Policy vs. No-Policy: Updated Estimates

Visualizing the risks and probabilities of success

In an attempt to better convey the uncertainty in climate change prediction, and thus the implied risk that the world faces, in 2001 the Joint Program developed The Greenhouse Gamble™. Depicted as a roulette wheel, the image portrays the MIT Program's estimations of climate change probability, or the likelihood of potential (global average surface) temperature change over the next hundred years, under different possible scenarios.

Estimates of the risks of climate change are based on the best available information at the time the estimates are made, and thus as continued observations are made and scientific investigation proceeds the likelihood estimates that underlie these wheels must be updated.

Based on new research we provide updated estimates of the likelihood of different amount of global warming over the century under reference case, in which it is assumed "no policy" action is taken to try to curb the global emissions of greenhouse gases, and a "policy case" that limits cumulative emissions of greenhouse gases over the century to 4.2 trillion metric tons of greenhouse gases (GHGs) measured in CO2-equivalent.

No-Policy Case

Policy Case

The notion is that as humans allow global emissions of greenhouse gases to continue to increase, the roulette wheel continues to spin. We can control emissions — the policy case represents one choice for cumulative allowed emissions over the century — and by doing so we can limit risk. Uncertainties in the Earth system response to increasing emissions are given by nature; we can learn more about these responses but we can not directly control them. The results show much higher likelihood of higher temperature increases than for the previous wheels.