Playing roulette with the climate – everybody loses

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The “roulette wheels” created by MIT researchers to show the range of probability of potential global temperature change over the next 100 years

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May 22, 2009 Research carried out by the MIT's Center for Global Change Science has predicted that global warming will be roughly double previous estimates – and could be even worse than that. While a major 2003 study indicated a median projected increase in earth surface warming of 2.4 degrees Celsius, the new study points to a median probability of surface warming of 5.2 degrees Celsius by 2100, unless drastic action is taken.

The worrying new projections were reached using the most comprehensive modeling yet carried out on the likelihood of how much hotter the Earth's climate will get this century. While other research groups have previously estimated the probabilities of various outcomes, based on variations in the physical response of the climate system itself, the model used by the researchers is the only one that also takes into account possible changes in human activities - such as the degree of economic growth and its associated energy use in different countries.

The study used the MIT Integrated Global Systems Model, a detailed computer simulation of global economic activity and climate processes that has been developed and refined since the early 1990s. The new research involved 400 runs of the model with each run using slight variations in input parameters, selected so that each run has about the same probability of being correct based on present observations and knowledge. The results not only indicated a median probability of surface warming of 5.2 degrees Celsius by 2100, but also a 90% probability of surface warming in a range of 3.5 to 7.4 degrees Celcius.

This figure is more than double the estimates provided by the 2003 study, with the difference attributed to 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.

These factors and a variety of other changes based on new measurements and new analyses changed the odds on what could be expected in this century in the "no policy" scenarios - that is, where there are no policies in place that specifically induce reductions in greenhouse gas emissions. While the outcomes in the "no policy" projections now look much worse than before, the team says there is less change from previous studies in the projected outcomes if strong policies are put in place now to drastically curb greenhouse gas emissions.

The team also admits that the odds indicated by their modeling may actually understate the problem, because the model does not fully incorporate other positive feedbacks that can occur. For example, if increased temperatures caused a large-scale melting of permafrost in arctic regions and subsequently released large quantities of methane, a very potent greenhouse gas. Taking such feedback events into account would only make the odds much worse.

To provide a simple illustration of the range of probabilities revealed by the 400 simulations, the team produced a "roulette wheel" that reflects the latest relative odds of various levels of temperature rise. The wheel provides a very graphic representation of just how serious the potential climate impacts are.

The MIT team do stress that, due to the large number of uncertainties in the climate change equation, there is no way to make absolutely accurate predictions. All they can do is collect the best data they can, run the simulations and look at the spread of the odds. What the team is certain about, however, is that the least-cost option to lower the risk of climate change is to start now and steadily change the global energy system over the coming decades to low or zero greenhouse gas-emitting technologies. Let’s hope the odds of that happening rapidly improve.

Darren Quick

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