assignment3_2023t1_students

Oct. 12 2023 EOSC340 Assignment 3 page: 1/2 (1) Assignment 3: feedbacks In this assignment you will perform calculations similar to those climate scientists use to estimate the strengths of the various climate feedbacks. For questions 1 and 2 you will require the MODTRAN model (using the default settings). Access the model through this website: https://forecast.uchicago.edu/modtran/ Q1. Start with the default configuration (tropical, 70 km, looking down, at 400, at 1.7, Trop. Ozone at 28ppb and all other gas scales set to 1). Defining as the change in net energy flux INTO the climate system in response to a forcing, adjust the methane concentration in Modtran to calculate . Methane has increased since pre-industrial times - it was about 1 ppm CH4 in 1870 and is close to 1.9 ppm now (approximate methane levels were obtained from https://www.methanelevels.org , which also includes information about the sources of data). For a change of from 1ppm to 1.9ppm, what is ? Give your answer in Q2. The concentration of methane is projected to increase to 3.5 ppm by 2100 if we do little about the problem between now and then. Using modtran again, calculate as you change from 1.9 ppm to 3.5 ppm. Your answer should be in . Q3. As we increase methane (positive ) the greenhouse effect increases, i.e. this is a positive forcing (positive ), but you should have found in the first two questions that the value of depends on the methane concentration - the amount of increased radiative forcing for a given increase in is not constant for all values of starting . Why does change when the methane concentration gets larger? Make sure to mention both the name of the phenomenon as well as a brief description of its effect. ( 1 - 2 sentences ) Q4. An amplifying feedback mechanism that is not yet operating on a large scale is the methane feedback: as global temperatures increase, methane frozen in the high-latitude permafrost regions may be released to the atmosphere as the permafrost begins to thaw. Assume that the current climate sensitivity, without this permafrost methane feedback, is λ=0.8 . Suppose this new permafrost methane feedback is added to the current climate sensitivity; the new feedback is such that, for every global temperature increase of 1 °C, atmospheric methane increases by 1.75 ppm. a. Find the smallest value of that would drive the total feedback to runaway greenhouse effect ( ). Your answer should be in . (hint: use the climate sensitivity to work out the total feedback factor , and recall how we add feedback factors from different feedbacks ) b. Using your understanding of from the first two questions (we can set = as the response of the climate to increased is the same whether the methane comes from a forcing, or a feedback), would this new methane feedback lead us to a runaway greenhouse effect, given that current levels are already at 1.9ppm? Explain why/why not. Q5) Methane is supplied to the atmosphere at a rate and it is removed by natural processes at a rate proportional to the current atmospheric concentration of Methane with a time constant of about ten years. In other words, the rate of flow OUT of the atmosphere for methane is in and so the stock and flow equation for methane is: Currently, atmospheric methane concentration is about = 1.9 ppm. If we assume is in equilibrium, i.e. (this isn’t quite true, as discussed above methane values are increasing, but assume it is for this question), then: a. what is ? b. Currently, the contribution to value from Arctic lakes and oceans is about 10% of the total input rate: = 0.1 * . Assuming the Arctic methane emission rate doubled, but all other sources and sinks of methane remain unchanged, what value of atmospheric methane concentration, , would the new equilibrium climate have?