Acid History: As human activity continues to pump nearly 50-fold more CO ₂ into the atmosphere than any existing natural sources, the oceans absorb it. Over time, this vast quantity of excess oceanic CO ₂ is expected to decrease oceanic pH and have marked effects on calcifying marine species. Looking to the past for records of the consequences, other instances of ocean acidification in geologic history caused by large natural events, such as volcanism, may help predict the oceans' response to contemporary CO ₂ levels. Hönisch et al. (p. 1058 ) review the geological events that potentially altered oceanic pH, from the last deglaciation to the largest mass extinction in Earth's history. The current rate of anthropogenic CO ₂ input into the oceans is much faster than at any other instance in the past, but yet it is unclear whether or not future ocean pH will be significantly affected.

Past sea surface temperature (SST) evolution in the Alboran Sea (western Mediterranean) during the last 50,000 years has been inferred from the study of C₃₇ alkenones in International Marine Global Change Studies MD952043 core. This record has a time resolution of ∼200 years allowing the study of millennial‐scale and even shorter climatic changes. The observed SST curve displays characteristic sequences of extremely rapid warming and cooling events along the glacial period. Comparison of this Alboran record with δ¹⁸O from Greenland ice (Greenland Ice Sheet Project 2 core) shows a strong parallelism between these SST oscillations and the Dansgaard‐Oeschger events. Five prominent cooling episodes standing out in the SST profile are accompanied by an anomalous high abundance of Neogloboquadrina pachyderma sinistral which is confined to the duration of these cold intervals. These features and the isotopic record reflect drastic changes in the surface hydrography of the Alboran Sea in association with Heinrich events Hl–5.

The oceans are becoming more acidic due to absorption of anthropogenic carbon dioxide from the atmosphere. The impact of ocean acidification on marine ecosystems is unclear, but it will likely depend on species adaptability and the rate of change of seawater pH relative to its natural variability. To constrain the natural variability in reef-water pH, we measured boron isotopic compositions in a ∼300-year-old massive Porites coral from the southwestern Pacific. Large variations in pH are found over ∼50-year cycles that covary with the Interdecadal Pacific Oscillation of ocean-atmosphere anomalies, suggesting that natural pH cycles can modulate the impact of ocean acidification on coral reef ecosystems.

29 pagex, 9 figures, 2 tablesThe Catalan Sea, located between the eastern Iberian coast and the Balearic Islands, is a representative portion of the western Mediterranean basin and provides a valuable case study for climate change effects on Mediterranean ecosystems. Global warming is reflected regionally by a rise in sea level over the last century, an increase in surface temperature of around 1.1°C in the last 35 yr, a progressive salinisation of intermediate and deep waters and a strengthening of the stratification. A likely scenario of what we can expect in the Mediterranean Sea is a considerable decrease in rainfall and wind, warmer surface waters and a prolonged stratification period. The effects on Mediterranean ecosystems are evident in: (1) a meridionalisation of the algal, invertebrate and vertebrate species, which favours the more thermophilic species over the temperate species; (2) mass mortality events of sessile invertebrates of the coralligenous communities owing to anomalous warm waters during the period when food is scarce; (3) increases in the smallest phytoplankton due to the prolongation of the water stratification period; (4) proliferation of gelatinous carnivores, including jellyfish, due to the temperature rise and the lack of rainfall; (5) a faster acidification of seawater, compared with the global oceans, accompanied by a decrease in the capacity to absorb atmospheric CO2. In order to anticipate and mitigate these predicted changes, we recommend investing in research and observation, conserving areas that serve as indicators of climate change and reducing other anthropogenic pressures such as habitat destruction, overfishing or pollution, which may act synergistically to accelerate these changes.E.C., C.P., M.R. and R.C. acknowledge funding\ud from the Spanish Ministerio de Ciencia e Innovación through grants CTM2009-08849/MAR, CTM2006-01463 and CGL2007-66757-C02-01/BOS and a Ramón y Cajal contract to E.C. This paper is a contribution from the Marine Biogeochemistry and Global Change research group, funded by Generalitat de Catalunya (Catalan Government) through grant 2009SGR142.Peer reviewe