Michael Mann Dr. Michael Mann Department of Environmental Sciences University of Virginia USA If you would like to interview this scientist, please contact Leah Christen. > biographical information > abstract > paper

Biographical Information:

Dr. Michael E. Mann teaches in the Department of Environmental Sciences at the University of Virginia. He received his Ph.D. in Geology & Geophysics from Yale University. Current research includes paleoclimate reconstruction using climate “proxy” data networks, and model/data comparisons aimed at understanding the long-term behavior of the climate system and its relationship with possible external “forcings”' of climate.

Dr. Mann was a Lead Author on Chapter 2 of the IPCC Third Scientific Assessment Report, Organizing Committee Chair for the National Academy of Sciences 'Frontiers of Science' and Committee Member or Advisor to other NAS panels. He served as Editor for the 'Journal of Climate', has been a member of numerous international and U.S. scientific advisory panels and steering groups, and has been the recipient of several fellowships and prizes, including selection as one of the 50 leading visionaries by Scientific American, and the NOAA outstanding publication award.

Abstract:

Insights from Comparing Empirically-estimated and Modeled Climate Change in Past Centuries

Both reconstructions from climate 'proxy' data (e.g., tree rings, ice cores, corals) and climate model simulations, suggest that late 20th century warmth is anomalous in the context of the past 1,000-2,000 years. Various alternative reconstructions differ in their details however. Many of these differences appear to be related to issues of seasonality and spatial representativeness. Statistical methodologies for reconstructing past large-scale temperatures from proxy data have now been tested using a long forced simulation of the NCAR CSM 1.4 coupled model. Analyses of synthetic 'proxy' networks produced from the model suggest that existing proxy-based climate reconstructions are likely to yield reliable estimates of past temperature variations within estimated uncertainties. Important differences between estimates of extratropical and full (combined tropical and extratropical) hemispheric mean temperature changes in past centuries appear consistent with seasonal and spatially-specific responses to climate forcing. Forced changes in large-scale atmospheric circulation such as the NAO, and internal dynamics related to El Nino, may play an important role in explaining regional patterns of variability and change in past centuries.

Paper:

Insights from Comparing Empirically-Estimated and Modeled Climate Change in Past Centuries

Overview
Several research groups have developed annually-resolved reconstructions of Northern Hemisphere mean temperature changes over the past 500-2000 years suggesting anomalous late 20th century warmth at hemispheric or global scales. These reconstructions are generally based on the use of a variety of different types of proxy data, and a variety of statistical approaches (see Jones and Mann, 2004 for a review). In addition, a large number of simulations of Northern Hemisphere mean temperature spanning the past millennium or longer have been performed over the past few years using the full hierarchy of available climate models and estimates of past natural and anthropogenic radiative forcing histories (Jones and Mann, 2004). The comparison of empirical reconstructions with model simulations (Figure 1) demonstrates that natural factors appear to explain the major surface temperature changes of the past millennium reasonably well, at least through to the 19th century. Only anthropogenic forcing of climate, however, can explain the recent anomalous large-scale warming in the late-20th century.

Testing of Statistical Methods
The statistical approaches commonly used to reconstruct hemispheric temperature series from proxy data were recently tested by Mann et al (2005b) using networks of synthetic or 'pseudoproxy' data derived from a simulation of the past millennium (AD 850-1999) with the NCAR coupled ocean-atmosphere model. The experiments closely followed a previous study by other researchers (von Storch et al, 2004). By contrast with this latter study, however, the NCAR model was forced with more conventional estimates of radiative forcing histories, and the model's surface temperature field was corrected for any long-term 'drift' artifacts. From these experiments, Mann et al (2005b) showed (Figure 2) that commonly used statistical approaches, applied to networks of pseudoproxies constructed to have properties similar to actual proxy data, display no evidence of any systematic underestimate of long-term variations. Further experiments are underway to investigate how von Storch et al (2004) come to somewhat different conclusions from this.

Role of 'Modes' of Climate Variability
The substantial cooling in large parts of Europe during the late 17th and early 18th century (i.e., the height of the so-called 'Little Ice Age') appears closely related to long-term variations in the "North Atlantic Oscillation" pattern of climate variability which influences the pattern of winds in mid-latitudes during the Northern Hemisphere winter. These variations appear to have been forced by a combination of solar and volcanic radiative forcing (Shindell et al, 2003).

A recent study by Mann et al (2005a) finds that long-term variability in El Niño over the past 1000 years appears to have resulted from a combination of solar and volcanic forcing, based on numerical experiments employing the Zebiak-Cane (Zebiak and Cane, 1987) model of the tropical Pacific ocean and atmosphere. Previous findings suggesting a statistically significant tendency towards El Niño conditions in response to past volcanic radiative forcing are reproduced in the model experiments. These results imply that forced changes in El Niño have likely played an important role in patterns of climate variability in past centuries, leading to a non-intuitive, offsetting pattern of cooling of large parts of the tropical Pacific during the so-called 'Medieval Warm Period' of AD 800-1200 when mean hemispheric conditions (e.g. Figure 1) were relatively mild, and warming during the 'Little Ice Age' of approximately AD 1400-1900, when hemispheric conditions were relatively cold.



Figure 1: Model-based estimates of Northern Hemisphere temperature variations over the past two millennia (from Jones and Mann, 2004). Series have been smoothed to highlight variations on timescales greater than 40 years. The simulations employ a range of climate models with differing sensitivities to radiative forcing, and employ various different estimates of radiative forcing histories including natural (solar+volcanic) and modern anthropogenic (greenhouse gas and sulfur aerosol) impacts. Shown for comparison is the instrumental northern hemisphere record 1856-2003, and the proxy-based estimate of Mann and Jones (see Jones and Mann, 2004) with its 95% uncertainty band (yellow shading).



Figure 2: "Pseudoproxy" reconstruction of Northern Hemisphere mean temperature compared with actual (NCAR model) Northern Hemisphere series (from Mann et al, 2005b). The Reconstruction is based on the use of a spatial climate field reconstruction approach (see Rutherford et al, 2005. A signal-to-noise ratio ('SNR') of 1, similar to that estimated for actual proxy data, was chosen in the experiment shown. Similar results were obtained for lower SNR values. As shown, similar results are shown using both long (1856-1980) and short (1900-1980) modern calibration intervals. Yellow shading indicates 95% certainty band.

References

Jones, P.D., Mann, M.E., Climate Over Past Millennia, Reviews of Geophysics, 42, RG2002, doi:10.1029/2003RG000143, 2004.

Mann, M.E., Cane, M.A., Zebiak, S.E., Clement, A. Volcanic and Solar Forcing of the Tropical Pacific Over the Past 1000 Years, Journal of Climate, 18, 447-456, 2005a.

Mann, M.E., Rutherford, S., Wahl, E. and Ammann, C., Testing the Fidelity of Methods Used in Proxy-Based Reconstructions of Past Climate, Journal of Climate (in press), 2005b.

Mann, M.E., Jones, P.D. Global Surface Temperatures over the Past two Millennia, Geophysical Research Letters, 30 (15), 1820, doi: 10.1029/2003GL017814, 2003.

Rutherford, S., Mann, M.E., Osborn, T.J., Bradley, R.S., Briffa, K.R., Hughes, M.K., Jones, P.D., Proxy-based Northern Hemisphere Surface Temperature Reconstructions: Sensitivity to Methodology, Predictor Network, Target Season and Target Domain, Journal of Climate (in press), 2005.

Shindell, D.T., Schmidt, G.A., Miller, R., Mann, M.E., Volcanic and Solar forcing of Climate Change During the Pre-Industrial era, Journal of Climate, 16, 4094-4107, 2003.

Von Storch, H., E. Zorita, J.M. Jones, Y. Dimitriev, F. Gonzalez-Rouco, F., and S.F.B. Tett, 2004: Reconstructing Past Climate from Noisy Data, Science, 306, 679-682.

Zebiak, S. E., Cane, M.A., A model El Niño-Southern Oscillation, Monthly Weather Review, 115, 2262-2278, 1987.