1. Climatic Research Unit University of East Anglia, UK
Comparisons of Observed Paleoclimate and Model-Based Studies of Climate Changes Over the Past Two Millennia
Michael E. Mann
Department of Environmental Sciences, University of Virginia, Charlottesville, VA
Keith Briffa
Climatic Research Unit
University of East Anglia, UK
Collaborators: B. Adams, C. Ammann, R. Bradley; M. Cane; A. Clement, T. Delworth; M.K. Hughes, T. Osborn; P. Jones, S. Rutherford; G. Schmidt; D. Shindell; S. Zebiak
CLIVAR 2004 Baltimore, MD
June 23, 2004

2. Mann, M. E. , Ammann, C. , Bradley, R. S. , Briffa, K. , Crowley, T. J
Mann, M.E., Ammann, C., Bradley, R.S., Briffa, K., Crowley, T.J., Hughes, M.K., Jones, P.D., Oppenheimer, M., Osborn, T.J., Overpeck, J.T., Rutherford, S., Trenberth, K.E., Wigley, T.M.L., On Past Temperatures and Anomalous late 20th Century Warmth, Eos, 84, , 2003 [MODIFIED: Boreholes—Rutherford and Mann (2004); Crowley & Lowery (updated)]

3. Greater variability in extratropics and summer!
Mann, M.E., Ammann, C., Bradley, R.S., Briffa, K., Crowley, T.J., Hughes, M.K., Jones, P.D., Oppenheimer, M., Osborn, T.J., Overpeck, J.T., Rutherford, S., Trenberth, K.E., Wigley, T.M.L., On Past Temperatures and Anomalous late 20th Century Warmth, Eos, 84, , 2003 [MODIFIED: Boreholes—Rutherford and Mann (2004); Crowley & Lowery (updated)]

4. Empirical Proxy-Based Climate Reconstructions

5. 1. Sensitivity to scaling/calibration
Osborn, T.J., and K.R. Briffa, Comments on the paper of R. N. Harris and D.S. Chapman "Mid-latitude (30'N-60'N) climate warming inferred by combining borehole temperatures with surface air temperatures", Geophysical Research Letters, 29 (16), (45)1-2, 2002.

6. 2. Sensitivity to target region and season
Comparison is difficult because each reconstruction uses a different season, region and calibration period
Example shows a comparison after re-scaling every series against annual or warm-season temperature averaged over all land north of 20°N over a common period
Figures and comparison from Briffa, Osborn and Schweingruber (2003) Global and Planetary Change paper. All series were rescaled using simple linear regression against the target temperature series and over the calibration period. All series were taken from the published records, except that the Mann1999 series was not the published NH series, but the spatial reconstructions obtained during Mann et al. (1998/9)’s procedure were subsampled to extract the reconstruction just for land boxes north of 20N, and then this time series was rescaled in the same way as all the others were. This extraction makes little difference to the results, because the full NH and the land north of 20N series in Mann et al. (1998/9) are highly correlated.
Mann1999 (land north of 20N extracted from spatial reconstructions)
Esper2002
Briffa2000
Briffa2001
Jones1998
From Briffa, Osborn and Schweingruber (2003) Glob Planet Change

7. 2. Sensitivity to target region and season
Comparison between Briffa et al. NH extratropical warm-season NH mean temperature reconstruction with RegEM reconstruction
(a) unmasked
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, Journal of Climate (in revision), 2004.

8. 2. Sensitivity to target region and season
Comparison between Briffa et al. NH extratropical warm-season NH mean temperature reconstruction with RegEM reconstruction
(a) unmasked
(b) masked
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, Journal of Climate (in revision), 2004.

9. 2. Sensitivity to target region and season
Comparison between Briffa et al. NH extratropical warm-season NH mean temperature reconstruction with RegEM reconstruction
(a) unmasked
(b) masked
(c) based on Esper et al (2002) locations only
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, Journal of Climate (in revision), 2004.

10. Documentary evidence of change in seasonal range
2. Sensitivity to target region and season
Documentary evidence of change in seasonal range
Manley/Parker
Van Engelen
Pfister
Brazdil
Wang
Figure from Jones et al. (2003) showing annual mean and the seasonal range (summer minus winter) for CET instrumental and various documentary records, all expressed as anomalies from their means. 50-year filters have been applied. Note that winters warm relative to summers over the last 150 years, which is why the seasonal range values are generally above zero (i.e., above the mean).
From Jones, Osborn and Briffa (2003) J Geophys Res

11. Model/Data Comparisons

12. Detection of “unusual” warming trends
Compare observed trends during the 20th century with the 95th percentile of pre-20th century “natural variability” trends
Observed warming over various periods ending in 2000
“Unusual” warming is detected for cases with observed warming exceeding all three 95th percentiles of “natural” variability The temperature in this example is extended summer temperature averaged over all land north of 20N.
trend
trend
trend
Three estimates of “natural” variability:
Reconstruction1 (from tree-rings with low-frequency standardisation)
Reconstruction2 (from tree-rings with
traditional standardisation)
Model (1000-yr HadCM3 control run)
trend
Long control run with the HadCM3 model, and pre-1900 reconstructions using traditionally-standardised (Briffa et al., 2002) and age-band-decomposition (Briffa et al., 2001) processing of tree-ring density network, are used to obtain estimates of the 95th percentile of the distributions of trends of various lengths (from 5-year trends up to 100-year trends). These are then compared with the observed temperature trends of various lengths ending in the year The value being analysed here is the mean temperature from April-September averaged over all land boxes north of 20N. This was chosen because we have reconstructions of this time series. The reconstructions have a shaded range of 95th percentiles, depending on how the reconstruction error term is modelled. Unusual warming is detected for all trend lengths, except for and (i.e., those starting from the peak of mid-century warmth) and even those are only undetected against the highest of the 3 natural variability estimates and assuming a red noise model for the reconstruction error term.
From unpublished analyses by Tim Osborn and Keith Briffa, using model data from Simon Tett.
Analysis from Tim Osborn (CRU, UEA, UK)

13. Natural CLIMATE FORCINGS Volcanism Solar
1000
1200
1400
1600
1800
2000
Volcanism
Solar
Natural
Crowley, T.J., Causes of Climate Change Over the Past 1000 Years, Science, , 2000.

14. Anthropogenic CLIMATE FORCINGS Industrial Aerosols Greenhouse Gases
Climate Change 2001: The Scientific Basis, Houghton, J.T., et al. (eds.), Cambridge Univ. Press, Cambridge, 2001

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

16. Northern extratropics summer temperature
Last 500 years, 10-year smoothing
HadCM3
Observations
ECHAM4-HOPE
Comparison of HadCM3 and ECHAM4-HOPE simulations of April-September temperature averaged over land north of 20N, with the observed record from Jones et al., and the age-band-decomposition reconstruction of Briffa et al. (2001) using a tree-ring density network. Both model series and the reconstruction are expressed as anomalies from their means. The observed series is adjusted so that its mean matches the reconstruction over the period
Tree-ring density reconstruction
Data comparison from Tim Osborn (CRU, UEA, UK)

17. European Winter Cooling During the Little Ice Age
Empirical
LIA winter cooling in Europe associated with an NAO trend due to solar irradiance changes, interacting w/ stratospheric atmospheric dynamics and chemistry
NASA/GISS Model
Shindell, D.T., Schmidt, G.A., Mann, M.E., Rind, D., Waple, A., Solar forcing of regional climate change during the Maunder Minimum, Science, 294, , 2001.

18. Annual average late 17th vs late 18th Century Temperature change
Proxy based
reconstruction
GCM Solar +
Volcanic patterns
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, , 2003.

19. Volcanic Radiative Summer Cooling
Ensemble average summer (Jun-Aug) surface temperature response following eruption of Pinatubo.
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, , 2003.

20. Volcanic Winter Dynamical Warming
Ensemble average winter (Dec-Feb) surface temperature response following eruption of Pinatubo.
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, , 2003.

21. Mid-Latitude Summers Greater variability?
Esper, J., Cook, E.R., Schweingruber, F.H., Low-Frequency Signals in Long Tree-Ring Chronologies for Reconstructing Past Temperature Variability, Science, 295, , 2002.

22. Mid-Latitude Summers Greater variability? 1000 1200 1400 1600 1800
2000

23. Simulated and reconstructed changes in seasonal range
Comparison of documentary-reconstructed annual temperature and seasonal range with simulated values from the “All forcings” runs of ECHAM4-HOPE and then HadCM3. For Netherlands “land” grid boxes, compared with van Engelen record.
ECHAM4-HOPE
Thick lines = documentary record
Thin lines =
HadCM3
Data comparison from Tim Osborn (CRU, UEA, UK)

24. Simulated and reconstructed changes in seasonal range
Comparison of documentary-reconstructed annual temperature and seasonal range with simulated values from the “All forcings” runs of ECHAM4-HOPE and then HadCM3. For Czech “land” grid boxes, compared with Brazdil record.
ECHAM4-HOPE
Thick lines = documentary record
Thin lines =
HadCM3
Data comparison from Tim Osborn (CRU, UEA, UK)

25. Volcanic Forcing of El Nino?
Adams, J.B., Mann, M.E., Ammann, C.M., Proxy evidence for an El Nino-like Response to Volcanic Forcing, Nature, 426, , 2003.

26. Theoretical mechanism for natural radiative forcing of El Nino
Cane, M.A., A.C. Clement, A. Kaplan, Y. Kushnir, D. Pozdnyakov, R. Seager, S.E. Zebiak, and R. Murtugudde, Twentieth-Century Sea Surface Temperature Trends, Science, 275, , 1997.

27. Theoretical mechanism for natural radiative forcing of El Nino
Combined response to Solar +Volcanic Forcing
Palymra coral isotopes (standardized to have same mean and standard deviation as Nino3 composite series)
Ensemble mean Nino3 (100 realizations of CZ model)
40 year smooth
Mann, M.E., Cane, M.A., Zebiak, S.E., Clement, A., Volcanic and Solar Forcing of El Niño Over the Past 1000 Years, in revision, 2004.

28. Theoretical mechanism for natural radiative forcing of El Nino

29. Theoretical mechanism for natural radiative forcing of El Nino
Courtesy of E.Cook

30. CONCLUSIONS
Proxy reconstructions and model simulations both suggest that late 20th century warmth is anomalous in the context of the past years
Primary source of differences between various reconstructions appear to be related to issues of seasonality and spatial representativeness
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.

31. Northern extratropics summer temperature
Last 500 years, 10-year smoothing
Last 1000 years, 20-year smoothing
HadCM3
Observations
ECHAM4-HOPE
Comparison of HadCM3 and ECHAM4-HOPE simulations of April-September temperature averaged over land north of 20N, with the observed record from Jones et al., and the age-band-decomposition reconstruction of Briffa et al. (2001) using a tree-ring density network. Both model series and the reconstruction are expressed as anomalies from their means. The observed series is adjusted so that its mean matches the reconstruction over the period
Tree-ring density reconstruction
Data comparison from Tim Osborn (CRU, UEA, UK)

32. Simulated changes in seasonal range over last 150 years
Trends in seasonal range (absolute difference between JJA and DJF temperature) from the “all forcings” simulations of ECHAM4-HOPE (=ECHO-G) and then from HadCM3. Note warming of winters relative to summers (BLUE COLOURS) over northern sea-ice and snow-cover regions. Note also the difference between North America and Eurasia, with less change (or even opposite change) over North America.
Analysis from Tim Osborn (CRU, UEA, UK)