Most climate proxies of sea surface temperatures suffer from severe limitations when applied to cold temperatures that characterize Arctic environments. These limitations prevent us from constraining uncertainties for some of the most sensitive climate tipping points that can trigger rapid and dramatic global climate change such as Arctic/Polar Amplification, the disruption of the Atlantic Meridional Overturning Circulation, sea ice loss, and permafrost melting. Here, we present an approach to reconstructing sea surface temperatures globally using paired Mg/Ca - δ<jats:sup>18</jats:sup>O<jats:sub>c</jats:sub> recorded in tests of the polar to subpolar planktonic foraminifera <jats:italic>Neogloboquadrina pachyderma</jats:italic>. We show that the fidelity of Mg/Ca-based paleoclimate reconstructions is compromised by variations in seawater carbonate chemistry which can be successfully quantified and isolated from paleotemperature reconstructions using a multiproxy approach. By applying the calibration to the last glacial maximum, we show that marine polar amplification has been underestimated by up to 3.0 ± 1.0 °C in model-based estimates.