January/February 2008

Wheel of Global Fortune

Continued from page 4

By Katherine Bourzac, SM '04

Which Wheel Would You Rather Spin?
Betting how hot the planet will get if we do­­--or don't--cap greenhouse-gas emissions.

The MIT Joint Program on the Science and Policy of Global Change created a pair of spinning gambling wheels to demonstrate the potential impact of policies designed to mitigate global warming. Program researchers estimated the probability of negligible, moderate, and dramatic global average temperature increases between 1990 and 2100 if the original signers of the Kyoto Protocol (including the U.S.) enact its provisions by 2010 or earlier and implement even more stringent policies lowering emissions to less than double their preindustrial levels by 2100. They also calculated the probabilities assuming no policy change and created pie charts showing the likelihood of various global temperature increases under both scenarios. (Each piece of pie represents a range of temperature ­increases; its size reflects its probability.)

The "no cap" wheel has only a sliver representing a temperature increase of less than 1 ºC; if the wheel is spun, there's just a 4.1 percent probability (1 chance in 24) that it will stop there. The odds of landing on the wedge showing an increase greater than 5 ºC are about 1 in 26, or 3.8 percent.

In the "cap" scenario, milder warming is more probable; the two largest temperature increases from the "no cap" wheel are not included. The largest increase, greater than 3 ºC, represents odds of about 1 in 29, or 3.5 percent. The odds of milder increases are much better: 1 in 6, or 15.8 percent, for less than 1 ºC and 36 percent for between 1 and 1.5 ºC.

Both wheels are due for sobering revisions, however, in view of new findings that oceans don't slow global warming as much as previously thought.