The Greenhouse Gamble
("No Policy" Reference Case)
- The wheel depicts the MIT Joint Program's estimation of the range of probability of potential global warming over the next hundred years, assuming a scenario in which "no policy" action is taken to try to curb the global emissions of greenhouse gases.
The face of the wheel is divided into eight slices, with the size of each slice representing the estimated probability of the temperature change in the year 2100 falling within that range.
For comparison, note that if a "policy" to reduce greenhouse gas emissions is enacted, the resulting change in probabilities is shown by the altered size of the representative slices:
The temperature change is the difference in the global average surface temperature predicted for 2100, with respect to the 1990 value.
For further discussion of this type of uncertainty and probability prediction, see Forest et al., 2004, Webster et al., 2003, Webster, 2003, Forest et al., 2002, Reilly et al., 2001, and Reilly et al., 1999.
For example, the size of the slice for greater than 5 degrees Celsius warming (shown in red), which represents the largest temperature increase predicted in this study, has a probability of 3.8%. Or, if stated in another way, that probability has the same likelihood as the "odds" of (about) 1 chance in 26. The slice representing the smallest predicted change, less than 1 °C (shown in blue), is of a similar size, with a probability of 4.1% (1 in 24 odds).The MOST LIKELY values for warming predicted by the MIT Joint Program for this reference case (no policy) scenario are in the mid-range: 2 to 2.5 °C (22.5%, yellow: 1 in 4 odds), and 1.5 to 2 °C (20.6%, light green: 1 in 5 odds) . The probabilities of the temperature change falling within the other ranges are the following: 2.5 to 3 °C (16.8%, yellow-orange: 1 in 6 odds), 3 to 4 °C (16.2%, orange: 1 in 6 odds), and 4 to 5 °C (4.6%, pink: 1 in 22 odds).
