Using Borehole Temperature Profiles to Reconstruct and Test Surface Temperature Scenarios Over the Last Millennium David S. Chapman 1, Michael G. Davis 1, and Robert N. Harris 2 1 Department of Geology and Geophysics, University of Utah 2 College of Ocean and Atmospheric Sciences, Oregon State University Reconstructing surface temperature histories Borehole temperatures A hybrid (SAT, borehole temperature) method Proxies and pre-proxy mean temperature (PPM)

Disadvantages Short records (post 1860) Non climatic biases Station moves Instrument changes Air - Ground Tracking Advantages Direct measure of temperature Good fidelity Good spatial coverage Instrumental Record

Advantages Long records (millennia) Annual resolution Disadvantages Temperature calibration issues Seasonal sensitivity Poor low frequency resolution + Multiproxy Reconstructions

Measuring Temperature in the Earth

The Geothermal Method

Evidence From Alaska

Northern Hemisphere

Advantages Long records (centuries) Annual sensitivity Direct measure of temperature Disadvantages Loss of temporal resolution with time Arbitrary offset with SAT + Borehole Temp. Reconstruction

Borehole-SAT Hybrid Method

Advantages Long records (centuries) Direct measure of temperature Directly tied and compared to SAT record Disadvantages Loss of temporal resolution with time + Hybrid Method

Convergence of Reconstructions

Borehole Test for Reconstructions

Borehole Temperatures and Climate Change 1.Ramp inversion of borehole temperatures yields 1.2 °C of warming from ca 1500 to year The hybrid (SAT + borehole T) model yields 1.1 °C of warming from ca 1750 to year Several proxy reconstructions are converging on 1 °C of warming. 4.Cautions: frequency sensitivity; spatial coverage; seasonality; etc. 5.Borehole temperatures provide tests for long term climate reconstructions.