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Climate Variability in Past Centuries: Regional and Climate Mode Response

Project Abstract

The "Little Ice Age" component of the Weather and Climate Assessment Science Initiative focuses on the uncertainty characteristics in forcing as well as climate reconstructions of past centuries. Both signal versus noise issues for forced variability as well as fundamental assumptions at the base of reconstruction techniques are assessed. Additionally, regional expressions of forced and unforced climate variability as generated by a coupled climate model are evaluated and compared to reconstructions with multi-proxy networks. New developments in climate mode response to external forcing are explored as well.


Natural External Forcing in the Climate System

  • Volcanic and solar forcing: Statistical characteristics

  • Volcanic EruptionBased on the previously identified forcing series, this component deals with the search for fingerprints in the climate system left by the external forcing. Two steps lead to increasingly comprehensive signal detection: First we restrict the search to temporal dimension in which we analyze hemispheric or global climate time series in both proxy reconstructions as well as climate model output. Second, instead of evaluating single hemispheric series, we then explore the spatial information to see if the noise can be further suppressed and if the spatial patterns are compatible with different potential physical processes that translate the forcing into a response in the climate system.

  • Spatio-temporal fingerprinting

  • The goal is to identify and describe the temporal evolution of the major external forcings (solar variations and explosive volcanism) using a comprehensive approach based on available independent proxy records. Despite uncertainty in physical basis or proxy based reconstruction, we attempt to isolate the primary characteristics and statistical properties of the forcing series.

  • Climate Mode response to external forcing

  • The Earth's climate system has a set of preferred modes of variability. Under unforced conditions, it's the inherent behavior in these modes that guide magnitude and spatio-temporal structure of climate variability. A key question for understanding both past and potential future climate is if and how these internal modes of climate respond to external radiative forcing. In the past, primary forcing factors include solar irradiance changes and explosive volcanism. In the future, it is increasingly the anthropogenic greenhouse gas signal that will impact the climate. Only recently have improved and extended proxy reconstructions offered a basis to study spatio-temporal mode responses to forcing. At the same time, coupled Ocean-Atmosphere General Circulation Models are not only ready to run over many centuries, they can now also be realistically forced with external forcing. Therefore, the impact of the regional and climate mode studies is geared both for understanding the past but also to potentially improve regional predictions in the future.

    • Centennial scale ocean variability in CSM
    • E-Asia climate response to the AD 1815 Tambora eruption
    • European circulation regimes over the last 500 years
    • Tropical climate variability in response to radiative forcing

Stationarity in the Climate System

  • Real world and simulated proxy series: Teleconnection fidelity

  • Tree RingsProxy climate records are based on a regression relationship between the proxy and particular climate variable(s) over a calibration period. Subsequently, it is a fundamental assumption in paleoclimate research that this relationship remains constant (and often linear) in time. But this issue is not limited to individual proxy representation of local climate, it often is also used for interpreting dynamical links to large-scale climate circulation, particularly since the importance of large scale climate modes has been recognized. Potentially, these so called teleconnections provide one of the strongest tools in paleoclimate research as climatic changes are thought to project onto the major modes of climate variability. So far, little research has dealt with verifying the fundamental assumption of stationarity, both for local climate as well as for teleconnetion fidelity. It is tested if in a model environment the stationarity assumption for the relationship between known proxy locations and the tropical El Nino/Southern Oscillation system uniformly holds or if the 150-year instrumental record is too short for a calibration.

  • Characterization and optimal reduction of uncertainty in climate and ecosystem reconstructions

  • Receiver Operating Characteristics analytical methodology is used to systematically explore the joint minimization of false positive and false negative identifications of modern analogs for fossil assemblages (pollen, forams, etc.). For the first time, this procedure offers a rational basis for accepting more of one kind of error to reduce the other in specific situations. These analyses are being recognized in the pollen-based paleoecological community as contributing one of the most important steps forward in application of the modern analog technique (MAT) in the last 20 years.


Education and Outreach

A teachers guide and additional in-class educational material are produced for complementing the new NCAR Mesa Lab exhibit "Climate Discovery" at the middle-school level. The hands-on material offers students and educators a stepwise introduction into proxy based climate reconstruction. The modules are designed to provide insight into climate variations during the Little Ice Age including background on the important role of solar variations and explosive volcanism for guiding past climates.


Project PI, Leads, and Staff

  • Caspar Ammann, PI, Project Lead
    Climate & Global Dynamics Division, NCAR
  • Susan Foster, Project Lead
    Education & Outreach, NCAR
  • Eugene Wahl, Project Lead
    Environmental & Societal Impacts Group, NCAR
  • Roberta Johnson
    Education & Outreach, NCAR
  • Philippe Naveau
    University of Colorado, Boulder
  • Doug Nychka
    Geophysical Statistics Project, NCAR
  • Robert Tomas
    Climate & Global Dynamics Division, NCAR

Project Collaborators

  • Brad Adams
    University of Virginia
  • James Bradbury
    University of Massachusetts, Amherst
  • Raymond Bradley
    University of Massachusetts, Amherst
  • Carlo Casty
    University of Bern, Switzerland
  • Kim Cobb
    Georgia Tech
  • Erich Fischer
    University of Bern, Switzerland
  • Nicholas Graham
    Scripps Institution of Oceanography
  • Malcolm Hughes
    University of Arizona
  • Fortunat Joos
    University of Bern, Switzerland
  • Juerg Luterbacher
    University of Bern, Switzerland
  • Michael Mann
    University of Virginia
  • Hee-Seok Oh
    University of Alberta, Canada
  • University of Alberta, Canada
    Goddard Institute for Space Studies, NASA
  • Wei-Chyung Wang
    State University of New York, Albany
  • Heinz Wanner
    University of Bern, Switzerland

For more information about this project, please contact Caspar Ammann at:, Susan Foster at:, or Eugene Wahl at:

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