PAST CLIMATES CAN be reconstructed by the use of the relationship between the climate and tree-growth parameters (dendroclimatology). Dendroclimatol-ogy is a sub-discipline of dendrochronology, which is the analysis of tree rings, including the dating of annual rings and study of patterns of ring characteristics, such as width, density, and isotopic composition. Annual growth rings of trees are natural recorders of climatic conditions (a proxy variable).
Outside the tropics, in the temperate and boreal zones with a distinct growing season and a strong seasonality in either temperature or rainfall, diameter growth of trees is distinguished by the formation of tree rings. Each growing season, new cells are produced by the vascular cambium (a cell layer that separates bark from wood).
While those cells formed toward the outside of the cambial layer become part of the bark, cells formed inward build up rigid wood and are the building blocks of tree rings. The actual tree rings are formed by a change in growth characteristics of wood formation through the season. Wood cells formed at the beginning of the growing season are large, thin-walled, and low in density, and are called earlywood. Toward the end of the growing season, cells become smaller and more thick-walled, forming the latewood. Finally, when the growth season ends, growth stops and cells die, with no new growth appearing until the next spring. The ring boundary is the contrast in cell size between the small latewood cells and the large earlywood cells.
Diameter growth of trees in arid and semi-arid environments strongly responds to changing soil water conditions and, thus, provides information on precipitation, while trees growing at high latitudes (poleward of 30 degrees), or high altitudes, are most sensitive to changing temperatures. In recent years, tree-ring studies have also been conducted in the tropical zone, where some tree species form distinct tree rings in reaction to dry and rainy seasons.
Analyses of tree rings have been carried out for several conifer species (such as spruce, pine, larch, fir, juniper, and cedar) and deciduous tree species (such as oak, beech, aspen, alder, birch, ash, and elm).
However, deciduous species have not been studied as thoroughly as conifers, because of the more complicated structure and variability in their annual tree-ring growth patterns. Generally, tree species suitable for dendroclimatology are those that are sensitive to climate and show a variation in ring-to-ring growth, while those that lack ring variability are called complacent. Yet, tree-ring characteristics are not only influenced by a number of climate-related factors (soil and air temperature, precipitation or soil moisture, sunlight, wind, snow accumulation, or the length of the growing season), but also by non-climate-related factors such as soil fertility, atmospheric composition, slope gradient, fires, pests and diseases, grazing, logging, tree age, competition, genetic differences, and growth in previous years.
Climate-related factors may alter cell enlargement and subsequent maturation of wood cells by regulating hormone activity or availability, by influencing net carbon gain via restricted photosynthesis, or by modifying metabolic pathways associated with the different growth processes. Variations in temperature and water availability can have direct effects on the rates or duration of cell expansion and wall thickening, and indirect effects on carbon availability and growth-regulator levels. Temperature represents the most critical factor at the beginning of the growing season, when sufficient water reserves are available. As the growing season progresses and soil moisture decreases, radial growth is increasingly exposed to stress due to water deficit. Adequate moisture and a long growing season will result in a wide ring, while drought and a short growing season will result in a narrow ring.
Andrew E. Douglass of the University of Arizona discovered the basic technique of dendroclimatology in the early 1900s. For a good record of climate conditions, many trees need to be sampled to avoid the possibility of all the collected data showing a missing or extra ring. Information is acquired by taking small-diameter radial cores or horizontal cross sections through the trunk of a tree. These are prepared and total ring width, latewood width, maximum late-wood density, and isotopic or chemical compositions are measured. Patterns of ring-growth characteristics are matched between several trees throughout a region in a process called cross-dating, which permits the assignation of exact calendar year dates to each individual ring. This matching is calculated by a number of computer programs.
Long-term growth trends associated with the aging of trees and forest dynamics represent an underlying disturbance for the purpose of climatic reconstruction and are removed statistically by detrending tree-ring growth series with a fitted smooth mathematical growth function (standardization). More complex modeling is used to remove the aftereffect of an adverse season to growth in the following years. Yearly growth measurements are then averaged from several trees in a region to form a master tree-ring chronology. This averaged tree-ring chronology enhances the common pattern of variations in growth (signal), while it dampens the non-common variance (confounding noise).
Cross-matching the patterns of a dated chronology with overlapping series of samples taken from dead trees of unknown age preserved in old buildings, river gravels, peat bogs, or lakes, allows chronologies to be extended into the past up to thousands of years (for example, chronology for Bristlecone pine greater than 8,500 years before the present time). By statistically comparing tree-ring chronologies with modern climate records, equations can be developed, which can be used in conjunction with the tree-ring data to reconstruct past climate values.
Tree-ring data are especially useful as climate proxies, in that they provide information of a high spatial and temporal resolution: the information is annually resolved, continuous, and abundant. However, tree ring data also have some limitations: confounding fac tors, geographic coverage (polar and oceanic climates are not covered), and annual resolution (dormant seasons are not covered). However, compared to corals and ice cores, tree-ring reconstructions show greater agreement with instrumental data over the last 100 years, both in terms of simple correlations and coherency on different timescales. Lake and ocean sediments have a lower time resolution, but provide climate information at multicentennial timescales that may not be captured by tree-ring data. By combining multiple tree-ring studies with the records taken from other climate proxy sources (multiproxy reconstruction), our understanding of past regional and global climates can be extended far beyond the instrumental record.
SEE ALSO: Climatic Data, Proxy Records; Climatic Data, Sediment Records; Detection of Climate Changes; Earth's Climate History; Forests.
BIBLIOGRAphY. H.C. Fritts, Tree Rings and Climate (Academic Press, 1976); F.H. Schweingruber, Tree Rings and Environment: Dendroecology (Haupt, 1996); F.H. Schweingruber, Annett Börner, and Ernst-Detlef Schulze, Atlas of Woody Plant Stems: Evolution, Structure, and Environmental Modifications (Springer, 2006).
Ina Christin Meier University of Göttingen
THE CLIMATIC research Unit is regarded as one of the world's leading institutions concerned with the study of natural and anthropogenic climate change. It was founded in 1972 at the University of East Anglia in Norwich, England, by Professor Hubert Lamb (1913-97). The Unit, often known simply as CRU, was the first research center of its kind with a focus on research into climatology, in particular climatic change, and on interdisciplinary and historical research on the climate of the past. Lamb, one of the best known climatologists of his time, remained CRU's Director until 1978 when he became emeritus professor. In August 2006, the distinctive circular CRU building was named the Hubert Lamb Building. Subsequent directors of the Unit have been Tom
Wigley, Trevor Davies and Jean Palutikof. The current Director is Phil Jones.
Lamb was employed by the United Kingdom's Meteorological Office from 1936 onwards. In 1950, he published a classic paper on weather types and natural seasons in Britain. The Lamb Weather Type (LWT) classification described in this paper began a new era in climatological research. In the 1960s, Lamb focused on reconstructions of monthly atmospheric circulation changes over the North Atlantic and Europe back to the 1750s. This research confirmed his growing conviction of the reality of natural climate change and its significance to humans. In 1970, he published a classic paper on the connections between volcanism and climatic change. His estimates of the dust ejected into the atmosphere by historical eruptions became known as the Lamb Dust Veil Index. Lamb was also one of the pioneers in the use of documentary evidence to reconstruct past changes in climate. This continues as a major focus of work in the CRU.
Although Lamb's work was crucial in the early years of the CRU, the Unit has had a number of other outstanding scientists on its staff. Their work has broadened the general aims of the CRU and helped to improve scientific understanding of past climate and its impact on humanity, the course and causes of climate change over the past century, and prospects for the future. The Unit currently employs 46 scientists and graduate students as well as hosting an M.Sc. course in Climate Change. CRU researchers have developed a number of data sets widely-used in climate research, including gridded data sets for surface temperature, precipitation, pressure, tree-ring records that have been important in reconstructing temperature variations over the past 1,000 years, and documentary records of climate and climate impacts. The user-friendly software package MAGICC/SCEN-GEN, used for making projections of future global-mean temperature, sea level, and patterns of climate change, was developed in the CRU (downloadable from http://www.cgd.ucar.edu).
Climate affects both social and natural systems through the occurrence of weather extremes, through inter-annual climate variability and through longer-term climate change. CRU researchers study aspects of all three of these scales of climate impact, with projects ranging from the United Kingdom to
Europe, Africa and Vietnam. A better understanding of the ways in which climate extremes and variability have affected society and the environment in the past is a pre-requisite for attempting to understand how serious the range of impacts associated with future climate change are likely to be. The results of analyses such as these feed directly into the design of climate-change response policies, whether these be mitigation or adaptation. Specific research areas include: paleoclimatology, dendroclimatology, present-day climate and climatological datasets, climate-change detection and attribution, construction of climate change scenarios, impacts of climate variability and change, links between atmospheric circulation and transport/deposition of air pollution, atmospheric sciences, and hydrology.
Climate Monitor, the seasonal data summary of work in the CRU changed from a printed to a online version in 1998. It is now available on http://www. cru.uea.ac.uk/cru/climon/ and brings together in one place regular updates of important climate and meteorological data, together with commentaries from the world's press and media. The CRU's website also contains a range of downloadable climate datasets and useful information pertaining to climate change.
SEE ALSO: Climatic Data, Historical Records; Climatic Data, Instrumental Records; Paleoclimates; Volcanism.
BIBLIOGRApHY. M. Hulme and E. Barrow, eds., Climates of the British Isles Present Past and Future (Routledge, 1997); H.H. Lamb, Climate Present Past and Future, vol. 1-2, (Methuen, 1972, 1977); A.E.J. Ogilvie "Lamb, Hubert Horace," Encyclopedia of Climate and Weather (Oxford University Press, 1996).
Astrid E.J. Ogilvie Institute of Arctic and Alpine Research University of Colorado
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