Global Change, Fall 2010 Problem Set #5: Forest-growth Offsets of Carbon footprints Due: October 7, 2010 Based on the results of homework #4, it appears that the carbon footprint of the average global change student is about 8 metric tons of CO2 per year (note: this is the mass of CO2, not mass of carbon. Each ton of CO2 contains 12/44 = 0.27 tons of carbon). (This is an average across carbon calculators, and there was actually a wide range of answers, depending on which carbon calculator was used). Let’s investigate the potential of growing new forests to offset the carbon footprint of the Global Change class, and what factors might limit our ability to do this at a large scale. 1. (2 points) Mature forests contain 200 metric tons of biomass per hectare. Assuming biomass is half carbon, what area would need to be set aside for new forest for each year of carbon emissions by the average Global Change student? What area would need to be set aside to offset a lifetime of carbon emissions? (hint: if, in this problem and the ones below, you consistently use and write down all your units throughout the problem, following the "multiply by one" rule of units conversion we discussed in the energy lecture on 9/14/10, this will give a good independent check on whether you are doing the problem correctly. For this problem 1, you want to end up in units of hectares per person per year, or just hectares per person. If you instead end up, for example, with persons per hectare, you know your number is likely wrong too!) 2. (2 points) Plants require nitrogen to grow. If, as in the example problem in the lecture on 9/30/2010, the C:N ratio of trees is on average 200, what is the annual nitrogen demand (in kg N/person/year) for growing this carbon offset crop? (2 points) 3. (4 points) As mentioned in the Biogeochemistry lecture #2, anthropogenic emissions of the greenhouse gas N2O arise partly from application of nitrogen fertilizer. Consider these facts: • N taken up by plants ≈ 40% of applied N (Galloway et al., 2004) • N emitted as N2O ≈ 10% of applied N (Crutzen et al. 2007) • Molecular weight of N=14, and molecular weight of N2O = 44 (so 1 mole of N2O weighs 44 grams, 28 grams of which are nitrogen). • N2O Global Warming Potential = 296. Recall that GWP is the radiative forcing of 1 kg of a substance (in this case, N2O), relative to that of 1 kg of CO2, (i.e., GWP is defined in terms of the total mass in N2O and CO2, not just the mass of N and C). (a) Assuming that all nitrogen required for growing trees is supplied by application of N fertilizer (a somewhat extravagant assumption, but we will use it), how much N2O (in kg N2O molecules) is emitted in offsetting the annual carbon footprint of the average Global Change student? (b) How much CO2 (in kg CO2 molecules) is this equivalent to in terms of radiative forcing? (c) What, consequently is the “effective C sequestration” of each kg of C actually sequestered in forest, accounting for associated N2O emissions? (in other words, for eachkg of CO2 or carbon actually sequestered in forest, how many kg-equivalents of CO2 or carbon are removed from the atmosphere, after weighting the offsetting N2O emissions as if they were CO2?) 4. (2 points) Now let’s consider policy implications of the above arithmetic. If the average individual in the U.S. had a carbon footprint like students in Global Change, which of the three factors above would have the biggest impact if we wanted to implement a policy of using forests to offset all CO2 emissions for the whole U.S. population? Would it be: • The demand for arable land? (Total arable land in the continental U.S. is ~130x106 ha), • The demand for N-fertilizer? (Current annual rate of U.S. N-fertilizer consumption is ~10x106 tons N) • The counteracting effect of N2O emissions associated with fertilizing