Climate: past ranges and future changes

Comparison of large-scale temperature reconstructions over the past millennium reveals agreement on major climatic episodes, but substantial divergence in reconstructed (absolute) temperature amplitude. We here detail several research priorities to overcome this ‘amplitude desideratum’, and discuss the relevance of this effort for the prediction of future temperature changes and the meaning of the Kyoto protocol. r 2005 Elsevier Ltd. All rights reserved. Persisting controversy (Regalado, 2005) surrounding a pioneering northern hemisphere temperature reconstruction (Mann et al., 1999) indicates the importance of such records to understand our changing climate. Such reconstructions, combining data from tree rings, documentary evidence and other proxy sources are key to evaluate natural forcing mechanisms, such as the sun’s irradiance or volcanic eruptions, along with those from the widespread release of anthropogenic greenhouse gases since about 1850 during the industrial (and instrumental) period. We here demonstrate that our understanding of the shape of long-term climate fluctuations is better than commonly perceived, but that the absolute amplitude of temperature variations is poorly understood. We argue that the knowledge of this amplitude is critical for predicting future trends, and detail four research priorities to solve this incertitude: (i) reduce calibration uncertainty, (ii) preserve ‘colour’ in proxy data, (iii) utilize accurate instrumental data, and (iv) update old and develop new proxy data. When matching existing temperature reconstructions (Jones et al., 1999; Mann et al., 1999; Briffa, 2000; Esper e front matter r 2005 Elsevier Ltd. All rights reserved. ascirev.2005.07.001 ing author. Tel.: +411 739 2510; fax: +41 1 739 2215. ess: [email protected] (J. Esper). et al., 2002; Moberg, et al., 2005) over the past 1000 years, although substantial divergences exist during certain periods, the timeseries display a reasonably coherent picture of major climatic episodes: ‘Medieval Warm Period’, ‘Little Ice Age’ and ‘Recent Warming’ (Fig. 1). However, when calibrated against instrumental temperature records, these same reconstructions splay outwards with temperature amplitudes ranging from 0.4 to 1.0 1C for decadal means (Moberg et al., 2005). Further, a comparison of commonly used regression and scaling approaches shows that the reconstructed absolute amplitudes easily vary by over 0.5 1C, depending on the method and instrumental target chosen (Esper et al., 2005). Overall, amplitude discrepancies are in the order of the total variability estimated over the past millennium, and undoubtedly confuse future modelled temperature trends via parameterisation uncertainties related to inadequately simulated behaviour of past variability. Solutions to reduce calibration uncertainty include the use of pseudo-proxy experiments (Osborn and Briffa, 2004; von Storch et al., 2004) derived from ensemble simulations of different models (Knutti et al., 2002; Stainforth et al., 2005) to test statistical calibration methods, e.g. principal component (Cook et al., 1994) and timescale-dependent (Osborn and Briffa, 2000)