Scenario for Limiting Future Warming
Climate Change mitigation is an example of the need for decision making in the face of uncertainty. We must take steps today to stabilize greenhouse gas concentrations if we are to prevent future warming of the globe, despite the fact that we do not know precisely how much warming to expect. Furthermore, it is a problem of risk management. We do not know precisely what potential impacts loom in our future, and where the threshold for dangerous anthropogenic impacts on the climate lies. Just like in nearly all walks of life, we must make choices in the face of uncertainty, and we must decide precisely how risk averse we are. Most homeowners have fire insurance, yet they don't expect their homes to burn down. They simply want to hedge against the catastrophe if it does happen. We can, in an analogous manner, think of climate change mitigation as hedging against dangerous potential impacts down the road. This project aims to integrate a number of themes we have already explored—energy balance and climate modeling, and our current lesson on carbon emissions scenarios—to quantify how to go about answering critical questions like, "How do we go about setting emissions limits that will allow us to hedge against the possibility of dangerous anthropogenic impacts (DAI) on our climate?"
For this assignment, you will need to record your work on a word processing document. Your work must be submitted in Word (.doc or .docx), or PDF (.pdf) format so the instructor can open it.
For this project, you will design your own fossil fuel emissions scenario that would limit future warming by the year 2100 to 2.0°C relative to the pre-industrial level.
- Defining the threshold for DAI with the climate is a value judgment, as much as a scientific one. The European Union has defined 2°C warming relative to pre-industrial conditions to be the threshold of DAI (and this has been adopted in the recent Paris agreement). Use a zero-dimensional EBM (i.e., Excel Spreadsheet for PS #3) with the standard, i.e., mid-range values of the gray body parameter (and default settings for solar constant and albedo) to estimate the concentration for which we would achieve such warming when equilibrium is reached. [Hint: you already did this particular calculation in Problem Set #3, and the gray body parameter here is term "B".]
- As discussed above, where we choose to set our emissions limits is also a matter of risk management. How risk averse are we? Let us assume that we want to limit the chance of exceeding DAI to only 5%, which is a typical threshold used in risk abatement strategies. Let us also assume that low end and high end sensitivities define our best estimate of the 90% confidence interval for climate sensitivity, that is, there is a 5% chance that the true sensitivity is lower than the low end value, and a 5% chance that the true sensitivity is higher than the high end value. Now, revisit the calculation that you did in Step 1, and use a high-end setting of the gray body parameter (i.e., increase by 50% term "B" used for mid-range) to calculate the highest concentration that we can reach and still keep the chance of exceeding DAI below 5%.
- That was the easy part! Now is where things really get interesting. You have to come up with your own emissions scenario to stabilize concentrations below the dangerous threshold that you calculated in Step 1 (i.e., using the mid-range values). Recall that based on Step 2 we are not being particularly risk-averse under this assumption. By stabilizing, we will mean that by the year 2100 the concentration curve should be flat or nearly flat, indicating that any peak in concentrations has been reached before 2100 - and that the value of that peak should not exceed the threshold found in Step 1. You should make use of the on-line Kaya calculator that we examined earlier in this lesson. Begin with the default settings in the Kaya calculator to figure out what baseline emissions scenario you are starting with and how much you need to do to achieve the required reduction in emissions to stabilize the concentration (given by the 'pCO2' curve when you choose 'ISAM pCO2' in the selector for the display on the right). You then have several knobs you can tune to achieve an alternative emissions scenario; specifically, you can do the following. Through some combination of tuning these knobs within the indicated ranges, it should be possible to stabilize concentrations at the necessary threshold level. (The concentration time-evolution curves corresponding to different specific stabilization scenarios are shown in the 'ISAM pCO2' plot.)
- You can take measures to control global population growth, which are consistent with the spread of the population trajectories of the various SRES scenarios (e.g., A1, A2, B1, B2); though note that you cannot set the limit below the current global population of 7.8 billion.
- You can change the rate of economic expansion (measured by GDP per capita) by up to 75% from the default value of 1.6% per year; that is, you can choose a value within the range of 0.4 to 2.8 % per year.
- You may change the rate of decline in energy intensity (measured in Watts per dollar) through new technology and/or the improvement of existing technology. You are permitted to change this value by 100% from its default setting of -1% per year, that is you can choose a value within the range of 0 to -2 % per year.
- You may change the decline in carbon intensity by 100% from the default value of -0.3 % per year, that is you can choose a value within the range of 0 to -0.6 % per year. This would reflect efforts to shift from the current reliance on fossil fuel sources, or the improved carbon efficiency of fossil fuel energy sources, e.g., through sequestration of , etc.
- As you write up your results, please discuss the reasoning that you used in arriving at the various choices for the factors in the Kaya identity, i.e., provide justification for why your choices reflect plausible policies that governments could in principle implement to achieve the necessary reductions. If your projections depart from what might be expected based on an extrapolation of past historic trends, provide some justification. Your discussion here might, for example, be guided by the plot of the required amounts of carbon-free energy to meet the requirements of your scenario, which is provided by the Kaya calculator tool. You will probably want to do some additional background reading. A good place to start would be the original 2001 IPCC report on SRES scenarios: Special Report on Emissions Scenarios.
- Save your word processing document as either a Microsoft Word or PDF file and upload to the appropriate location in Canvas.
Submitting your work
- Upload your file to the Project 1 assignment in Canvas by the due date indicated in the Syllabus.
The instructor will use the general grading rubric for problem sets to grade this project.