Energy is generated by the body's adjustments to differences in temperature between the skin and the outside air, which can be harvested.

WSJ: Harnessing Energy From the Body to Power Devices

Thermal imaging thus records slight differences in temperature.

ECONOMIST: Archaeology: What lies beneath | The

Such differences result from various assumptions regarding unknowns such as changing urbanization and other land use influences that contaminate surface temperature recordings.

FORBES: NASA's Inconvenient Ruse: The Goddard Institute For Space Studies