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Human influences on climate operate against a background of long-term natural climate variability. Our ability to characterize this long-term variability and to distinguish it from climate change due to human activities is limited by the relative shortness of the instrumental record. Thus, investigators turn to a combination of indirect paleoclimate proxy evidence and theoretical climate models to ascertain the nature and causes of climate changes on centennial and longer timescales.

Particularly relevant in this context is the time frame of the last few millennia, which is termed the ‘Late Holocene.’ During this period, the fundamental external boundary conditions on the climate, such as the configuration of the continents, the size and locations of the major ice sheets, and the mean radiative forcing due to changes in Earth-orbital geometry, are similar to those today. Study of this interval thus allows insights into the natural variability that might be expected today in the absence of human influences.