 Good morning. We've just crossed an important threshold in Earth history. Global CO2 concentrations now are above 400 parts per million. That is really significant because as we'll learn a lot about in this class, CO2 is a very potent greenhouse gas and it's responsible for the warming and climate that we're seeing. Now you have to go back many millions of years to find CO2 concentrations in the atmosphere as high as they are today. It is really critical to understand what happened in Earth's past because it can inform us about what's going to happen in the future. Now I'm standing here in Aquaria in rural Pennsylvania and these rocks behind me were deposited in a shallow ocean about 450 million years ago and geologists have worked really hard to establish chemicals and fossils that can tell us about conditions in the past. So by studying the chemicals and fossils in rocks such as these, we can learn what's in store for the future of the Earth.