As discussed several times previously, two of the key uncertainties relating to climate change is (a) how much temperature would increase in response to any particular change in the atmospheric concentration of greenhouse gasses and (b) what humanity will actually emit between now and the achievement of global carbon neutrality. One way to express those uncertainties colourfully is with the Roulette wheels the MIT Joint Program on the Science and Policy of Global Change has created.
The wheels are based on results from the MIT Integrated Global Systems Model and have shaded areas proportional in size to different possible levels of temperature increase. The projections were recently updated, and the new ones contain significantly higher estimates of the risks of high levels of warming:
The new projections, published this month in the American Meteorological Society’s Journal of Climate, indicate a median probability of surface warming of 5.2 degrees Celsius by 2100, with a 90% probability range of 3.5 to 7.4 degrees. This can be compared to a median projected increase in the 2003 study of just 2.4 degrees. The difference is caused by several factors rather than any single big change. Among these are improved economic modeling and newer economic data showing less chance of low emissions than had been projected in the earlier scenarios. Other changes include accounting for the past masking of underlying warming by the cooling induced by 20th century volcanoes, and for emissions of soot, which can add to the warming effect. In addition, measurements of deep ocean temperature rises, which enable estimates of how fast heat and carbon dioxide are removed from the atmosphere and transferred to the ocean depths, imply lower transfer rates than previously estimated.
The ‘policy’ wheel assumes aggressive mitigation action, while the ‘no policy’ wheel assumes a business-as-usual course. It is notable that the chances of keeping warming below 2°C are infinitesimal, on that wheel. Even with aggressive action, our changes of keeping below 2°C of increase are looking increasingly distant, with effects that may be severe for both human and natural systems.
In addition to being a good visual image, I like the conceptual linkage between climate change and gambling. We are certainly taking a chance, whatever we do, but science can help us to assess the odds we face and make choices that reduce the risks of unacceptable outcomes.