Update: See “Recent Climate Observations: Disagreement With Projections” 2009, Energy & Environment.

Rahmstorf et.al. claims that previous projections of the IPCC have underestimated climate change, particularly temperatures and sea level rise. The conclusion of the brief paper has been used to justify more vigorous action on global warming by the Garnaut review, on the grounds that global warming is ‘worst that we previously thought’. But does Rahmstorf et.al. hold water?

The method used is non-linear fitting of a trend to temperatures. The conclusion is from a marked increase in the trend in recent years beyond IPCC decadal projections. However, there appear to be many statistical black holes.

Lucia has been trying to verify the provenance of the IPCC projections as they are not described explicitly in paper, with no success.

The method is described only as

a nonlinear trend line computed with an embedding period of 11 years and a minimum roughness condition at the end

with a reference to Moore et.al. “New Tools for Analyzing Time Series Relationships and Trends“. The non-linear method appears to be singular spectrum analysis (SSA). This method decomposes a time series into orthogonal components, such a a periodic, linear trend and random noise parts. Rahmstorf et.al. also mentions a non-SSA ‘minimum roughness condition applied at the end’ without further elaboration. Any ad-hoc techniques appled to the end of the series should raise a red flag, as the part of the series of interest is the recent temperatures at the end.

Digging into Moore et.al. we find the ‘minimum roughness condition’ consists of ‘padding the series so the local trend is preserved’. The reference to the padding the data is none other than Mann 2004, “On smoothing potentially non-stationary climate time series“!. Figures. Mann has been outed for various dubious extrapolation activities at ClimateAudit here).

Second, the unique adjustment was not disclosed in the MBH98 footnotes. Worse, the start date of this series was actually misrepresented in the original supplementary information, which listed the series as starting at the “adjusted” start date rather than the true start date. We only noticed the extrapolation when we compared the Mann version to original data. We noted this in MM 2003, but were not then fully aware of the impact.

Amazing what you find with a bit of digging. It seems like the ‘minimum roughness condition’ is simply a fancy name for the extrapolation of the trend of the last few data points in the series. In that case, the conclusions of the paper — that observed trends surpass the IPCC projections — are based on only a few end points, raising the specter of ‘c