The Little Ice Age signature and subsequent warming seen in borehole temperature logs versus solar forcing model

The ‘low’ in the transient temperature versus depth borehole profiles around 120 m seen from deep temperature logs in the Canadian Prairies (southern Alberta–southern Saskatchewan), as well as in some of the European data, has been interpreted to be related to the Little Ice Age (LIA). Data point to the lowest ground surface and subsurface temperatures occurring in the very late eighteenth to nineteenth centuries. Inversion of these logs shows that surface temperature lows were followed by a recent warming period. Further, the synthetic profiles built on the basis of solar forcing history, stretching as far back as the beginning of the seventeenth century, suggest that the LIA signatures interpreted from the inversion of the borehole temperature logs would be difficult to be explained by known published models of past solar irradiation despite large range of assumed sensitivities for the couplings assumed, and that further forcing needs to be considered.