A couple of commentators (Pat Michaels, Roy Spencer) recently raised an issue about the standard scenarios used to compare climate models, in this case related to a study on the potential increase in hurricane activity.
The biggest uncertainty in what will happen to climate in the future (say 30 years or more) is the course that the global economy will take and the changes in technology that may accompany that. Since climate scientists certainly don’t have a crystal ball, we generally take a range of scenarios or projections of future emissions of CO2 and other important forcings such as methane and aerosols.
For each scenario, the models will project a course of climate change. Of course, this is not a prediction – climate change in the real world will depend on which of the scenarios turns out to be more accurate. However, there are a number of scenarios that have become de-facto standards – not because they are more likely than others, but due to historical reasons. i.e. they are simple, and there is substantial exisiting work that you can compare the results to.
The most common de-facto standard is the equilibrium doubled CO 2 run. i.e. you just double the amount of CO2 in the model and you see what happens. Only slightly more realistic is the 1% increasing equivalent CO2 case. i.e. every year the amount of CO2 increases by 1% for 100 years. By the time you get to about 2080, CO2 has doubled. Even if we take account of methane, N2 O and CFC contributions, this too is an unlikely scenario. The maximum growth rate occured in the 1980′s (due to the rapid growth of CFCs) and was equivalent to about 0.7% increase per year. Currently, the forcing is around 0.6% increase of equivalent CO2 per year. Does this mattter? In most cases, the answer is no.
Both these simple scenarios are used mainly to be able to characterise the behaviour of different models. The existence of a ‘date’ attached to the results is really rather misleading. The models are not going to be able to tell you what will happen in 2080, but more what may happen at the time of doubling of CO2 , whenever that may be. It turns out the much of the climate is only weakly dependent on the rate of change of the greenhouse gases (though there are some important exceptions). So the result at the time of doubling doesn’t much matter whether it takes 70 or 100 years to get there.
More complex scenarions that also include aerosol and other GHG changes have been developed by the IPCC. Models are using these (which range from no further change in greenhouse gases to a ‘Business as usual’ continuation of the past increases) to run simulations for the next IPCC report due 2007. In the meantime, the simple standards can continue to tell us a lot about the models, and hopefully, the real world.