Evidence of Climate Variability and Change from Tree-Ring Records

Paleoclimatic data provide a record of past climate variability and change, and a context from which to assess current and future climate change. Tree-ring data have proven to be useful for assessing climate over the past centuries to millennia, and are particularly valuable for their annual resolution. Tree rings document climate over large areas of the mid-to high latitudes, most notably in the Northern Hemisphere, and reflect both temperature and moisture-related climate variability. Reconstructions of Northern Hemisphere temperatures, based in large part on tree-ring data, show key features of hemispheric-scale temperature variability over the past nine centuries, and have played an important role in the detection of anthropogenic climate change. At continental and regional scales, tree-ring data document precipitation, drought, and hydrologic variability. A gridded network of drought for North America has provided a unique record of the temporal and spatial patterns of drought over the past several millennia, and has been the basis for studies that have investigated ocean/atmosphere drivers of drought. A period of particular interest has been the medieval period, during which drought appears to have been persistent over a large area of western North America over several centuries. At more regional scales, reconstructions of streamflow that document periods of low flow more severe and sustained than in the modern gage records are now being used as the basis for drought planning and water resource management. These and other tree-ring based reconstructions of climate provide evidence for the range of natural variability that has occurred in the past, and over which the influence of human-induced climate change will be imposed. Knowledge of this underlying natural variability is critical for understanding the current climate and future climate changes.