The consumption of fossil fuels in 2005 was 7.5 Gt carbon. It has been estimated that 7000 g of ancient biomass was needed to form 1 g of fossil fuel. The current net primary productivity of the Earth is estimated to be 105 Gt carbon per year. This is the average amount of biomass carbon that grows on the Earth per year. The current terrestrial phytomass (plant matter) stores on Earth are estimated to be 500-600 Gt carbon. Using this data, answer the following questions. 

1. Calculate the amount of ancient biomass needed to produce the fuel consumed in 2005. 

2. How many years would it take to produce this amount of biomass on the basis of the current net primary productivity? 

3. How does the quantity calculated in item 1 compare with the current terrestrial phytomass stores? 

4. What does this imply about the possibility of replacing our current rate of fuel use by biofuels?