evidence: Boreholes [edit]Borehole temperatures are used as temperature proxies. Since heat transfer through the ground is slow, temperature measurements at a series of different depths down the borehole, adjusted for the effect of rising heat from inside the Earth, can be "inverted" (a mathematical formula to solve matrix equations) to produce a non-unique series of surface temperature values. The solution is "non-unique" because there are multiple possible surface temperature reconstructions that can produce the same borehole temperature profile. In addition, due to physical limitations, the reconstructions are inevitably "smeared", and become more smeared further back in time. When reconstructing temperatures around 1500 AD, boreholes have a temporal resolution of a few centuries. At the start of the 20th century, their resolution is a few decades; hence they do not provide a useful check on the instrumental temperature record.[13][14] However, they are broadly comparable.[3] These confirmations have given paleoclimatologists the confidence that they can measure the temperature of 500 years ago. This is concluded by a depth scale of about 492 feet (150 meters) to measure the temperatures from 100 years ago and 1,640 feet (500 meters) to measure the temperatures from 1,000 years ago.[15]