Limits of historical reconstructions using

indirect evidence

Limits of historical reconstructions using

indirect evidence

Hans von Storch, Eduardo Zorita, Julie Jones, Institute for Coastal Research, GKSS Research Centre, GermanyFidel González-RoucoUniversity Complutense Madrid, Spain

Hans von Storch, Eduardo Zorita, Julie Jones, Institute for Coastal Research, GKSS Research Centre, GermanyFidel González-RoucoUniversity Complutense Madrid, Spain

9IMSC, Cape Town24-28. May 2004

ECHO-G simulations „Erik den Røde” (1000-1990)

and “Christoph Columbus” (1550-1990)

with estimated volcanic, GHG and solar forcing

1675-1710vs. 1550-1800

Reconstruction from historical evidence, from Luterbacher et al.

Late Maunder Minimum

Model-based reconstuction

Christoph CErik dR

Both, Erik den Røde and Christoph Columbus generate temperature variations considerably larger than standard reconstructions (Mann, Jones …).

The simulated temperature variations are of a similar range as derived from NH summer dendro-data and from terrestrial boreholes.

For the present context, it does not really matter how „good“ the historical climate is reproduced by Erik den Røde and Christoph Columbus

.

The model data provide a laboratory to test MBH and other methods.

pseudo-proxies: grid point SAT plus white noise

red: mimicking largest sample used in MBHblue: hypothetic additional data to obtain better coverage.

Testing the MBH method

Mimicking MBH?

Estimated NH SAT from terrestrial deep soil temperature (TDST) in Erik den Røde.

Conclusion, 1

• Erik den Røde, an effort to simulate the response to estimated volcanic, GHG and solar forcing, 1000-2000.

• Low-frequency variability in Erik den Røde > Mann, Jones, & mainstream, but ~ Esper, boreholes, (some) instrumental data

Conclusion, 2

• Erik den Røde-data used to test MBH and borehole approaches.

• Randomized grid-point SAT (i.e. white noise added) is used as pseudo proxy.

• MBH method, based on regression, gives significant underestimation of low-frequency NH mean SAT.

• Direct averaging of randomized local data results in considerably smaller errors in NH mean SAT.

Conclusion, 3

• When using Erik den Røde-data and pseudo-proxy data, which share a correlation of about 0.5 with the grid point data, the best guess resembles the MBH estimate (incl. the ±2σ confidence band).

• The problem is common to ALL regression based methods, trained with temporal high-resolution data – except if the correlations are really high.

• A reliable reconstruction of centennial time scales requires either really high correlation, or process-based inverted data (e.g., borehole inversions), or local instrumental data.

Testing with HadCM3 simulation

Erik

HadCM3

Data provided by Simon Tett.