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Journal ArticleDOI

Improved spectral comparisons of paleoclimate models and observations via proxy system modeling: Implications for multi-decadal variability

TL;DR: In this paper, a forward proxy modeling approach coupled with an isotope-enabled GCM is proposed to disentangle the various contributions to signals embedded in ice cores, speleothem calcite, coral aragonite, tree-ring width, and tree cellulose, and conclude that the paleoclimate record may exhibit larger low-frequency variability than GCMs currently simulate, indicative of incomplete physics and/or forcings.
About: This article is published in Earth and Planetary Science Letters.The article was published on 2017-10-15 and is currently open access. It has received 39 citations till now. The article focuses on the topics: Climate model.

Summary (2 min read)

2.1. GCM & PSM-Generated Pseudoproxies

  • Each proxy type employs its own unique PSM.
  • The complicated nature of proxy data (e.g. chronological uncertainties and impacts on phasing) precludes point-to-point comparisons of time series, and thus there is a strong case for comparing simulated proxy to the observations in the frequency domain.

3. Case Studies

  • Various approaches including downscaling or bias correction can help to minimize such problems, or paleoclimate data can be aggregated to match GCM grid cell size.
  • For each proxy type, the authors attempt to answer whether the mismatch arises from a lack of low-frequency variability simulated by the GCM SPEEDY-IER, or from a data-model comparison strategy problem.
  • For completeness, the authors report absolute variance for all case studies and the PAGES2k data in SI Section S3.

3.1. Spectral Fingerprinting of Proxy Systems

  • As a first pass, the authors forced each PSM with white noise climate inputs to assess the impact of proxy system processes alone on the shape of the spectra.
  • For ice cores, speleothems, and tree ring widths, the white noise +.
  • For all proxy types, the spectra revert to the shape of the white input climate signal on decadal and longer timescales.
  • Under different PSM formulations these spectra could change significantly, and this non-unicity proves a large source of uncertainty.

3.2.1. Corals

  • Shows that the corals are generally strong SST proxies (or, possibly, that the GCM completely underplays salinity variability).
  • Testing the effects of parametric uncertainty for the corals provides an example of how PSMs can be used to inform data-model comparison.
  • More interestingly, discrepancies exist between the simulated and observed power spectrum on decadal to centennial timescales.
  • Further, if the authors instead evaluate both in terms of absolute variance, the Palmyra record exhibits larger σ at the decadal band as compared to the PSM-simulated data (SI Section S3).
  • While the PSM-generated pseudo-coral captures interannual SST variability similar to observations, the PSM seems not to account for the larger variance in the observations on longer timescales, and this discrepancy remains even when uncertainties in the coral's sensitivity to salinity and δ 18 O S W are taken into account.

3.2.2. Ice Cores

  • On decadal to centennial timescales, differences in the observed vs. simulated spectral slopes are more modest than for interannual, but three of the records tend to increasingly diverge at low frequencies (see Fig. 3 ).
  • 18 O PRECIP vs. the observed ice core values exhibit some agreement on multi-decadal frequencies, but the model does not simulate comparable variance in the observations on longer (>centennial) timescales (see Fig. 3 ).
  • This suggests that neither the GCM, the water isotope physics in the GCM, nor the PSM can account for observed low frequency variability.

3.2.3. Speleothems

  • The speleothem PSM highlights the fact that on interannual to decadal timescales, the authors can essentially obtain a β value in agreement with observations simply as a function of the karst parameters.
  • On longer timescales, the simulated spectra tend to flatten while the observed spectra continue to show increased lowfrequency variance, potentially indicative of climate processes resulting in a spectrum similar to what the authors would expect from a power law system (see Fig. 5 ).

3.2.5. Tree Ring Width

  • Aggressive detrending methods tend to remove low frequency variability (demonstrated by Table 2 ).
  • Table 2 also illustrates the RCS method is most conservative in maintaining low-frequency TRW variability.
  • In general, using the same detrending method for both proxy and pseudoproxy is essential.

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Journal ArticleDOI
TL;DR: In this paper, the authors examined interdecadal GMST variability in Coupled Modeling Intercomparison Projects, Phases 3, 5, and 6 (CMIP3, CMIP5, and CMIP6) preindustrial control (piControl), last millennium, and historical simulations and in observational data.
Abstract: Attribution and prediction of global and regional warming requires a better understanding of the magnitude and spatial characteristics of internal global mean surface air temperature (GMST) variability. We examine interdecadal GMST variability in Coupled Modeling Intercomparison Projects, Phases 3, 5, and 6 (CMIP3, CMIP5, and CMIP6) preindustrial control (piControl), last millennium, and historical simulations and in observational data. We find that several CMIP6 simulations show more GMST interdecadal variability than the previous generations of model simulations. Nonetheless, we find that 100‐year trends in CMIP6 piControl simulations never exceed the maximum observed warming trend. Furthermore, interdecadal GMST variability in the unforced piControl simulations is associated with regional variability in the high latitudes and the east Pacific, whereas interdecadal GMST variability in instrumental data and in historical simulations with external forcing is more globally coherent and is associated with variability in tropical deep convective regions. Plain Language Summary Ongoing and future global and regional warming will progress as a combination of internal climate variability and forced climate change. Understanding the magnitude and spatial patterns associated with internal climate variability is an important aspect of being able to predict when, where, and how climate change will be felt around the globe. Here, we show that the latest climate model simulations, which will be used in the Intergovernmental Panel on Climate Change (IPCC) Assessment Report 6 (AR6), simulate a large range in magnitudes of internal global mean temperature variability. Although there are large unforced global temperature trends in some models, we find that even the most variable models never generate unforced global temperature trends equal to the recently observed global warming trends forced by greenhouse gas emissions. We examine the regions associated with internal climate variability and forced climate change in climate model simulations and find that only forced simulations show a pattern of warming consistent with instrumental data.

49 citations


Cites background from "Improved spectral comparisons of pa..."

  • ...…interdecadal GMST variability (Brown et al., 2015, 2017; Parsons & Hakim, 2019), with both instrumental (Laepple & Huybers, 2014a) and paleoclimate (Dee et al., 2017; Laepple & Huybers, 2014b; Parsons et al., 2017) evidence suggesting that climate models may underestimate local, low‐frequency…...

    [...]

Journal ArticleDOI
TL;DR: In this article, a review of the literature focused on proxy-based reconstructions of climate variability during the Holocene (i.e., the last 11.7 thousand years) with a special emphasis on i) proxybased reconstruction methods; ii) available proxy based reconstruction of the main modes of variability, i.e. El Nino Southern Oscillation, Pacific Decadal Variability, Atlantic Multidecadal Vectors, the North Atlantic Oscillations, the Southern Annular Mode and the Indian Ocean Dipole; iii) major interactions between these modes;

44 citations

Journal ArticleDOI
TL;DR: A replicated reconstruction of sea-surface temperature and salinity from a site sensitive to North Atlantic circulation in the Gulf of Mexico which reveals pronounced centennial-scale variability over the late Holocene and reveals that weakened surface-circulation in the Atlantic Ocean was concomitant with well-documented rainfall anomalies in the Western Hemisphere during the Little Ice Age.
Abstract: Surface-ocean circulation in the northern Atlantic Ocean influences Northern Hemisphere climate. Century-scale circulation variability in the Atlantic Ocean, however, is poorly constrained due to insufficiently-resolved paleoceanographic records. Here we present a replicated reconstruction of sea-surface temperature and salinity from a site sensitive to North Atlantic circulation in the Gulf of Mexico which reveals pronounced centennial-scale variability over the late Holocene. We find significant correlations on these timescales between salinity changes in the Atlantic, a diagnostic parameter of circulation, and widespread precipitation anomalies using three approaches: multiproxy synthesis, observational datasets, and a transient simulation. Our results demonstrate links between centennial changes in northern Atlantic surface-circulation and hydroclimate changes in the adjacent continents over the late Holocene. Notably, our findings reveal that weakened surface-circulation in the Atlantic Ocean was concomitant with well-documented rainfall anomalies in the Western Hemisphere during the Little Ice Age.

40 citations

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TL;DR: In this article, the authors proposed a geosciences at the University of Arizona using the Kartchner Caverns scholarship fund and the National Science Foundation EaSM2 grant.
Abstract: National Science Foundation EaSM2 Grant [AGS-1243125]; Directorate for Geosciences [3008610]; Graduate Research Fellowship [DGE-1143953]; Kartchner Caverns scholarship fund; Department of Geosciences at the University of Arizona

36 citations

References
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TL;DR: In this article, the authors introduce the principles, benefits and caveats of using climate models with embedded water isotopes as a support for the interpretation of isotopic climate archives, and a short overview of the limitations of empirical calibrations of isotope proxy records is presented.
Abstract: . Stable water isotopes have been measured in a wide range of climate archives, with the purpose of reconstructing regional climate variations. Yet the common assumption that the isotopic signal is a direct indicator of temperature proves to be misleading under certain circumstances, since its relationship with temperature also depends on e.g. atmospheric circulation and precipitation seasonality. Here we introduce the principles, benefits and caveats of using climate models with embedded water isotopes as a support for the interpretation of isotopic climate archives. A short overview of the limitations of empirical calibrations of isotopic proxy records is presented. In some cases, the underlying hypotheses are not fulfilled and the calibration contradicts the physical interpretation of isotopic fractionation. The simulation of climate and its associated isotopic signal, despite difficulties related to downscaling and intrinsic atmospheric variability, can provide a "transfer function" between the isotopic signal and the considered climate variable. The relationship between modelled temperature and isotopic signal is analysed under present-day, pre-industrial and mid-Holocene conditions. The linear regression relationship is statistically more significant for precipitation-weighted annual temperature than mean annual temperature, yet the regression slope varies greatly between the time-slice experiments. Temperature reconstructions that do not account for the slope variations will in this case underestimate the low-frequency variability and overestimate high-frequency variability from the isotopic proxy record. The spatial variability of the simulated δ18O-temperature slope further indicates that the isotopic signal is primarily controlled by synoptic atmospheric circulation rather than local temperature.

157 citations

Journal ArticleDOI
TL;DR: In this article, a linear correlation between the first principal component of the winter season stable isotope data and Greenland winter temperatures was found to be 0.71 for seasonally resolved data and 0.83 for decadally filtered data.

154 citations

Journal ArticleDOI
TL;DR: In this article, the spectral characteristics of temperature and precipitation fluctuations in observations, model simulations and proxy records across the globe were assessed, and it was shown that the observed dominance of inter-annual precipitation fluctuations is not reflected in the annually resolved hydroclimatic proxy records.
Abstract: Seamless quantification of past and present climate variability is needed to understand the Earth’s climate well enough to make accurate predictions for the future. This study addresses whether tree-ring-dominated proxy data properly represent the frequency spectrum of true climate variability. The results challenge the validity of detection and attribution investigations based on these data. External forcing and internal dynamics result in climate system variability ranging from sub-daily weather to multi-centennial trends and beyond1,2. State-of-the-art palaeoclimatic methods routinely use hydroclimatic proxies to reconstruct temperature (for example, refs 3, 4), possibly blurring differences in the variability continuum of temperature and precipitation before the instrumental period. Here, we assess the spectral characteristics of temperature and precipitation fluctuations in observations, model simulations and proxy records across the globe. We find that whereas an ensemble of different general circulation models represents patterns captured in instrumental measurements, such as land–ocean contrasts and enhanced low-frequency tropical variability, the tree-ring-dominated proxy collection does not. The observed dominance of inter-annual precipitation fluctuations is not reflected in the annually resolved hydroclimatic proxy records. Likewise, temperature-sensitive proxies overestimate, on average, the ratio of low- to high-frequency variability. These spectral biases in the proxy records seem to propagate into multi-proxy climate reconstructions for which we observe an overestimation of low-frequency signals. Thus, a proper representation of the high- to low-frequency spectrum in proxy records is needed to reduce uncertainties in climate reconstruction efforts.

130 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present one millennium-long (1171-year) and three 100 year long annually resolved tree-ring chronologies from ecologically varying Juniperus stands in the Karakorum Mountains (northern Pakistan), and evaluate their response to climatic and atmospheric CO2 changes.

129 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a comprehensive modeling framework for proxy systems, named PRYSM, which is based on water-isotope based climate proxies in ice cores, corals, tree ring cellulose, and speleothem calcite.
Abstract: Paleoclimate observations constitute the only constraint on climate behavior prior to the instrumental era. However, such observations only provide indirect (proxy) constraints on physical variables. Proxy system models aim to improve the interpretation of such observations and better quantify their inherent uncertainties. However, existing models are currently scattered in the literature, making their integration difficult. Here, we present a comprehensive modeling framework for proxy systems, named PRYSM. For this initial iteration, we focus on water-isotope based climate proxies in ice cores, corals, tree ring cellulose, and speleothem calcite. We review modeling approaches for each proxy class, and pair them with an isotope-enabled climate simulation to illustrate the new scientific insights that may be gained from this framework. Applications include parameter sensitivity analysis, the quantification of archive-specific processes on the recorded climate signal, and the quantification of how chronological uncertainties affect signal detection, demonstrating the utility of PRYSM for a broad array of climate studies.

125 citations

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Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "Improved spectral comparisons of paleoclimate models and observations via proxy system modeling: implications for multi-decadal variability" ?

In this paper the authors bridge this gap via a forward modeling approach, coupled to an isotope-enabled GCM. The paper addresses the following questions: ( 1 ) do forward modeled “ pseudoproxies ” exhibit variability comparable to proxy data ? The authors apply their method to representative case studies, and parlay these insights into an analysis of the PAGES2k database ( ? ). The authors conclude that, specific to this set of PSMs and isotope-enabled model, the paleoclimate record may exhibit larger low-frequency variability than GCMs currently simulate, indicative of ∗Corresponding author Email addresses: sylvia 11 dee @ brown. The authors find that current proxy system models ( PSMs ) can help resolve model-data discrepancies on interannual to decadal timescales, but can not account for the mismatch in variance on multi-decadal to centennial timescales.