Our Common Future Under Climate Change

International Scientific Conference 7-10 JULY 2015 Paris, France

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Tuesday 7 July - 14:30-16:00 UNESCO Fontenoy - ROOM IV

L1.1 - Climate variability and change over the last millennia

Large Parallel Session

Chair(s): V. Masson-Delmotte (LSCE, Gif-sur-Yvette, France)

Co-Convener(s): G. Hegerl (University of Edinburgh, Edinburgh, United Kingdom)


Model-data comparison over the last millennium: progress, uncertainties and challenges

F. Gondalez Rouco (Instituto de Geociencias, Madrid, Spain)

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Model-data comparison over the last millennium: progress, uncertainties and challenges

F. Gondalez Rouco (1)
(1) Instituto de Geociencias, Madrid, Spain

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J. F. González-Rouco

L. Fernández-Donado

E. García-Bustamante


Present day climate variability and change, including recent anthropogenic warming, poses questions that cannot be answered based solely upon instrumental records. The last two millennia (L2k), and specifically the last millennium (LM), are immediate temporal intervals that involve climate processes similar to nowadays. The last 2k and LM have the potential to expand our understanding of climate variability from inter-annual and decadal to multi-centennial timescales, place a wider context for current warming and explore internally induced and externally forced responses of the climate system. Knowledge about proxy-based climate reconstructions, paleoclimate model simulations and estimations of external radiative forcing emerge then as key elements to gain insights about the relative roles of internal versus forced variability.

Comparisons of last millennium simulations and reconstructions constitute opportunities for learning about pre-instrumental climate variability beyond the lessons that climate simulations or reconstruction efforts can offer by themselves. Model-data comparisons provide insight about the relative roles of internal variability and external natural or anthropogenic induced changes and the processes involved. The relatively short ranges of external forcing variability within the last 2k/LM nevertheless make these comparisons challenging and further complicated by the large uncertainties that affect both reconstructions and model simulations (Masson-Delmotte et al. 2013).

This work reports on the progress of about a decade of efforts in L2k/LM model-data comparison and discusses how model-data comparison exercises focused on the last millennium can improve our understanding of decadal to multi-centennial climate variability as well as contribute to our knowledge of present and future climate and/or associated projection uncertainties. For this purpose, the available continental, hemispherical and global L2k/LM temperature reconstructions, an ensemble of simulations including both Paleoclimate Modelling Intercomparison Project Phase III / Coupled Model Intercomparison Project Phase 5 (PMIP3/CMIP5; Taylor et al 2012) and non-PMIP3 model experiments, as well as the external forcing configurations applied (Schmidt et al 2012) are analysed. In addition, for each simulation considered, a total external forcing (TEF), including all individual forcing factors, is estimated as a simple approach to compare the total radiative forcing applied to each experiment (Fernández-Donado et al., 2013).

At hemispherical and global scales, simulations and reconstructions broadly agree on the major temperature changes and suggest, despite the important influence of the internal variability, an overall linear response to external forcing above multidecadal timescales. The rate of temperature response to LM changes in TEF is quantified as a metric of the transient climate response during the LM (LMTCR) and its distribution from the model and reconstructed ensembles are compared to other estimates of climate sensitivity and transient climate response. LMTCR also allows to frame a simple quantitative comparison between simulations and reconstructions where discrepant behaviors can be singled out. The uncertainties in reconstructions and model experiments that impact our understanding of simulated and reconstruction responses at these spatial scales are also discussed.

At regional/continental scales we focus on the assessment of PMIP3/CMIP5 experiments and temperature reconstructions developed within the PAGES 2k project (PAGES 2k consortium 2013) and their responses to forcing and report on their consistency across regions and timescales. Inter-regional behavior is more homogeneous in the simulated than in the reconstructed climates. Agreement between simulations and reconstructions is higher for Northern Hemisphere regions whilst models disagree more with the reconstructions in the Southern Hemisphere.


Fernández-Donado, L. et al., 2013: Temperature response to external forcing in simulations and reconstructions of the last millennium. Climate of the Past, 9, 393-421.

Masson-Delmotte, V., M. et al., 2013: Information from Paleoclimate Archives. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T. F., D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

PAGES 2k Consortium, 2013: Continental-scale temperature variability during the past two millennia. Nature Geoscience, 6, 339-346.

Schmidt, G. A., et al., 2012: Climate forcing reconstructions for use in PMIP simulations of the last millennium,  Geosci. Model Dev., 5, 185-191.

Taylor, K. E., R. J. Stouffer, and G. A. Meehl, 2012:  An overview of CMIP5 and the experiment design. Bull. Amer. Meteor. Soc., 93.


Variability of the North Atlantic Oscillation during the past millennium

P. Ortega (1LSCE/IPSL, UMR 8212 (CEA-CNRS-UVSQ), CEA Saclay , Gif-sur-Yvette, France)

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Variability of the North Atlantic Oscillation during the past millennium

P. Ortega (1) ; F. Lehner (2) ; D. Swingedouw (3) ; V. Masson-Delmotte (4) ; CC. Raible (5) ; M. Casado (6) ; P. Yiou (7)
(1) 1LSCE/IPSL, UMR 8212 (CEA-CNRS-UVSQ), CEA Saclay , Gif-sur-Yvette, France; (2) University of Bern, , Climate and environmental physics, physics institute,, Bern, Switzerland; (3) Universite de Bordeaux, UMR CNRS 5805 EPOC, Pessac, France; (4) IPSL, Paris, France; (5) University of Bern, Climate and environmental physics, physics institute, Bern, Switzerland; (6) 1LSCE/IPSL, UMR 8212 (CEA-CNRS-UVSQ), CEA Saclay, Gif-sur-Yvette, France; (7) Laboratoire des Sciences du Climat et de l'Environnement, Saclay, France

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The North Atlantic Oscillation (NAO) is the dominant mode of winter atmospheric circulation variability in the Northern Hemisphere. This atmospheric mode is characterized by a changing dipole of sea-level pressure between the Azores and Iceland, and has widespread impacts on temperature, precipitation, storm tracks and therefore on strategic sectors such as insurance, renewable energy production, crop yields and water management.

Recent developments of dynamical methods offer promising advances for seasonal NAO predictions. However, assessing potential predictability at multi-annual time scales requires a documentation of past NAO low-frequency variability. A recent bi-proxy NAO reconstruction spanning the last millennium suggests that long-lasting positive NAO conditions were established during medieval times, explaining the particularly warm conditions over Europe; however, this result is still debated. Here, we present a new annually-resolved NAO reconstruction for the last millennium based on an initial selection of 48 proxy records distributed around the Atlantic Ocean and surrounding continents and built through an ensemble of multivariate regressions. This approach has been validated in perfect model analyses, using climate simulations as physically consistent surrogates of the real world. The analysis makes evident that the multi-proxy reconstruction outperforms the bi-proxy index.

The final reconstruction shows no persistent positive NAO during the medieval period, but suggests that positive phases were dominant during the thirteenth and fourteenth centuries. It also reveals that a positive NAO emerges two years after strong volcanic eruptions, consistent with results obtained from models and satellite observations for the Mt Pinatubo eruption in the Philippines.


European summer hydroclimate variability during the last millennium

H. Linderholm (University of Göteborg, Göteborg, Sweden)

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European summer hydroclimate variability during the last millennium

H. Linderholm (1) ; K. Seftigen (2) ; E. Cook (3) ; D. Chen (4) ; T. Ou (4)
(1) University of Göteborg, Institute of geography, Göteborg, Sweden; (2) Swiss Federal Research Institute WSL, Birmensdorf, Switzerland; (3) Columbia University, Lamont-doherty earth observatory, Columbia, United States of America; (4) Gothenburg University, Department of earth sciences, Gothenburg, Sweden

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Associated with global warming, changes in extreme weather and climate events have been observed in Europe, including increased frequencies of heat waves as well as the frequency or intensity of heavy precipitation events. In a future warmer world, it is likely that the risk of hydroclimatological extremes will increase. Extreme hydroclimate events, such as droughts and floods, can have significant impacts on society, e.g. by affecting food availability, water quality, health, energy, infrastructure etc., but also on ecosystems. It is apparent that climate variability and change already pose a challenge to Europe’s economic sectors, production systems, and ecosystems. Increased drought frequency will significantly affect natural and human systems, and compared to other hazards, hey can persist for long periods and affect large areas. Floods, associated with heavy precipitation events, can pose threats to human life and property, and also affect water quality, e.g. by spreading pollutants and fertilizers.


Clearly, society must prepare for an intensification of hydroclimate extremes in the future. However, major uncertainties and knowledge gaps still exist in understanding and modeling hydroclimate, making it difficult to quantify future changes and their impacts on systems and sectors. A prerequisite to mitigate extreme hydroclimatological events is good understanding of their spatiotemporal characteristics as well as the mechanisms generating such events. However, the lack of instrumental observations limits the period of spatial analysis to the recent century, making it difficult to fully understand natural hydroclimate variability.


Tree rings can provide annually resolved environmental and climate information and are widely used as proxies of past climatic events, such as drought or floods. The wide geographical distribution of tree-ring chronologies, compared to most other high-resolution climate proxies, provides a potential to infer past climate change on large spatial scales. Recently, past hydroclimate variability has been reconstructed from networks of moisture-sensitive tree-ring chronologies in North America and Monsoon. These reconstructions have not only provided valuable information of past hydroclimatic characteristics, but they have also contributed essential background data for increasing the understanding of the underlying mechanisms of past drought variability.


This presentation will focus on a new reconstruction of summer hydroclimate variability in Fennoscandia during the last millennium. Using tree-ring data from a dense network, a point-by-point multiple nested regression approach was used to reconstruct June through August average Standardized Precipitation Evapotranspiration Index (SPEI) with a spatial resolution of 0.5°x 0.5°. We will show that the data provides highly useful information of regional natural hydroclimate variability in time and space in a long-term context, making it possible to assess the impact of global warming on hydroclimate in Fennoscandia. It also allows for identification of historical extreme hydrological events, including severity, duration and magnitude. Moreover, we will discuss potential drivers of hydroclimate regional drought variability on different time scales. Finally, we will take a broader, European, look on summer hydroclimate variability during the last 1000 years.


Atmospheric carbon dioxide tracks climate and land carbon changes during the past millennium

T. K. Bauska (University of Cambridge, Cambridge, United Kingdom)

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Atmospheric carbon dioxide tracks climate and land carbon changes during the past millennium

T. K. Bauska (1)
(1) University of Cambridge, Department of earth sciences, Cambridge, United Kingdom

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Co-authors :

Fortunat Joos (Climate and Environmental Physics, Physics Institute and Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland)

Alan C. Mix (College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331)

Raphael Roth (Climate and Environmental Physics, Physics Institute and Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland)

Jinho Ahn (School of Earth and Environmental Science, Seoul National University, Seoul 151-742, Korea)

Edward J. Brook (College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331)



The land carbon reservoir is predicted to turn into a net source of carbon to the atmosphere if global warming continues unabated. Multi-decadal, global-scale observations needed to test this predication are difficult adding uncertainties to projections of atmospheric CO2 and climate.  Ice core records of the last millennium document atmospheric CO2 variations on multi-decadal to centennial timescales but attempts to constrain the underlying drivers of atmospheric CO2  using the stable isotopic composition of atmospheric carbon dioxide (δ13C-CO2) have been limited by the precision and temporal resolution of existing data.  This spurred discussion on the magnitude of climate-carbon feedbacks and emissions from past anthropogenic land use change . 

We developed a new high-resolution, high-precision ice core record of δ13C-CO2 and use it to show that terrestrial organic carbon likely controlled multi-decadal scale atmospheric CO2 variability from 760-1850 C.E.  Our results put strong limits on the net source of land carbon to the atmosphere prior to the Industrial period. If significant long-term carbon emissions came from pre-industrial anthropogenic land-use changes, they must have been offset by some natural 13C depleted land sink, plausibly peatlands.  On multi-decadal timescales, carbon cycle changes appear to covary with reconstructed regional climate changes, consistent with climate as an important driver of land carbon storage on these time scales. 

Our new observations present a challenging benchmark for models attempting to simulate the climate and carbon cycle of the past in order to understand the projections for the future.   However, reducing the uncertainties in past temperature reconstructions and developing stronger constraints on pre-Industrial anthropogenic emissions are likely needed to provide further insight into climate-carbon cycle interactions.


Q&A session

V. Masson-Delmotte (LSCE, Gif-sur-Yvette, France)

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Q&A session
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