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

古气候重建扩展了我们的知识，使我们了解在广泛的测量仪器网络出现之前，气候是如何变化的。这些重建是建立在对理论和统计关联的理解的基础上的，这些关联是通过比较古气候代用物的平行行为和变化气候的测量而获得的。基于这种理解，对过去气候变化的推断必然假设，我们现在观察到的关联在进行重建的整个时期都是正确的。这就是均变说原理的本质。在世界北部的一些地区，最近对树木生长的观测和测量的温度趋势在最近几十年似乎出现了分歧，即所谓的“分歧”现象。人们有很多猜测，也提出了许多理论来解释为什么这些地区树木生长的温度敏感性明显被打破了。散度的存在使人们对支持许多重要的基于树木年轮的（树木气候）重建的均变论假设产生了怀疑。它表明，过去某些时期，特别是中世纪时期的变暖程度可能被低估了，或者至少受到比目前所接受的更大的不确定性的影响。由于缺乏对这一现象的清晰概述和普遍接受的原因，一些人对目前的科学共识提出了挑战，这一共识在2007年IPCC的报告中得到了体现，该报告指出，从过去1300年的代理证据（主要来自树木）来看，20世纪末半球平均变暖可能是前所未有的。该项目将寻求系统地重新评估和量化北半球周围许多树木年轮数据集的差异证据。它将建立一个更清晰的理解发散现象的本质，特征的空间模式和时间演变。根据提议者最近发表和未发表的工作，很明显，在可能的解释中，最重要的是需要考虑到用于构建许多树木年轮年表的方法中潜在的系统性偏见，包括许多被认为表现出这种现象的年表。该方案的设计基于最近的树轮年表制作技术的创新，该技术也是由提案人开发的。这些新方法将产生树木年轮年表，其可变性是无偏的，无论是通过组成样本系列的年龄结构的时间变化，还是通过使用以前应用的技术时可能出现的数据失真。广泛的重新处理和改进的数据集将构成许多详细的、逐地点比较当地气候和各种树木生长参数的基础，以便重新描述气候/生长关联的性质、强度和时间稳定性。这将代表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，该项目将利用统计和过程建模技术，探索所有当前的理论，以找出分歧的原因。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具有不同空间细节水平的重要气候重建，并仔细评估新特征的散度效应对重建不确定性的影响。该项目将提供的结果将为国际科学辩论提供信息，特别是为基于树木年轮的气候重建的可靠性提供广泛的公众认知，以及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.