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671 Earth’s climate has varied dramatically over its long history, from snowball glaciations to greenhouse extremes. Ancient warming is one reason we study the past—to get the fullest possible picture of what might lie ahead. No episode of past warming was more dramatic than the Paleocene-Eocene Thermal Maximum (PETM) ~55 m.y. ago: a rapid temperature increase of ~5–8 °C unfolded over a mere tens of thousands of years, driving severe perturbations to the marine and terrestrial carbon cycles and concomitant impacts on ecologies in both settings. As we anticipate the impacts possible via modern climate trends, with similar levels of projected carbon release and temperature rise but over only centuries, the PETM has become a conservative poster child for why we look back in time to inform our understanding of the future. Among the growing list of concerns in the face of the current warming trend are widespread decreases in the dissolved oxygen content of seawater, or ‘ocean deoxygenation’ (Keeling et al., 2010), in part because O 2 is less soluble in warmer water. In this regard, however, the PETM’s potential as an ancient window to view modern climate has been only partly realized. Oxygen loss has been implicated in perturbations to organisms living on the deep seafl oor, but little is known about the intensity and global extent of those O 2 -challenged conditions. Dickson et al. (2012, p. 639 in this issue of Geology) set their sights on fithis gap through the use of molybdenum isotope measurements applied to an exceptional set of drill-core samples collected in the Arctic Ocean. Their data point convincingly to low-O 2 conditions during the PETM that were more expansive than today’s. The causes and consequences of O 2 loss in the ocean during PETM

Deoxygenation in warming oceans--Looking back to the future