The dendroclimatic divergence phenomenon: reassessment of causes and implications for climate reconstruction

古气候重建扩展了我们在测量仪器广泛网络可用之前气候如何变化的知识。这些重建是建立在理解的理论和aerodically派生协会通过比较古气候代理和测量不同的气候的平行行为。基于这种理解，对过去气候变化的推断必然假定，我们现在观察到的关联在整个重建时期都是正确的。这就是均变论原理的本质。在世界上的某些北方地区，最近对树木生长的观测和对温度趋势的测量似乎在最近几十年出现了分歧，即所谓的“分歧”现象。有很多猜测和许多理论提出，以解释为什么以前的温度敏感性树木生长在这些地区显然是打破。分歧的存在使人怀疑均变论的假设，支持了一些重要的树木年轮为基础的（树木气候）重建。它表明，过去某些时期的温暖程度，特别是在中世纪时期，可能被低估，或者至少比目前接受的更大的不确定性。由于缺乏对这一现象的明确概述，也缺乏普遍接受的原因，一些人对目前的科学共识提出了挑战，这一共识体现在政府间气候变化专门委员会（IPCC）2007年的报告中，该报告认为，在过去1300年的替代证据（主要来自树木）的背景下，20世纪世纪末的平均半球温暖度可能是前所未有的。该项目将寻求系统地重新评估和量化北方半球许多树木年轮数据集差异的证据。它将建立一个更清楚的理解的分歧现象的性质，特点的空间模式和时间演变。基于最近发表和未发表的工作的提议者，它已经变得很明显，最重要的可能的解释是需要考虑系统的偏见可能固有的方法来建立许多年轮年表，包括许多被认为表现出这种现象。这一提议旨在建立在树木年轮年表制作技术的最新创新基础上，这些技术也是提议者开发的。这些新方法将产生树木年轮年表，其变异性是无偏的，无论是由组成样本系列的年龄结构的时间变化，或任何失真的数据，可能会出现时，使用以前应用的技术。然后，经过广泛再处理和改进的数据集将成为对当地气候和各种树木生长参数进行许多详细逐点比较的基础，以便重新确定气候/生长关联的性质、强度和时间稳定性。这将是对不同森林环境中差异的证据进行系统和客观的重新评估。然后，该项目将探索所有目前的理论，采用统计和过程建模技术的差异的原因（S）。该项目将继续使用重新处理的树木年轮数据集重新校准许多重要的气候重建，具有不同程度的空间细节，并仔细评估新表征的发散效应对重建不确定性的影响。该项目将提供的结果，将通知国际科学辩论和广泛的公众对基于树木年轮的气候重建的可靠性的看法，特别是，而且我们目前对全新世晚期高分辨率温度变化的现有证据的可靠性的理解。

Palaeoclimate reconstructions extend our knowledge of how climate varied in times before expansive networks of measuring instruments became available. These reconstructions are founded on an understanding of theoretical and statistically-derived associations acquired by comparing the parallel behaviour of palaeoclimate proxies and measurements of varying climate. Inferences about variations in past climate, based on this understanding, necessarily assume that the associations we observe now hold true throughout the period for which reconstructions are made. This is the essence of the uniformitarian principle. In some northern areas of the world, recent observations of tree growth and measured temperature trends appear to have diverged in recent decades, the so called 'divergence' phenomenon. There has been much speculation, and numerous theories proposed, to explain why the previous temperature sensitivity of tree growth in these areas is apparently breaking down. The existence of divergence casts doubt on the uniformitarian assumption that underpins a number of important tree-ring based (dendroclimatic) reconstructions. It suggests that the degree of warmth in certain periods in the past, particularly in medieval times, may be underestimated or at least subject to greater uncertainty than is currently accepted. The lack of a clear overview of this phenomenon and the lack of a generally accepted cause had led some to challenge the current scientific consensus, represented in the 2007 report of the IPCC on the likely unprecedented nature of late 20th century average hemispheric warmth when viewed in the context of proxy evidence (mostly from trees) for the last 1300 years. This project will seek to systematically reassess and quantify the evidence for divergence in many tree-ring data sets around the Northern Hemisphere. It will establish a much clearer understanding of the nature of the divergence phenomenon, characterising the spatial patterns and temporal evolution. Based on recent published and unpublished work by the proposers, it has become apparent that foremost amongst the possible explanations is the need to account for systematic bias potentially inherent in the methods used to build many tree-ring chronologies including many that are believed to exhibit this phenomenon. This proposal is designed to build on recent innovations in tree-ring chronology production techniques, also developed by the proposers. These new methods will produce tree-ring chronologies whose variability is unbiased, either by temporal changes in the age structure of the constituent sample series, or by any distortion in the data that can arise when using the previously applied techniques. The extensive reprocessed and improved data sets will then form the basis for many detailed, site-by-site comparisons of local climate and various tree-growth parameters in order to re-characterise the nature, strength and temporal stability of the climate/growth associations. This will represent a systematic and objective re-assessment of the evidence for divergence in different forest contexts. The project will then explore all of the current theories for the cause(s) of divergence employing both statistical and process-modelling techniques. The project will go on to use the reprocessed tree-ring data sets to re-calibrate many important climate reconstructions, with varying levels of spatial detail, and carefully assess the implications of the divergence effect, as newly characterised, on reconstruction uncertainty. This project will provide results that will inform the international scientific debate and widespread public perception of the reliability of tree-ring-based climate reconstructions in particular, but also our current understanding of the reliability of current evidence for high-resolution temperature changes during the late Holocene.