CLIMATE CHANGE: RATE VARIATIONS DURING THE PAST 90 YEARS AND QUANTIFICATION OF INTERNAL FORCING

Multiple temperature vs. depth measurements acquired during the past three decades in boreholes that were drilled and completed for heat flow and geothermal research in 1984 in the north central US contain an important record of the rate of climate change. Heat conduction analyses using the surface air temperature (SAT) as a proxy for the ground surface temperature (GST) show a close correlation between the SAT and the GST. The analyses use the 1984 borehole temperature profiles as initial conditions, and the results yield SAT-forced temperature vs. depth curves that closely match the borehole profiles observed in 1996, 2002, 2007, 2011, 2012 and 2014. The rates of GST change between the observation dates, based on temperatures at 20 meters depth, were: 0.115, 0.070, 0.016, 0.002, 0.080, and 0.035 ⁰C y^-1 respectively. The SAT and GST data, not models or statistical analyses, show that warming slowed significantly between 1996 and 2012. The warming rate calculated by forward modeling of the 1984 borehole temperature profile was 0.038 ⁰C y^-1 for the preceding 60 years. The 1984-2011 borehole temperatures were also compared with forcing by solar irradiance using the ACRIM data as a proxy for surface temperature. A temperature proxy of 0.24 ⁰C per W yielded a warming rate of 0.008 ⁰C y^-1 due to solar irradiance while the SAT and GST records show a rate of 0.042 ⁰C y^-1 during the 25 y period. The results show that solar irradiance had a small warming effect but it was approximately 1/5 of the change observed in the GST and SAT. It is proposed that the difference between the GST and solar irradiance signals permits quantification of the internal forcing of the surface temperature.