<p>Boron isotopes in planktonic foraminifera are a widely used proxy to determine ancient surface seawater pH, and by extension atmospheric CO<sub>2</sub> concentration and climate forcing on geological time scales. Yet, to reconstruct absolute values for pH and CO<sub>2</sub>, we require a δ<sup>11</sup>B<sub>foram-borate</sub> to pH calibration and independent determinations of ocean temperature, salinity, a second carbonate parameter, and the boron isotope composition of seawater. Although δ<sup>11</sup>B-derived records of atmospheric CO<sub>2</sub> have been shown to perform well against ice core-based CO<sub>2</sub> reconstructions, these tests have been performed at only a few locations and with limited temporal resolution. Here we present two highly resolved CO<sub>2</sub> records for the late Pleistocene from ODP Sites 999 and 871. Our δ<sup>11</sup>B-derived CO<sub>2</sub> record shows a very good agreement with the ice core CO<sub>2</sub> record with an average offset of 4.6 ± 49 (2σ) ppm, and a RMSE of 25 ppm, with minor short-lived overestimations of CO<sub>2</sub> (of up to ~50 ppm) occurring during some glacial onsets. We explore potential drivers of this disagreement and conclude that partial dissolution of foraminifera has a minimal effect on the CO<sub>2</sub> offset. We also observe that the general agreement between δ<sup>11</sup>B -derived and ice core CO<sub>2</sub> is improved by optimising the δ<sup>11</sup>B<sub>foram-borate</sub> calibration. Despite these minor issues a strong linear relationship between relative change in climate forcing from CO<sub>2</sub> (from ice core data) and pH change (from δ<sup>11</sup>B) exists over the late Pleistocene, confirming that pH change is a robust proxy of climate forcing over relatively short (<1 million year) intervals. Overall, these findings demonstrate that the boron isotope proxy is a reliable indicator of CO<sub>2</sub> beyond the reach of the ice cores and can help improve determinations of climate sensitivity for ancient time intervals.</p>