This wheel depicts the MIT Joint Program's estimation of the range of probability of potential global warming over the next hundred years, under the following policy scenario:
In this example, the size of the slice for greater than 3° Celsius warming, which represents the largest temperature increase predicted under this policy scenario, has a probability of 10%, or odds of 1 in 10. In the no policy case there is only a 1% chance of less than 3 °C warming, but with this policy in place the likelihood of staying below 3 °C temperature change increases to 90% (9 in 10 odds). The median warming level (50% chance of remaining below, or even odds) is 2.3 °C.
Note that compared to the reference case, this policy scenario not only increases the chances that the warming will be less (the cooler-range slices are larger), but it also removes the warmest four slices from the no policy wheel.
The likelihood of each temperature-change range is: 1 to 2 °C, 20%, (1 in 5 odds); 2 to 2.5 °C, 43% (about 2 in 5 odds); 2.5 to 3 °C, 27% (about 1 in 4 odds); 3 to 4 °C, 10% (1 in 10).
For more technical detail see Webster et al., 2009 and Webster et al., 2008.
For comparison, note that in the reference case, in which "no policy" action is taken to try to curb the global emissions of greenhouse gases, the probabilities of projected global average surface temperature change in 2100 are shown below.
For discussion of this type of uncertainty and probability prediction, see Webster et al., 2008, Webster, 2007, Stott & Forest, 2007, Webster et al., 2003, Webster, 2003, and Reilly et al., 2001.
