By the year 2400, it is predicted that humans will have released about 5,000 gigatonnes of carbon (Gt C) to the atmosphere since the start of the industrial revolution if fossil-fuel emissions continue una- bated and carbon-sequestration efforts remain at current levels1. This anthropogenic carbon input, predominantly carbon dioxide (CO2), would eventually return to the geosphere through the deposition of calcium carbonate and organic matter2. Over the coming mil- lennium, however, most would accumulate in the atmosphere and ocean. Even if only 60% accumulated in the atmosphere, the par- tial pressure of CO2 (pCO2) would rise to 1,800 parts per million by volume (p.p.m.v.) (Fig. 1). A greater portion entering the ocean would decrease the atmospheric burden but with a consequence: significantly lower pH and carbonate ion concentrations of ocean surface layers1 (Fig. 1). A marked increase in atmospheric pCO2 would increase mean global temperature, thereby affecting atmospheric and oceanic circulation, precipitation patterns and intensity, the coverage and thickness of sea ice, and continental ice-sheet stability. However, forecasting the tim- ing and magnitude of these responses is challenging because they can be nonlinear. Of particular concern are potential positive feedbacks that could amplify increases in the concentrations of greenhouse gases ��� water, CO2, methane and nitrous oxide (N2O) ��� effectively esca- lating climate sensitivity to initial anthropogenic carbon input3. For example, ocean surface warming and freshwater discharge at high lati- tudes could slow the exchange of shallow and deep water in the ocean, impeding both abiotic and biotic removal of anthropogenic carbon from the atmosphere. Potential negative feedbacks are also garnering great interest. As a possible counterbalance to decreased density of surface water on a warmer Earth, stronger zonal winds might increase ocean overturning (see page 286). Observations of modern and Holocene (the past 10,000 years or so) climates have provided essential constraints for understand- ing climate dynamics and a baseline for predicting future responses to carbon input. But such observations can provide only limited insight into the response of climate to massive, rapid input of CO2. To evaluate climate theories more thoroughly, particularly with regard to feedbacks and climate sensitivity to pCO2, it is desirable to study samples obtained when CO2 concentrations were high (approaching or exceeding 1,800 p.p.m.v.) and to make observations for intervals longer than those of ocean overturning and carbon cycling (more than 1,000 years)4. Earth scientists have therefore turned increasingly to ancient time intervals, particularly those in which pCO2 was much higher than now, in which pCO2 changed rapidly, or both. Recent reconstructions of Earth���s history have considerably improved our knowledge of known ���greenhouse��� periods and have uncovered several previously unknown episodes of rapid emissions of greenhouse gases and abrupt warming. Cenozoic greenhouse climates The Cenozoic era, the last 65 million years of Earth���s history, provides an ideal backdrop from which to understand relationships between carbon cycling and climate. In contrast to the present day, much of the early Cenozoic was characterized by noticeably higher concentrations of greenhouse gases, as well as a much warmer mean global temperature and poles with little or no ice5,6 (Fig. 2). The extreme case is the Early Eocene Climatic Optimum (EECO), 51���53 million years ago, when pCO2 was high and global temperature reached a long-term maximum. Only over the past 34 million years have CO2 concentrations been low, temperatures relatively cool, and the poles glaciated. This long- term shift in Earth���s climatic state resulted, in part, from differences in volcanic emissions, which were particularly high during parts of the Palaeocene and Eocene epochs (about 40���60 million years ago) but have diminished since then. Changes in chemical weathering of silicate rocks were also important7. On long timescales, this process sequesters CO2, preventing concentrations from rising too high or from falling too low. As the atmospheric CO2 concentration rises, temperature and precipitation increase and thereby enhance chemical weathering as the concentration declines, temperature and precipitation decrease, slowing weathering. Whereas other processes (such as the oxidation and burial of organic carbon) change CO2 concentrations, the negative weathering feedback loop maintains Earth���s climate within a habitable range over millions of years and longer7. On shorter timescales, atmospheric CO2 concentration and tem- perature can change rapidly, as demonstrated by a series of events dur- ing the early Cenozoic known as hyperthermals. These were relatively brief intervals (less than a few tens of thousands of years) of extreme global warmth and massive carbon addition but with widely differing scales of forcing and response. During the most prominent and best-stud- ied hyperthermal, the Palaeocene���Eocene Thermal Maximum (PETM about 55 million years ago), the global temperature increased by more than 5 ��C in less than 10,000 years6 (Fig. 3). At about the same time, more than 2,000 Gt C as CO2 ��� comparable in magnitude to that which could occur over the coming centuries ��� entered the atmosphere and ocean. Evidence for this carbon release is found in sedimentary records across the event. This includes a rapid and pronounced decrease in the 13C/12C ratio of carbonate and organic carbon across the globe (that is, a negative carbon isotope excursion) and a prominent drop in the carbonate content of marine sediment deposited at several thousands of metres water depth (that is, a deep-sea dissolution horizon)8. The first observation indicates injection into the atmosphere or ocean of a very large mass of 13C-depleted carbon, affecting the composition of the global carbon cycle. The second observation is a telltale signature of ocean acidification. The entire event lasted less than 170,000 years. Given the residence time of carbon (the average time a carbon atom spends in the ocean about 100,000 years), this is consistent with a fast An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics James C. Zachos, Gerald R. Dickens & Richard E. Zeebe Past episodes of greenhouse warming provide insight into the coupling of climate and the carbon cycle and thus may help to predict the consequences of unabated carbon emissions in the future. 279 YEAR OF PLANET EARTH FEATURE NATURE|Vol 451|17 January 2008|doi:10.1038/nature06588

ocean overturning and increased surface temperatures should have decreased the flow of dissolved oxygen to deep water. Several direct lines of evidence, such as laminated sediment in cores from the Car- ibbean and central Arctic regions, suggest that dissolved oxygen did indeed decrease across the PETM. Moreover, the PETM coincided with a major extinction of benthic foraminiferans, with widespread oxygen deficiency in the ocean as a possible cause17. With such ocean conditions, greater preservation and burial of solid organic carbon in deep-sea sediments might be predicted, effectively countering the decreased carbon flux from surface waters. However, this has not been documented. Two largely unexplored processes involving the microbial decomposition of organic carbon, both functioning as additional positive feedbacks, might operate during times of massive carbon input and rapid warming. Carbonate dissolution in the deep ocean decreases sedimentation rates, exposing organic carbon at or near the sea floor for a longer duration, and warming of deep waters will accelerate overall microbial activity and the consumption of organic carbon. Future investigations might therefore focus specifically on the evidence for changes in ocean overturning, oxygen deficiency and the burial of organic carbon. The positive feedbacks of greatest concern for understanding overall global warming may be those that could release hundreds to thousands of gigatonnes of carbon after initial warming11���13. The large masses of organic carbon stored in soils (for example, as peat) or sediments of shal- low aquatic systems (for example, wetlands, bogs and swamps) represent a potential carbon input, should regions that were humid become drier. Rapid desiccation or fire could release carbon from these reservoirs at rates faster than carbon uptake by similar environments elsewhere. By contrast, regions that once were dry might emit methane as they become wetter18. Methane might also enter the ocean or atmosphere through the