Greenhouse Gamble
Comparison of Projections: 2009 update vs. 2002
In February 2009 the MIT Joint Program published updated estimates of the range of probability of potential global warming over the next hundred years, along with new Greenhouse Gamble wheel images to depict the updated projections. Here we note differences in the estimates made in 2009 as compared with the prior projections that have appeared on the Program's website and elsewhere since 2002.
No-Policy Case: The differences are greatest for the reference or "no policy" wheels. In the previous wheel the likelihood of exceeding 5°C was about 4%, but in the new wheels that likelihood is 57%. There is no single revision that is responsible for this change. In our more recent global model simulations, the ocean heat-uptake is slower than previously estimated, the ocean uptake of carbon is weaker, feedbacks from the land system as temperature rises are stronger, cumulative emissions of greenhouse gases over the century are higher, and offsetting cooling from aerosol emissions is lower. No one of these effects is very strong on its own, and even adding each separately together would not fully explain the higher temperatures. Rather than interacting additively, these different affects appear to interact multiplicatively, with feedbacks among the contributing factors, leading to the surprisingly large increase in the chance of much higher temperatures.
- Addendum: Note that the colorscale used in the new and old graphics is different, as indicated by the temperature change values labeled on each segment. The scale was shifted for the new graphics to improve the portrayal and comparision of the new Policy vs. No Policy cases &mdash not the new vs. old projections. To aid in the visual comparison of the new vs. old estimates, an additional graphic for each case is provided below: the third wheel image (at far right) depicts the new wheels portrayed in the old colorscale to help clarify the difference.
 NEW No-Policy Case |
 OLD No-Policy Case |
 NEW – USING OLD SCALE |
Policy Case: The difference in the "policy" wheel is much less. If greenhouse gas emissions are controlled to relatively low levels then the Earth systems feedbacks are much lower and the slight difference in Earth system properties is not so important — again a result of the way in which these different factors interact multiplicatively. If any single important feedback is removed, it greatly reduces the resulting effect on climate.
 NEW Policy Case |
 OLD Policy Case |
 NEW – USING OLD SCALE |
There is also a slight difference in the way the temperature increases were calculated. The old wheels were calculated using the period 1990-2000 as the base year. The new wheels were calculated with the period 1981-2000 as the base year to be consistent with current practice of the Intergovernmental Panel on Climate Change (IPCC) in its comparison of model projections. This explains some part of the increased warming in the new wheels but only a few tenths of a degree increase in the median value.
More technical detail on the emissions and the climate results are available. See also MIT News Release.