> 1) We know that the Earth was much warmer during the Great Warming Period. We also know that the Earth was much colder during the Ice Age. How do we know that

> 1) We know that the Earth was much warmer during the Great Warming Period. We also know that the Earth was much colder during the Ice Age. How do we know that right now it's because of CO2 and not because of some process by the planet? Incoming solar electromagnetic radiation can be measured. Outgoing electromagnetic radiation can be measured. From the difference we can see that Earth is gaining energy. We can also measure incoming and outgoing radiation at the surface, and at various layers of the atmosphere. We can see that when the surface is heated by absorbing incoming visible and ultraviolet solar radiation and then later re-radiates some of that energy it does so by radiating infrared. We can measure the transparency of atmospheric gases to radiation and see that CO2 is transparent to that incoming visible and ultraviolet but is opaque to the outgoing infrared and hence blocks some energy that would have otherwise been radiated away into space. Increasing the CO2 levels increases how much of the outgoing infrared is blocked. CO2 that comes from sources that are geologically old has a mix of C isotopes different from CO2 that comes from sources that aren't geologically old. From the isotope mix in the atmosphere and how it changed we can tell that almost all of the increase is from geologically old carbon sources. Fossil fuels are geologically old sources. We know reasonably accurately the annual consumption of fossil fuels. The amount of CO2 from them is very close to the total amount of old source CO2 that is being added to the atmosphere yearly. Hence we know that most of the CO2 increase comes from human activity.