Purpose:
The most prominent feature of the pre-industrial period is a rapid ~7 ppm decrease of CO2 in a span of ~20-50 years at ~ 1600 A.D. This observation confirms the timing of an abrupt atmospheric CO2 decrease of ~10 ppm observed for that time period in the Law Dome ice core CO2 records, but the true magnitude of the decrease remains unclear. Atmospheric CO2 variations over the time period 1000-1800
... A.D. are statistically correlated with northern hemispheric climate and tropical Indo-Pacific sea surface temperature. However, the exact relationship between CO2 and climate remains elusive due to regional climate variations and/or uneven geographical data density of paleoclimate records. We observe small differences of 0~2% (0~6 ppm) among the high-precision CO2 records from the Law Dome, EPICA Dronning Maud Land and WAIS Divide Antarctic ice cores. However, those records share common trends of CO2 change on centennial to multi-centennial time scales, and clearly show that atmospheric CO2 has been increasing above preindustrial levels since ~ 1850 A.D. 
