The heat exchange at the air/ground interface is determined by many complex processes making the energy balance at the earth's surface extremely difficult to quantify and model. A new methodology allows heat flux at the Earth's surface to be estimated using ground surface temperature history reconstructed from geothermal data. We found that over a large region in eastern and central Canada, the average heat flux into the ground during the last 1000 years was on the order of 2.8 mWm-2. Our results suggest that significant change in the ground heat flux occurred in the last two centuries. The 200 years averaged heat flux since 1765 is 17.0 mWm-2, while the average heat flux over the latest 100 years is 74.0 mWm-2. The sensitivity of the subsurface to very small energy imbalances makes these type of data and analysis useful complements to the paleoclimatic record; they also provide constrains for general circulation model land-surface parameterization over a wide range of spatial-temporal scales. ¿ 2000 American Geophysical Union