GEOS 578, Fall 2010 Graduate Assignment #3 Assigned: October 12, 2010 Due: October 26, 2010 (via email to [email protected]) Question 1. We learned some things about the water cycle in the lecture on 10/5/2010 that will help us think about a claim that is sometimes put forward by climate contrarians. The claim is that since it is water vapor, not CO2, that is the most important greenhouse gas, and since humans emit lots of water vapor, then one of two things must be true: either we should be equally or more concerned about human emissions of water vapor as we are about emissions of CO2 and other non-water greenhouse gases, or, alternatively, if water vapor emissions are not something to worry about, than neither are emissions of CO2. (e.g., see http://www.geocraft.com/WVFossils/greenhouse_data.html or http://www.ecoenquirer.com/EPA-water-vapor.htm). Below (a and b) are two possible responses to this contrarian claim. Are these accurate responses, or not? Explain why or why not. (a) First: "The contrarian claim is just wrong. Water vapor is not the most important greenhouse gas by a long shot." (b) Second: "The contrarian claim is not directly false, just highly misleading. We don't worry about anthropogenic water vapor emissions as much as we do about CO2 because humans just don't emit a flux of water vapor that is anywhere close to the flux of CO2 we emit." (hint: a simplified formula for combustion of fossil fuel is: C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy ) (c) Let's consider a third possible response: i. Use the facts given on the slide summarizing the global water cycle (lecture notes on 10/5) to answer the question: What is the residence time of water vapor in the atmosphere? How does this compare to the residence time of CO2 or other greenhouse gases? Hint: for a pool or stock of size S that, like water vapor, is in steady state with its sources and sinks (i.e. where the inflow from its sources = outflow to its sinks), and those flows are each of size F, the average amount of time (residence time, τ) that each molecule spends in the pool is equal to S / F. This makes sense unit-wise, because dividing S (in gazillions of stuff) by F (units of gazillions of stuff per year) gives an answer in years. ii. What is the flow of water from combustion sources (calculable from knowledge of the total emission of CO2 and from information in b)? How does this flow compare to natural background flows of water from precipitation or evapotranspiration? iii. Using the answers to (a) and (b), together with the stock-flow relation defined there (S = F⋅τ), calculate the new steady state stock Snew for atmospheric water vapor that would arise if the "new" extra emission (∆F) of water from combustion remained steady at today's rates (and assuming the residence time does not change). What fractional increase is this over the original (pre-anthropogenic) steady-state stock of atmospheric water vapor? iv. In light of these calculations, write a third possible response to the contrarian's claim. Imagine that your audience is the general public, and explain the elements of your response as if you are writing an op-ed for a newspaper. Question 2. Future changes in the water cycle may interact with temperature increases. For example, as mentioned in one of the lectures, Piñon Pine forests in the southwestern U.S. recently experienced a large scale drought-associated die-off that was substantially more severe than the mortality associatedwith a comparably strong drought in the 1950's (Breshears et al., 2005). A recent experiment in UofA's Biosphere 2 (Adams et al., 2009) sought to test whether this higher drought mortality could be due to warmer temperatures that make piñon pine trees more susceptible to drought, and if so, what the mortality mechanism might be, and the implications for a warmer world. a. Briefly characterize the observations of Breshears et al. (2005), to set the context for the experiment and analysis of Adams et al., (2009). What are the findings of Adams et al., and what do you think are the study's strengths and weaknesses? (do this before you read the exchanges with critics cited in part b, below). b. Now consider the criticisms of the Adams et al. study by Sala (2009) and Leuzinger et al. (2009), and the author responses (all compiled in the file Adams.09_exchange_PNAS.pdf). What are the main specific issues are being criticized, what do you make of the criticisms, and of the original study, in light of this exchange? Based on what you have learned overall from these readings (and others you may want to look up to assess the perspectives found in these), what is your take on the future of Piñon Pine in the Southwestern U.S., and on the risks of climate-change induced tree mortality in general? Your answers should be in the form of a concise, scholarly essay of order 4-5 pages double-spaced (about 1,500 words). REFERENCES Adams HD, Guardiola-Claramonte M, Barron-Gafford GA, et al. 2009. Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought. Proc Natl Acad Sci. 106(17): 7063-7066. Breshears, D.D. N.S. Cobb, P. M Rich, K.P. Price, C.D. Allen, et al. (2005) Regional vegetation die-off in response to global-change type drought. Proc Natl Acad Sci USA 102:15144–15148. Leuzinger S, Bigler C, Wolf A, Körner C (2009) Proc Natl Acad Sci USA, 10.1073/pnas.0908053106. Sala A (2009). Proc Natl Acad Sci USA