Documenting the large-scale surface temperature changes of the past millennium is necessary in order to place recent climate change in an appropriate long-term context. Because the fundamental boundary conditions on the climate (the parameters of the Earth's orbit in relation to the Sun, and global patterns of vegetation and continental ice cover) have not changed appreciably, the major variations in climate over this time frame are representative of the range of natural climate variability that might be expected in the absence of any human influence on climate.

Placing global warming of the twentieth and twenty-first centuries in this longer-term context can aid in our ability to determine the role of anthropogenic factors, such as increases in greenhouse gas concentration produced by humans, in past and future climate change.