cement production was subtracted.11 The conversion factor from CO2 emissions to fossil fuel generated energy production was obtained by taking a ratio of the fossil fuel consumption to the CO2 emissions. The year 1986 was used because it was the most recent year for which data was available. The relationship between atmospheric concentration and global warming was based on an equation from Berner, et. al.,12 and is given by: where C(t) = atmospheric CO2 concentration t years from today C(0) = atmospheric CO2 concentration today n = rate of temperature change in degrees per year s = climate sensitivity in degrees Celsius. Note that here, the climate sensitivity is the temperature change that results from the CO2 concentration increasing by a factor of e, not 2. In our modelling, the best estimate of 2.5°C for CO2 doubling was used? To obtain s for Equation 4, this was divided by ln(2) to give 3.6°C. For the inverse model, n was specified as an input, CO2 concentration was calculated from Equation 4, emission was calculated by using the inverse of Equation 1, and then emission was converted to energy. Here, C(0) is the concentration for the year 1990. The results are plotted in Figure 4. For years before the present, the concentration is based on observed data, so that the graph shows implied fossil fuel energy production. For 1991 and beyond, the graph shows the maximum allowable fossil fuel energy production which would limit global warming to the rates indicated. Note that in order to hold the globally averaged temperature to its 1990 value, fossil fuel energy production must immediately be cut by approximately two-thirds, and then must continue to decrease over the next century. This scenario confirms Wigley's emission results for the case in which the future CO2 concentration is held to its 1990 value. If we allow a 1°C per century global warming, then fossil fuel energy production must immediately be cut by approximately one-third, but it can then be allowed to slowly increase over the next century. These energy scenarios show a discontinuity at the year 1990 because we are now using fossil fuels at a rate consistent with a global warming greater than 1°C per century. By comparing the slope of the 2°C per century curve with the slope of the curve prior to 1990, it is seen that at current rates of fossil fuel use, a considerable amount of global warming is likely during the next century. Of the three curves in Figure 4, only the 2°C per century scenario represents an energy scenario that can be achieved without an immediate and drastic energy production decrease. Setting strict limits on global warming thus results in some rather contrived and unrealistic fossil fuel energy production profiles. It would therefore make more sense to set limits on fossil fuel energy production and then examine the effect on climate. This was done using Wigley's forward model, with an input fossil fuel use that varies by a fixed percent per year. To accomplish this, Equation 4 was solved for n; this temperature increase rate was then integrated over time to give a temperature difference from 1990. Thus,
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