Several episodes of abrupt and transient warming, each lasting between 50,000 and 200,000 years, punctuated the long-term warming during the Late Palaeocene and Early Eocene (58 to 51 Myr ago) epochs 1,2. These hyperthermal events, such as the Eocene Thermal Maximum 2 (ETM2) that took place about 53.5 Myr ago 2 , are associated with rapid increases in atmospheric CO 2 content. However, the impacts of most events are documented only locally 3,4. Here we show, on the basis of estimates from the TEX 86 proxy, that sea surface temperatures rose by 3-5 • C in the Arctic Ocean during the ETM2. Dinoflagellate fossils demonstrate a concomitant freshening and eutrophication of surface waters, which resulted in euxinia in the photic zone. The presence of palm pollen implies 5 that coldest month mean temperatures over the Arctic land masses were no less than 8 • C, in contradiction of model simulations that suggest hyperthermal winter temperatures were below freezing 6. In light of our reconstructed temperature and hydrologic trends, we conclude that the temperature and hydrographic responses to abruptly increased atmospheric CO 2 concentrations were similar for the ETM2 and the better-described Palaeocene-Eocene Thermal Maximum 7,8 , 55.5 Myr ago. At the onset of Eocene Thermal Maximum 2 (ETM2), the stable isotopic composition of sedimentary carbon (δ 13 C) shows a >1.5 negative excursion, interpreted as a geologically rapid injection of 13 C-depleted carbon into the ocean-atmosphere system 2,4,9. A marked calcium carbonate dissolution horizon in deep-sea sediments reflects ocean acidification resulting from this carbon input 2,3. Evidence for surface warming during ETM2 is, however, available only from the subtropical southeastern Atlantic Ocean, where a ∼0.8 negative oxygen isotope excursion in calcite of surface-dwelling foraminifers was interpreted as a ∼3 • C warming 2. Hence, it remains uncertain whether ETM2 was associated with warming on a global scale and whether climate response was similar to the well-studied Palaeocene-Eocene Thermal Maximum (PETM). Uppermost Palaeocene to Lower Eocene sediments were recovered from the Lomonosov Ridge, Arctic Ocean, at ∼85 • N palaeolatitude, during Integrated Ocean Drilling Program Expedition 302 (Supplementary Fig. S1). This ridge represents a fragment of continental crust that rifted from the Eurasian margin during the latest Palaeocene 10. Upper Palaeocene and Lower Eocene sediments