Summary

Assessment Summary

As before, let’s try to recap some of the main points from this activity.

1. Earth’s climate system is an energy balance system in which the energy absorbed from the Sun is equal to the energy emitted (in the form of infrared radiation) by the Earth.
2. The simple fact that the energy emitted is directly related to the temperature provides a powerful negative feedback mechanism that drives the system to a steady state (where the energy in is equal to the energy out).
3. Although the system has a tendency to find a steady state, changes in the solar input, the albedo, and the greenhouse effect can change the steady state and the system takes a while to adjust and get into the new steady state.
4. The influence of CO₂ on Earth’s climate is a logarithmic one, meaning that for each doubling, the amount of extra heat retained in our system increases by 4 W/m² — so changing the concentration from 200 ppm to 400 ppm has more of an effect than changing it from 400 ppm to 600 ppm.
5. We hypothesize that over the past 1000 years, there are four main forcings that cause our climate to change — solar variability, volcanic eruptions, aerosols (or pollutants), and greenhouse gases. By applying these forcings to our simple climate model, we can see that combined, they provide a fairly good match with the observed temperature record (supporting our hypothesis). Of these forcings, the greenhouse gases are by far the most important in terms of matching the observed temperatures over the past 100 years. Thus, we conclude that the increase in CO₂ over this time period is the primary cause of the warming. None of the other forcings are capable of explaining this warming.