I’m now hopelessly confused about the distinction between climate projection and prediction.
I used to be happy with what I thought was the case: that given the range in model results, and no good way of knowing the best, calling them predictions seemed too precise; so use a weaker word like projection instead. But.
The IPCC glossary says “A climate prediction or climate forecast is the result of an attempt to produce a most likely description or estimate of the actual evolution of the climate in the future, e.g. at seasonal, interannual or long-term time scales”
That isn’t a very good definition, because its near meaningless. Indeed, it appears to make the outcome dependent on the intention of the researcher(s) producing the runs.
And “A projection of the response of the climate system to emission or concentration scenarios of greenhouse gases and aerosols, or radiative forcing scenarios, often based upon simulations by climate models. Climate projections are distinguished from climate predictions in order to emphasise that climate projections depend upon the emission/concentration/ radiative forcing scenario used, which are based on assumptions, concerning, e.g., future socio-economic and technological developments, that may or may not be realised, and are therefore subject to substantial uncertainty.”
I read that as saying that all that distinguishes pred from proj is knowning the forcing scenario; if we knew future GHG (and solar, and volcanic) accurately, we would call them predictions. That doesn’t seem right either.
Over at RC, discussing the recent Smith 10-y forecasts, Gavin says “the kinds of simulations used in AR4 are all ‘projections’ i.e. runs that attempt to estimate the forced response of the climate to emission changes, but that don’t attempt to estimate the trajectory of the unforced ‘weather’.
This is at least a meaningful distinction – predictions attempt to forecast the actual trajectory of the ‘weather’. Or it would be meaningful if I was sure what ‘weather’ meant in this context. I presume not actual weather, but year-to-year fluctuations, including El Nino etc. But then thats not really “climate” prediction, since climate averages out the ‘weaher’.
Anyone have a more exact definition?