Relative Amplitudes of Surface Temperature Anomalies for the Medieval Warm Period, Little Ice Age, and 20th Century Warming Determined from Borehole Temperatures

Borehole temperatures constitute one of the few direct measures of Earth’s surface temperature that extend more than 1000 years into the past. The process of thermal diffusion, however, smears the subsurface temperature response so that borehole temperature profiles are most useful in constraining centennial length surface temperature events. Aware of these limitations, we generalize the past millennium of Earth surface temperatures in terms of three long-lived events superimposed on a millennial mean temperature (Figure - top): the Medieval Warm Period (MWP) from 900-1200 AD, the Little Ice Age (LIA) from 1500-1800, and principally 20th century recent warming (RW) from 1850 to present. The events are modeled as two boxcars and a ramp respectively; amplitudes of these anomalies are left as free parameters. Each event has a clear expression in the subsurface temperature profile (Figure - bottom). Recent warming produces a clear and unambiguous signature that is a maximum at the surface and extends to about 200 m depth. The peak anomalies for the LIA and MWP are at 150 m and 250 m respectively, governed by the timing of these events. The sum of the signals for LIA and MWP have considerable overlap and may be close to zero depending on the specific timing and amplitudes of the forcing signals. If one of the forcing signals dominates in a region of the world, the borehole temperature profile should record it. Temperature profiles from boreholes sufficiently deep to encompass the full millennium of time being examined exist in central Europe, northeast North America, South Africa, and central Asia. Of these regions, the central Europe borehole profiles show clearest evidence of strong LIA and MWP signals. These events, on the other hand, are either both missing, or largely cancel each other in the North American, South African, and central Asian profiles. Twentieth Century warming is dramatically apparent in all regions.