1505_sejda-DYJ.pdf - Chapter 5 Greenhouse Gas Emissions and...

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Chapter 5 Greenhouse Gas Emissions and Climate Change 5-24 based on the climate ensemble data of the RCP 23 scenarios for sea level and temperature. Simple equations relating projected changes in sea level to projected changes in temperature are developed for each scenario using a regression model. The regression models for the RCP4.5 and GCAM6.0 scenarios are developed directly from the RCP4.5 and RCP6.0 data, while the regression model for the GCAM Reference scenario uses a hybrid relation based on the RCP6.0 and RCP8.5 data, as there is no equivalent IPCC scenario. The hybrid relation employs a weighted average of the relationship between RCP6.0 and RCP8.5 sea-level rise and temperature data based on a comparison of the radiative forcings. The temperature outputs of the MAGICC RCP4.5, GCAM6.0, and GCAM Reference simulations are used as inputs to these regression models to project sea-level rise. 5.3.3.3 Ocean pH NHTSA projected changes in ocean pH using the CO 2 System Calculations (CO2SYS) model, which calculates parameters of the CO 2 system in seawater and freshwater. This model translates levels of atmospheric CO 2 into changes in ocean pH. A lower ocean pH indicates higher ocean acidity, while a higher pH indicates lower acidity. The model was developed by Brookhaven National Laboratory and Oak Ridge National Laboratory and is used by both the U.S. Department of Energy and EPA. Orr et al. (2015) compared multiple ocean carbon system models, and found that the CO2SYS model was more efficient at analyzing observed ocean chemistry data than other models. This model uses two of four measurable parameters of the CO 2 system, total alkalinity, total inorganic CO 2 , pH, and either fugacity or partial pressure of CO 2 to calculate the remaining two input parameters. NHTSA used the CO2SYS model to estimate the pH of ocean water in the year 2040, 2060, and 2100 under the No Action Alternative and each of the action alternatives. For each action alternative, total alkalinity and partial pressure of CO 2 were selected as inputs. The total alkalinity input was held constant at 2,345 micromoles per kilogram of seawater and the projected atmospheric CO 2 concentration (ppm) data was obtained from MAGICC model runs using each action alternative. NHTSA then compared the pH values calculated from each action alternative to the No Action Alternative to determine the impact of the Proposed Action and alternatives on ocean pH. 5.3.3.4 Global Emissions Scenarios MAGICC uses long‐term emissions scenarios that represent different assumptions about key drivers of GHG emissions. The reference scenario used in this EIS is the GCAM Reference scenario (formerly MiniCAM), which does not assume comprehensive global actions to mitigate GHG emissions. NHTSA selected the GCAM Reference scenario for its incorporation of a comprehensive suite of GHG and pollutant gas emissions, including carbonaceous aerosols and a global context of emissions with a full suite of GHGs and ozone precursors.
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