Comments on “Reconstruction of the Extratropical NH Mean Temperature over the Last Millennium with a Method That Preserves Low-Frequency Variability”

Moberg, Anders

doi:10.1175/jcli-d-11-00404.1

Cited by 11 publications

(16 citation statements)

References 23 publications

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“…As in the case of Christiansen and Ljungqvist (2011), reconstructions using the LOC method aim to preserve lowfrequency variability, perhaps to the detriment of high frequencies (Christiansen, 2011). An overestimation of variability can not be ruled out, and some studies suggest these reconstructions may be taken as an estimation of maximum bounds for low-frequency amplitude changes during the last millennium (Tingley and Li, 2012;Moberg, 2012;Christiansen, 2012;Christiansen and Ljungqvist, 2012b). This reconstruction shows noticeably more variance at low frequencies not only in the pre-industrial period but also during the 20th century.…”

Section: Hemispheric and Global Reconstructed Temperaturesmentioning

confidence: 96%

Large-scale temperature response to external forcing in simulations and reconstructions of the last millennium

et al. 2013

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Abstract. Understanding natural climate variability and its driving factors is crucial to assessing future climate change. Therefore, comparing proxy-based climate reconstructions with forcing factors as well as comparing these with paleoclimate model simulations is key to gaining insights into the relative roles of internal versus forced variability. A review of the state of modelling of the climate of the last millennium prior to the CMIP5–PMIP3 (Coupled Model Intercomparison Project Phase 5–Paleoclimate Modelling Intercomparison Project Phase 3) coordinated effort is presented and compared to the available temperature reconstructions. Simulations and reconstructions broadly agree on reproducing the major temperature changes and suggest an overall linear response to external forcing on multidecadal or longer timescales. Internal variability is found to have an important influence at hemispheric and global scales. The spatial distribution of simulated temperature changes during the transition from the Medieval Climate Anomaly to the Little Ice Age disagrees with that found in the reconstructions. Thus, either internal variability is a possible major player in shaping temperature changes through the millennium or the model simulations have problems realistically representing the response pattern to external forcing. A last millennium transient climate response (LMTCR) is defined to provide a quantitative framework for analysing the consistency between simulated and reconstructed climate. Beyond an overall agreement between simulated and reconstructed LMTCR ranges, this analysis is able to single out specific discrepancies between some reconstructions and the ensemble of simulations. The disagreement is found in the cases where the reconstructions show reduced covariability with external forcings or when they present high rates of temperature change.

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Section: Hemispheric and Global Reconstructed Temperaturesmentioning

confidence: 96%

Large-scale temperature response to external forcing in simulations and reconstructions of the last millennium

et al. 2013

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“…However, pseudo-proxies are generated by degrading the local temperature by adding noise and therefore include assumptions about the temporal structure of the noise (Moberg, 2012;Christiansen and Ljungqvist, 2012). Christiansen and Ljungqvist (2012) discuss how the cutoff between exaggerated high-frequency variability and well reconstructed low-frequency variability depends on the degree of auto-correlation in the noise.…”

Section: Spatial Averagingmentioning

confidence: 99%

The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability

2012

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Abstract. We present two new multi-proxy reconstructions of the extra-tropical Northern Hemisphere • N) mean temperature: a two-millennia long reconstruction reaching back to 1 AD and a 500-yr long reconstruction reaching back to 1500 AD. The reconstructions are based on compilations of 32 and 91 proxies, respectively, of which only little more than half pass a screening procedure and are included in the actual reconstructions. The proxies are of different types and of different resolutions (annual, annual-to-decadal, and decadal) but all have previously been shown to relate to local or regional temperature. We use a reconstruction method, LOCal (LOC), that recently has been shown to confidently reproduce low-frequency variability. Confidence intervals are obtained by an ensemble pseudo-proxy method that both estimates the variance and the bias of the reconstructions. The two-millennia long reconstruction shows a well defined Medieval Warm Period, with a peak warming ca. 950-1050 AD reaching 0.6 • C relative to the reference period 1880-1960 AD. The 500-yr long reconstruction confirms previous results obtained with the LOC method applied to a smaller proxy compilation; in particular it shows the Little Ice Age cumulating in 1580-1720 AD with a temperature minimum of −1.0 • C below the reference period. The reconstructed local temperatures, the magnitude of which are subject to wide confidence intervals, show a rather geographically homogeneous Little Ice Age, while more geographical inhomogeneities are found for the Medieval Warm Period. Reconstructions based on different subsets of proxies show only small differences, suggesting that LOC reconstructs 50-yr smoothed extra-tropical NH mean temperatures well and that low-frequency noise in the proxies is a relatively small problem.

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“…These new reconstructions have not changed the overall picture, although some show amplitudes of variations larger than those with the largest temperature amplitudes used in AR4. This larger amplitude is mainly a result of a particular choice of calibration technique and it is debated whether the resulting reconstruction has more low-frequency variance than in the real temperature variations (Moberg, 2012;Christiansen and Ljungqvist, 2012b). On the contrary, most previous methods have been shown to potentially result in too little variance (Christiansen et al, 2009).…”

Section: Nh Temperature Reconstructionsmentioning

confidence: 99%

Comparisons of simulated and observed Northern Hemisphere temperature variations during the past millennium – selected lessons learned and problems encountered

Moberg

2013

Tellus B: Chemical and Physical Meteorology

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A B S T R A C T Comparison of simulated and reconstructed past climate variability within the last millennium provides an opportunity to aid the understanding and interpretation of palaeoclimate proxy data and to test hypotheses regarding external forcings, feedback mechanisms and internal climate variability under conditions close to those of the present day. Most such comparisons have been made at the Northern Hemispheric scale, of which a selection of recent results is briefly discussed here. Uncertainties in climate and forcing reconstructions, along with the simplified representations of the true climate system represented by climate models, limit our possibility to draw certain conclusions regarding the nature of forced and unforced climate variability. Additionally, hemispheric-scale temperature variations have been comparatively small, wherefore the last millennium is apparently not a particularly useful period for estimating climate sensitivity. Nevertheless, several investigators have concluded that Northern Hemispheric-scale decadal-mean temperatures in the last millennium show a significant influence from natural external forcing, where volcanic forcing is significantly detectable while solar forcing is less robustly detected. The amplitude of centennial-scale variations in solar forcing has been a subject for much debate, but current understanding of solar physics implies that these variations have been small Á similar in magnitude to those within recent sunspot cycles Á and thus they have not been a main driver of climate in the last millennium. This interpretation is supported by various comparisons between forced climate model simulations and temperature proxy data. Anthropogenic greenhouse gas and aerosol forcing has been detected by the end of Northern Hemispheric temperature reconstructions.

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Comments on “Reconstruction of the Extratropical NH Mean Temperature over the Last Millennium with a Method That Preserves Low-Frequency Variability”

Cited by 11 publications

References 23 publications

Large-scale temperature response to external forcing in simulations and reconstructions of the last millennium

Large-scale temperature response to external forcing in simulations and reconstructions of the last millennium

The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability

Comparisons of simulated and observed Northern Hemisphere temperature variations during the past millennium – selected lessons learned and problems encountered

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