Seminar EAS 8001
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Date Created: 11/02/15
Comment on quotReconstructing Past Climate from Noisy Dataquot Eugene R Wahl1 David M Ritson2 Caspar M Ammann3 von Storch et 11 Reports 22 October 2004 p 679 criticized the ability of the quothockey stickquot climate field reconstruction method to yield realistic estimates of past variation in Northern Hemisphere temperature However their conclusion was based on incorrect implementation of the reconstruction procedure Calibration was performed using detrended data thus artificially removing a large fraction of the physical response to radiative forcing etention of century scale temperature Rvariations in proxy based climate re constructions is important for under standing real world natural climate variability and to estimate climate sensitivity Both are fundamental benchmarks for climate model simulations used to examine human induced climate change A recent study by von Storch et al VS04 1 purported to apply as real istically as possible the m odology f Mann et al MBH 2 3 to reconstruct Northem Hemisphere surface temperatures from cli mate model output Comparin these emulated reconstructions based on pseudoproxy data constructed by adding white noise to Euro ean Centre Hamburg 4 Hamburg Ocean Primitive Equation G ECHO G surface temperatures at MBH proxy sites to the actual model temper atures VS04 found that the MBH style recon structions underestimated the amplitude of true simulated northern hemisphere temperatures by a factor ofup to three or more figure 2A in 1 the exact factor depends on the amount of noise included in the pseudoproxies VS04 us reasoned that MBH could have systematically underestimated past temperature excursions by similar factors This critique has assumed political importance being cited in a congres sional inquiry concerning the MBH reconstruc tion 4 It has gone unnoted that the VS04 analysis differed critically from the procedures used by MBH which bears directly on the validity of the VS04 critique MBH see Fig 1A calibrated proxies against time series of dominant in tem er atures patterns over 1902 to 1980 in a procedure guaranteeing by construction retention of sam ple mean and variance and thus the calibration period trend 2 3 MBH additionally validated the reconstructions ove an indepen ent time span 1854 to 1901 called the verification jEnvironmental Studies and Geology Division Science Center Alfred University Alfred NY 14802 USA 2Physics Department Stanford University Stanford CA 94305 USA 3Climate and Go a D namics Division National Center for Atmospheric Research Boulder CO 80307 USA To whom correspondence should be addressed E mail wahlealfrededu period 2 3 during which at least mean low frequency tracking of instrumental temper atures must also be demonstrated Figure 1B shows the corresponding VS04 results with two pseudoproxy based estimates of the true model temperatures The 75 noise curve is the case from VS04 gure 2A in 1 that shows proxy based reconstructions underestimating the amplitude of true ECHO G temperatures by more than a factor of three Althoughthere is strong agreement in MBH between observed and reconstructed temperatures in the 1902 to 1980 calibration period and good perform ance in capturing mean temperature during the verification period Fig 1A the results in VS04 are very different Fig 1B Large sys tematic amplitude losses appear between the reconstructed and true simulated temperatures over both the calibration and verification periods even though their temporal structures remain similar In fact the VS04 results could be closely mimicked by applying scaling factors to the ECHO G output that re ect the amounts of noise added to construct the seudoproxies factors MBH od would necessarily assimilate in calibration The systematic ampli tude losses in calibration and verification in VS04 indicate highly unsuccessful validation which would have led to dismissal of the re construction results in a real world paleoclimate analysis and clearly demonstrate a fun ental discrepancy from the MBH algorithm There fore the VS04 results 1 cannot speak to the question of whether and if so why the MBH procedure causes large losses of ow frequency on variability in climate reconstructi A later 2005 conference report by Zorita and von Storch ZV S05 5 acknowledged that VS04 had altered the MBH procedure to base their reconstructions on detrended data training the model on year to year variability ZVS05 showed results for the same analysis using non detrended data which calibrate and verify far more realistically gure 3 in 5 These results indicate still some but much smaller amplitu e loss in the MBH method at most 02 for the perfect pseudoproxy case which VS04 suggest shows loss of low frequency variance induced A by the method alone in relation to a total ex cursion of 13 over the 1000 year simulation What causes the difference in the VS04 ZVSOS results and is it indeed statistically prudent ZVS05 to use detrended data for calibration see also various experiments in 6 Calibration with detrended data arti cially dampens low frequency climate variations and largely removes effects from the mos fun en tal physical processes responsible for climatic changes The MBH reconstruction recombines spatial modes of temperature variability called empirical ortho onal functions or EOFs which in uence globalhemispheric mean tem erature e g the phase of El Nifio Southern Oscillation mostly contained in EOF2 in MBH whereas others are of more regional importance But over past centuries and the millennium and particu larly over the 20th century global and hemi spheric temperature changes are not simply due to a recombination of internal modes of variabil ity but largely result from externally imposed perturbations to the planet s energy balance 7 The 20th century warming trend at its core contains necessary information for the recon struction algorithm to identify the climate system s primary response to large scale radia tive forcing Remov39ng this physical process contained in MBH EOF1 effectively dismisses a large portion of the central physical mecha nism necessary to represent climate in both pre industrial and recent times Statistically the MBH procedure allows a century scale trend such as the radiatively in duced warming trend or a possible linear com ponent in the trend contributed by any other physical mode of variability to be mathemat ically separated from other climatic variations T e proxy series will still calibrate against and add weight to all of the EOFs retained in the reconstruction with which they have a relation ship Detrending is therefore not statistically required and in fact will arti cially dampen low frequency signals associated with any mode of variability that contributes to EOF1 in MBH The VS04 results have been interpreted to cast serious doubt on the MBH reconstruction Note that a newer method has since been presented and evaluated 8 9 However these results are in large part dependent on a de end ing step not used by MEIL which is physically inappropriate and statistically not required The e away message for the climate community should be strong encouragement for more vigor ous cross comparisons of the various recon struction implementations based on real world proxy series model emulations and simulated modifications to real world data Such a step climate change research to important scienti c and policy questions wwwrsciencemagrorg SCIENCE VOL 312 28 APRIL 2006 U k I Downloaded from wwwsciencemagorg on September 13 2007 529b