Method: The ice found in mountains and in polar ice caps is made of snow that has been slowly compressed over millenia. New snow falling on top of old creates pressure, so each layer contains trapped gas, stable isotope ratios and pollen captured when the snow fell. From these materials, scientists can determine the composition of the atmosphere and thus an estimated temperature. Ice cores can reveal climate conditions back to 650,000 years ago.

Uncertainty: Temperatures are read indirectly, so the results depend on an accurate understanding of physics and chemistry as they relate to the atmosphere.