Its shooting fish in a barrel, of course, but you must go and read Another uncertainty for climate models – different results on different computers using the same code [WebCitation].
The issue here is a well-known one – it dates back to Lorenz’s original stuff on chaos. That trivial differences in initial conditions, or in processing methods, will lead to divergences in weather forecasts. The (entirely harmless) paper that has sparked all this off is an Evaluation of the Software System Dependency of a Global Atmospheric Model by Song-You Hong et al. and sez
There exist differences in the results for different compilers, parallel libraries, and optimization levels, primarily due to the treatment of rounding errors by the different software systems.
This astonishes the Watties, as though it was a new idea. To them I suppose it is. But it’s exactly what you’d expect, within a numerical weather prediction framework (though I’d expect you not to care within NWP. If differences in optimisation level have lead to error growth large enough to see, I’d have expected uncertainties in initial conditions to have grown much more and made the whole output unreliable). I don’t think you’d expect it within a climate projection framework, at least atmospheric-wise. You might expect more memory from the ocean. JA and I have a post on RC from 2005 that might help, originating from a post on old-stoat by me where I was playing with HadAM3.
In the comments at WUWT Nick Stokes has done his best to explain to the Watties their mistake – but AW has just rubbed out NS’s comments, because they were too embarrassing.
There’s an important distinction to make here, which is that climate modelling isn’t an Initial Value Problem, as weather prediction is. Its more of a Boundary Value Problem, with things like GHGs being the “boundary”s. Or at least, that’s the assumption and that is how people are approaching it (RP Sr disagrees, and you could discuss it with him. Except you can’t, becasue he doesn’t allow comments at his blog. RP Sr is too wise to value anyone else’s opinion). Potentially, there’s an interesting debate to be had about whether climate modelling can indeed be considered largely free of its initial conditions. But you can’t start such a debate from the level of incoherent rage displayed at WUWT.