Millennial-length tree-ring records: A basis for climate reconstruction and assessment of climate extremes and trends at local to global scales

Abstract

Over the last two decades, tree-rings have become one of the most important paleoclimatic archives in terms of studying high-resolution climate over the Common Era (CE) in the mid-to-high latitudes. Such analyses are crucial, not only to put current temperature and hydroclimatic changes in a longterm context, but also to assess the full range of natural and anthropogenic climate forcing. However, estimates of the first millennium remain poorly constrained due to a paucity of millennial-length treering records, causing a spatial under-representation of some regions in larger-scale climate reconstructions and increasing uncertainty in the interpretation of climatic trends and extremes. Thus, a major activity towards the assessment of long-term climate dynamics at local to regional scales is the development of new millennial-length proxy records. This thesis addresses the aforementioned topic by (i) developing two millennial-length climate reconstructions based on tree-ring width (TRW) and maximum latewood density (MXD) measurements from Pinus heldreichii trees growing in the highelevation environment at the tree line of Mount Smolikas in the Pindus Mountains in northern Greece and by (ii) assessing millennial-length temperature trends in tree-ring records in a global proxy database compiled by the PAGES 2k consortium to identify potential drivers that limit long-term temperature trends in tree-ring data. Calibration studies showed that P. heldreichii TRW is controlled by June-July precipitation and April temperatures, with differences in the climate signal of four sites arising from varying meltwater supply and exposure effects. MXD formation is controlled by summer temperatures. Climate signal strength of both parameters is particularly strong in the high-frequency spectrum, mirroring inter-annual climate variability patterns and thus allowing for a detection of climate extremes. Based on the standardized precipitation index and TRW, regional summer drought variability is reconstructed over the period 730-2015 CE. MXD of the trees is translated in an August- September temperature reconstruction covering the period 738-2014 CE. Regional high-resolution, annually resolved climate information is extended for the first time back into the first millennium, thus both reconstructions fill a temporal and spatial gap in a larger-scale hydroclimate and temperature reconstruction network in Europe. The records help to advance our understanding of climate variability over Europe, associated atmospheric processes, and external forcings. The PAGES 2k multiproxy database offers a new and unique opportunity to study the lack of long-term cooling trends in tree-ring data which can be expected in Northern Hemisphere summers, particularly in the high latitudes, due to orbitally driven changes in solar irradiance. Tests of different influencing factors reveal that preserving millennial-scale cooling trends related to orbital forcing is not feasible in most tree-ring datasets. This result is of great importance as it touches the current debate by showing that tree-ring data do not lack this trend as a consequence of inappropriate tree-ring standardization. Evaluation of the database provides a basis not only to improve future climate reconstructions, but also to enhance our understanding of long-term climate variations as well as to reduce associated uncertainties.