by the energy emitted (energy out) (via heat or infrared radiation).
  • The energy in varies with latitude and season (and also orbital cycles like precession, axial tilt, and eccentricity mentioned in Module 1 regarding the Ice Ages). The energy in also varies as a function of the albedo (fraction of sunlight reflected); ice and snow have high albedo, water has low albedo, and the land surface is in between, varying according to the type and density of vegetation.
  • The energy out depends on temperature and the greenhouse effect, or emissivity, as described by the Stefan-Boltzmann Law.
  • The rate at which different parts of the Earth warm and cool is a function of the heat capacity; a larger heat capacity means that things warm and cool more slowly, and they also store much more heat.
  • The greenhouse effect is not a theory — it is directly measured by satellites and represents a kind of energy recycling mechanism wherein particular gases in the atmosphere absorb heat emitted from the surface and then re-radiate some of that heat back to the surface.
  • Without the greenhouse effect, our planet would be about 33 °C colder!
  • Water, carbon dioxide, and methane are the main greenhouse gases. Water contributes the greatest amount of warming, but the atmosphere is saturated with water, and it cycles through the atmosphere very fast, so it cannot drive climate change (even though it is a very important part of the climate system). Carbon dioxide, on the other hand, is not close to saturation, and it cycles through the atmosphere more slowly, so it can drive climate change. Methane is far less abundant in the atmosphere and is quickly converted to carbon dioxide, so it is less important as a greenhouse gas.
  • Our climate system is filled with feedback mechanisms. Positive feedback mechanisms like the ice-albedo feedback are triggered by a small climate change and then enhance the strength of that climate change. Negative feedbacks like the weathering feedback are triggered by a small climate change and then act to counter that change — these tend to stabilize the climate.
  • If we look at the Earth as a whole, we see that the tropics get more heat than they emit back to space, and the polar regions emit more heat back to space than they get — the balance on a global scale comes about from the transport of heat through the winds and ocean currents.
  • Winds and ocean currents are initiated by density differences that create pressure differences (or gradients). Pressure gradients (change in pressure over a certain distance) drive these flows, but the flows are modified by the Coriolis effect caused by the spinning of the Earth.
  • Finally, we looked at the effects of historical variations in the four major drivers of the Earth’s climate system — the amount of sunlight, volcanoes, greenhouse gases, and pollutants (aerosols) — on a simple climate model. We saw that combined, they make our climate model warm and cool in a pattern that is pretty close to the reconstructed temperature history, and that among these, the greenhouse gas effect is by far the most important factor in the climate change of the last 100 years or so.


You should have read the contents of this module carefully, completed and submitted any labs, the Yellowdig Entry and Reply, and taken the Module Quiz. If you have not done so already, please do so before moving on to the next module. Incomplete assignments will negatively impact your final grade.


  • Lab 3: Climate Modeling.