 The model that you're going to use for problems six and seven is this one here. It's the same climate model that we've worked with before, in essence. But here, we're running it for a much longer time from the year 1,000 up until 1998. And over that time, we're applying to the model the best estimates of four different climate forcings, things that can change the climate. One is greenhouse gas concentrations. Another is aerosols. These are fine particulates, essentially pollution in the atmosphere. Then there are volcanic forcing. So whenever there's an eruption, all the particles thrown up into the atmosphere tend to block sunlight and cause cooling. And then there's solar variability. That's another forcing. And that changes over the course of time as the sun gets brighter and dimmer. So each of these forcings has a switch associated with it. And you can turn them. They're in the on position now. You can turn them off here. And we'll just run the model real quickly here. And you'll see two things on this graph. One is in red, the model temperature, so our climate model temperature, about 15 degrees, steady through time. And then in blue is the observed temperature. This is the reconstructed temperature over this time period based on all sorts of studies of different climate proxies. And so the idea is that if we have a relatively good climate model and we apply these four principal forcings to it, we should be able to kind of closely match this observed temperature curve here in blue. So we can turn these things on and see what they do. There's the greenhouse gas concentration. Here's the aerosol concentration combined with greenhouse gases. And now I'm going to combine the volcanic forcing and finally the solar forcing. So we see all four kind of principal climate forcings added in here. One variable here is the ocean depth, the depth of the ocean water that's involved in relatively short-term climate change. And watch what happens if I increase that. Just increase it to 400 and something. I'll run it again and watch what happens to these abrupt little cooling spikes that are associated with volcanic eruptions. And you see they get diminished greatly if the ocean depth is greater. And that's just because these volcanic eruptions are such short-lived forcings that if the ocean that's involved in climate is very deep, it doesn't change much. It doesn't have time to change much because these volcanic events are so short. So that really dampens the cooling effect there. And you've got a much closer match to the curve here. So in these questions, you'll be asked to try out various combinations of these forcings and evaluate the match between the model temperature and the observed temperature here. And there are two questions to answer with respect to this model.