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Surface Temperature Reconstructions for the last 2,000 Years
The reliability of large-scale temperature time series derived from observations at a small number of sites and with varying levels of chronological precision is still unresolved. It is widely agreed that fewer sites are required for defining century-to-century fluctuations than year-to-year fluctuations, but errors in the reconstructions that are specifically attributable to the limited spatial sampling are difficult to quantify.
The committee identified the key strengths of large-scale surface temperature reconstructions as:
Proxy records are meaningful recorders of environmental variables. These records are selected and sampled on the basis of established criteria, and the connections between proxy records and environmental variables are well justified in terms of physical, chemical, and biological processes.
Tree rings, the dominant data source in many large-scale surface temperature reconstructions, are derived from regional networks with extensive replication that reflect temperature variability at the regional scale.
Most surface temperature reconstructions incorporate proxy evidence from a variety of sources and wide geographic areas, and the resulting temperature estimates are often robust with respect to the removal of individual records.
The same general temperature trends emerge from different reconstructions. Some reconstructions focus on temperature-sensitive trees, others focus on geochemical and sedimentary proxies, and others infer the temperature signal by exploiting the spatial relationship between temperature and precipitation fields.
Our overall confidence in the general character of the reconstructions for the period from around A.D. 1600 onward is high because different reconstructions based on different types of proxy evidence, different selections of proxy data of a given type, and different methodologies yield similar results. Our confidence in statements concerning how temperature may have varied before 1600, and in particular concerning the warmth of the Northern Hemisphere during medieval times compared to that of the last few decades, is lower because of the limited amount of proxy evidence available and the uncertainties in reconstructing a large-scale average temperature from such limited datasets.
The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on ice caps and the retreat of glaciers around the world. Not all individual proxy records indicate that the recent warmth is unprecedented, although a larger fraction of geographically diverse sites experienced exceptional warmth during the late 20th century than during any other extended period from A.D. 900 onward.
Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium. The substantial uncertainties currently present in the quantitative assessment of large-scale surface temperature changes prior to about A.D. 1600 lower our confidence in this conclusion compared to the high