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The fine print:

The results from our calculator are produced from climate change calculations performed using the MAGICC climate model simulator (MAGICC: Model for the Assessment of Greenhouse‐​gas Induced Climate Change). MAGICC was developed by scientists at the National Center for Atmospheric Research under funding by the U.S. Environmental Protection Agency.

We are not creative enough to have made that acronym up. MAGICC is itself a collection of simple gas‐​cycle, climate, and ice‐​melt models to efficiently emulate the output of complex climate models. MAGICC produces projections of the global average temperature and sea level change under user configurable emissions scenarios and model parameters. MAGICC is run using its default model parameter settings except for climate sensitivity, which you can choose from between 1.5°C and 4.5°C.

The baseline emissions scenario against which all climate dioxide reductions were measured is scenario A1B from the IPCC’s Special Report on Emissions Scenarios (SRES). Scenario A1B is a middle‐​of‐​the‐​road emissions pathway which assumes rapid carbon dioxide emissions growth during the first half of the 21st century and a slow CO2 emissions decline thereafter. Emissions are prescribed by country groups. Our “Industrialized Countries” group is the OECD90 countries (which includes North America, Western Europe, and Australia, New Zealand and Japan.) In order to obtain the baseline emissions from the United States to which the emissions reduction schedule could be applied, the U.S. emissions were backed out from the OECD90 country grouping. To do so, the current percentage of the total group emissions that are being contributed by the United States was determined—which turned out to be right around 50%. We assume that this percentage will be constant over time. In other words, that the U.S. contributed 50% of the OECD90 emissions in 2000 as well as in every year between 2000 and 2100. In this way, the future emissions pathway of the U.S. was developed from the group pathway defined by the IPCC for the A1B scenario. From these baselines (either the U.S. baseline or the OECD90 baseline), carbon dioxide emissions reductions were applied linearly from 2005 to 2050 to obtain the user‐​specified total reduction. The new (reduced) emissions were recombined with the other (unadjusted) IPCC country groupings to produce the global emissions total. It is the total global emissions that are entered into MAGICC to yield global temperature projections. The results using the reduced emissions pathway were then compared to the results using the original A1B pathways as prescribed by the IPCC, with the baseline against which temperature changes were calculated set to the year 2010.

We assume that a carbon tax would only be applied to reduce carbon dioxide emissions. In practice however, the only way to reduce carbon dioxide emissions is to reduce the burning of fossil fuels. Reducing the burning of fossil fuels will have co‐​impacts such as reducing the emissions of carbon monoxide (CO), volatile organic compounds (VOCs), nitrogen oxides (NOx), and sulfur oxides (SOx). The first three chemical compounds generally enhance warming while the latter generally retards it. Sensitivity tests using MAGICC indicate that for the OECD90 countries under the A1B pathway, the effect of collective changes in these co‐​emissions is largely compensative.

Additional fine print on precision: The temperature savings are presented to three significant digits in order to tell the results apart. In the real world, the impacts from the emissions reduction pathways are not nearly so precise and, in fact, the temperature savings from most of the different carbon dioxide emissions reduction pathways are scientifically impossible to tell apart from each other, and in many cases, are impossible to tell apart from the original A1B scenario, i.e., they are same thing as doing nothing.