. Go to the website: http://climatemodels.uchicago.edu/geocarb/ This site runs the GEOCARB model of Earth's carbon cycle. Click on About this model at upper right, read the Overview and How To sections from the menu at left, and watch the short video. In the left side menu, set the Transition CO2 spike to zero. [Later we will use this model to explore the consequences of introducing anthropogenic CO2 spikes] a. Over very long periods of time, the rate of weathering reactions that remove CO2 from the atmosphere balances the amount of CO2 outgassing from volcanoes and deep sea vents. If the CO2 degassing rate (in units of 1012 mol/year) is increased at the transition between the spin-up and transient phase, what happens to the level of atmospheric CO2, and how long does it take to reach a new equilibrium? b. Run the model at a range of degassing rates, and make a table of the equilibrium CO2 concentration as a function of CO2 degassing rate. To do this leave the spinup value alone at 7.5 X 1012 mol/year and vary the value in the simulation. c. The rate of weathering also increases with the level of sunlight. Run a simulation of the Earth as it was 500 million years ago, when the Sun was cooler. How does the steady state CO2 level compare to today's level? Can you explain why? d. Plants affect the amount of CO2 stored in the soil in complex ways. What happens to the atmospheric CO2 level when plants are added to the model? Where does the carbon go? What happens to the rate of chemical weathering when plants are added: does it increase or decrease, and is this affected by the amount of time that the simulation runs?