CGD's Dr. Caspar Ammann

Solar Role in Climate of the Last Millennium

Ammann, C.M., F. Joos, D.S. Schimel, B.L. Otto-Bliesner, and R.A. Tomas, 2007: Solar influence on climate during the past millennium: results from transient simulations with the NCAR Climate System Model, Proc. National Academy Sci., 104, 3713-3718.

Abstract

The potential role of solar variations in modulating recent climate has been debated for many decades and recent papers suggest that solar forcing may be less than previously believed. Because solar variability before the satellite period must be scaled from proxy data, large uncertainty exists about phase and magnitude of the forcing. We used a coupled climate system model to determine whether proxy-based irradiance series are capable of inducing climatic variations that resemble variations found in climate reconstructions, and if part of the previously estimated large range of past solar irradiance changes could be excluded. Transient simulations, covering the published range of solar irradiance estimates, were integrated from 850 AD to the present. Solar forcing as well as volcanic and anthropogenic forcing are detectable in the model results despite internal variability. The resulting climates are generally consistent with temperature reconstructions. Smaller, rather than larger, long-term trends in solar irradiance appear more plausible and produced modeled climates in better agreement with the range of Northern Hemisphere temperature proxy records both with respect to phase and magnitude. Despite the direct response of the model to solar forcing, even large solar irradiance change combined with realistic volcanic forcing over past centuries could not explain the late 20th century warming without inclusion of greenhouse gas forcing. Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of greenhouse gases have dominated since the second half of the last century

Figure caption: Comparison of NCAR CSM simulations with proxy reconstructions and instrumental data. (a) Reconstructed NH average surface temperature anomalies over the past millennium. All series are as published originally and no additional scaling has been performed, but annual records have been smoothed with a 50-year-long Gaussian filter. All series are relative to 1901-1960 averages computed from original data. (b) Northern hemisphere surface temperature from the low- (green), medium- (red), and high-scaled (blue) solar forcing simulations compared with the range spanned by the annual proxy-based reconstructions. This range does not include a systematic error analysis, it only illustrates the current debate regarding the amplitude of hemispheric multidecadal to century-scale temperature variations of the past. (c) Simulated versus the instrumental (gray) record of global average surface temperature (gray, thick solid line). The time series of the low (green), medium (red), and high (blue) solar forcing experiments were smoothed by using an 11-year Gaussian filter. Anthropogenic forcings were included in the primary experiments (solid lines) but held at 1870 AD conditions in 1870-2000 AD branch experiments (dashed lines). See publication for references.

Climate in medieval times over wester N-America

Graham N.E., M.R. Hughes, C.M. Ammann, K.M. Cobb, M.P. Hoerling, D.J. Kennett, J.P. Kennett, B. Rein, L. Stott, P.E. Wigand, and T. Xu, 2007: Tropical Pacific - Mid-Latitude Teleconnections in Medieval Times. Climatic Change. doi:10.1007/s10584-007-9239-2.

Abstract

Terrestrial and marine late Holocene proxy records from the western and central US suggest that climate between approximately 500 and 1350 A.D. was marked by generally arid conditions with episodes of severe centennial-scale drought, elevated incidence of wild fire, cool sea surface temperatures (SSTs) along the California coast, and dune mobilization in the western plains. This Medieval Climate Anomaly (MCA) was followed by wetter conditions and warming coastal SSTs during the transition into the "Little Ice Age" (LIA). Proxy records from the tropical Pacific Ocean show contemporaneous changes indicating cool central and eastern tropical Pacific SSTs during the MCA, with warmer than modern temperatures in the western equatorial Pacific. This pattern of mid-latitude and tropical climate conditions is consistent with the hypothesis that the dry MCA in the western US resulted (at least in part) from tropically forced changes in winter NH circulation patterns like those associated with modern La Niña episodes. We examine this hypothesis, and present other analyses showing that the imprint of MCA climate change appears in proxy records from widely distributed regions around the planet, and in many cases is consistent with a cool medieval tropical Pacific. One example, explored with numerical model results, is the suggestion of increased westerlies and warmer winter temperatures over northern Europe during medieval times. An analog technique for the combined use of proxy records and model results, Proxy Surrogate Reconstruction (PSR), is introduced.

Figure caption: Geographically distributed prosy records that show MCA-LIA variability like see in the western US and tropical Pacific. The England winter precipitation, Bermuda Rise SST, Lake Naivasha level, and Chile flood deposition [Laguna Aculeo, after Jenny et al. (2002)] records are unsmoothed, the Alps winter temperature (Spannagel Cave), Scotland (Uamh an Tartair Cave) T/P, temperature/precipitation, Panama (Chilibrillo Cave) δ¹⁸O, Peru (core SO147-106KL) river discharge and central California PDSI records are smoothed over sliding 25-year, the Laguna Chichancanab δ¹⁸O record over 51-year windows. Values are standard deviations (σ) for the smoothed records over the period of record shown; vertical axis tick marks are separated by 1σ.

Statistical evaluation of the "warmest decade in the last millennium" statement

Li B., D. Nychka and C.M. Ammann, in press: The "Hockey Stick" and the 1990s: A statistical perspective on reconstructing hemispheric temperatures. Tellus, doi:10.111/j.1600-0870.2007.00270.x.

Abstract

The short instrumental record of about 100-150 yr forces us to use proxy indicators to study climate over long timescales. The climate information in these indirect data is embedded in considerable noise, and the past temperature reconstructions are therefore full of uncertainty, which blurs the understanding of the temperature evolution. To date, the characterization and quantification of uncertainty have not been a high priority in reconstruction procedures. Here we propose a new statistical methodology to explicitly account for three types of uncertainties in the reconstruction process. Via ensemble reconstruction, we directly obtain the distribution of decadal maximum as well as annual maximum. Our method is an integration of linear regression, bootstraping and cross-validation techniques, and it (1) accounts for the effects of temporal correlation of temperature; (2) identifies the variability of the estimated statistical model and (3) adjusts the effects of potential overfitting. We apply our method to the Northern Hemisphere (NH) average temperature reconstruction. Our results indicate that the recent decadal temperature increase is rapidly overwhelming previous maxima, even with uncertainty taken into account, and the last decade is highly likely to be the warmest in the last millennium.

Importance of the geophysical context for statistical climate reconstructions

Ammann C.M., and E.R. Wahl, 2007: Importance of the Geophysical Context for Statistical Evaluation of Climate Reconstruction Procedures. Climatic Change, doi:10.1007/s10584-007-9276-x.

Abstract

A portion of the debate about climate reconstructions of the past millennium, and in particular about the well-known Mann-Bradley-Hughes ("MBH" 1998, 1999) reconstructions, has become disconnected from the goal of understanding natural climate variability. Here, we reflect on what can be learned from recent scientific exchanges and identify important challenges that remain to be addressed openly and productively by the community. One challenge arises from the real, underlying trend in temperatures during the instrumental period. This trend can affect regression-based reconstruction performance in cases where the calibration period does not appropriately cover the range of conditions encountered during the reconstruction. However, because it is tied to a unique spatial pattern driven by change in radiative balance, the trend cannot simply be removed in the method of climate field reconstruction used by MBH on the statistical argument of preserving degrees of freedom. More appropriately, the influence from the trend can be taken into account in some methods of significance testing. We illustrate these considerations as they apply to the MBH reconstruction and show that it remains robust back to AD 1450, and given other empirical information also back to AD 1000. However, there is now a need to move beyond hemispheric average temperatures and to focus instead on resolving climate variability at the socially more relevant regional scale.

Figure caption: Correction of MBH99: our emulation of the real world proxy-based MBH99 reconstruction containing full-period proxy PC-centering corrections and omission of the Gaspé-series during 1400-1449 (solid grey line) is compared to the original MPH99 reconstruction (black line).