When trying to understand what factors could impact long-term temperature trends, scientists must first identify natural variability in the Earth system before attributing any changes to human activity. 1. What natural processes explain the glacial/inter-glacial cycles, as recorded in Antarctic ice cores, over the past 500,000 years and why don't scientists use this to explain the current rate of warming? 2. It is sometimes claimed that the planet has never experienced CO2 concentrations as high as they are now. Is this true? Please explain. 3. CO2 concentrations undergo regular, yearly fluctuations. What causes these fluctuations and explain what time of year we would expect CO2 levels to be the highest and why.