Recent climate variations in the subarctic inferred from three borehole temperature profiles in northern Quebec, Canada

Three temperature depth profiles recorded in permafrost in northern Quebec, Canada, were used to infer the ground surface temperature history (GSTH) of the region. The site is located in a barren rock desert on the Katinniq plateau at an elevation of 600 m, near the northern tip of the Ungava Peninsula. The boreholes were logged more than 3 yr after drilling was completed, insuring that the holes had returned to thermal equilibrium. Thermal conductivity measurements were made on core samples. Radiogenic heat production is small and can be neglected. The temperature depth profiles show marked deviations from steady state in the upper 200 m that are assumed to be caused by recent (< 300 yr) variations in ground surface temperature. The GSTHs obtained by inversion of the three temperature profiles consistently show warming by ∼ 2.5 K, but differ significantly in the details. One profile which is least affected by topographic effects and thermal conductivity changes was analyzed in great details with different inversion methods; direct methods were also used to verify how well the GSTH can be resolved by the data. The results show a marked warming (≈ 1.4 K) between the mid-1700s and 1940, followed by a cooling episode (≈ 0.4 K) which lasted 40-50 yr, followed by a sharp ≈ 1.7 K warming over the past 15 yr. The borehole temperature measurements suggest that most of this warming occurred over the past 15 yr. These results are in agreement with the available meteorological records and proxy data.