The greenhouse effect works because the air in the upper atmosphere is colder than the ground, so that absorption and re-emission of IR by greenhouse gases decreases the amount of energy leaving the planet to space. Here we explore the physics responsible for keeping the upper atmosphere cold.

Another property of the real world, missing in our model so far, is that the real world is not everywhere the same temperature, and the heat fluxes to and from space do not necessarily balance at any given time or location. This is because the winds in the atmosphere and the currents in the ocean carry heat around, in general from the hot tropics up to the cold high latitudes.

Feedbacks are loops of cause-and-effect that can either stabilize Earth's climate or amplify future climate changes. There is an exercise in Part II of this class where you solve for a planet's temperature by iteration, and in the process demonstrate a runaway ice albedo feedback that might have led to the Snowball Earth climate state 700 million years ago.

Now we shift gears in a major way — away from climate physics (you now have seen its main ingredients) to the emergent miracle that is the carbon cycle on Earth. Not only is carbon the chemical element of life, it is also the means of storing life's energy. We will look at how carbon cycles through the land, the oceans, and the deep earth, going in and out of the atmosphere -- and how that stabilizes the earth's climate.

On the carbon locked up in fossil fuels and what happens when we burn those fuels. In Part II of this class, you can create a simple but somewhat realistic model of Earth's temperature evolution in the coming decades, in response to the release of CO2 (or in the sudden stop of emissions in a scenario called "The world without us").

You have now seen the ideas behind the forecast for a human impact on Earth's climate. The next question is: Do we see it happening today? It turns out that the "smoking gun" for a human impact on climate is the global average temperature record since about the 1970's. In order to interpret that temperature change, we need to consider it within the context of natural climate changes in Earth's geologic past.

This unit we focus on the potential impacts of continued business-as-usual CO2 emissions. This is also the topic of the Working Group 2 volume of the IPCC reports (the Working Group 1 report is on the scientific basis, which is what we've been studying so far this course). You may find this material distressing, but hang on, because next week we'll go over "Mitigation", which is what it takes to avoid climate change (treated in the Working Group 3 report). Remember that most of the carbon we're worried about is still in the ground, so these impacts are inevitable only if we continue to decide to make them so. In Part II of this class, you can create a simple ice sheet model of your own.

The last unit of the class finds us considering the options for avoiding, or "mitigating," a human impact on Earth's climate. Bottom line: I think it would be a challenge that humankind could beat if we decided to. If there hypothetically were no more coal on Earth, our potential to alter the climate would be much less. Finding energy sources in that world would not be an existential threat would just be a business opportunity. The hard part, in my opinion, is making that decision.