The IGSM results are expressed in terms of probabilities or probability distributions. This is because the calculations incorporated into the model include starting values for both climatic and economic parameters that are selected by a random process; the calculations are then repeated several hundred times and the results are assembled into graphs of probabilities or frequencies of occurrence of a given value of the output. (These are similar to histogram bar charts, which are used when the number of data points is small. In the probability distributions each value, for example of temperature increase, has an associated frequency of occurrence, which varies as the temperature value moves across an essentially continuous range.)
IGSM forecasts for temperature changes are shown in the graphic below for a “no mitigation policy” case (others call this “business-as-usual”), which leads to an atmospheric concentration of CO2 and equivalent contributions from other greenhouse gases of 1330 parts per million (ppm) CO2 equivalents (CO2-eq) in the decade 2091-2100. The forecasts also include mitigation policies of increasing stringency which are modeled to constrain GHGs to 890, 780, 660 and 560 ppm CO2-eq.
Frequency distributions for each value of the variability (SD) found for successive ten-year global average temperatures. The frequency is plotted on the vertical axis. The temperature variability is plotted on the horizontal axis as the deviation from the mean of the black “bell-shaped curve”, in units of the statistical standard deviation. These curves may be considered as highly compressed histograms showing the fractional occurrence for each value of SD. (All curves sum to 1.000.) The curve for a set of numbers that would be found from fully random valuations about the average (“bell-shaped curve”) is shown in black. Any deviation from the bell-shaped curve shows the existence of a bias in the distribution of the values. Decades in the base period are: crimson, 1951-1961; yellow, 1961-1971; and green, 1971-1981. Decades showing warming are aqua, 1981-1991; dark blue, 1991-2001; and magenta, 2001-2011.