A perspective on climate and marine biogeochemistry (ocean acidification) change from paleo proxy evidence

Understanding the ocean's role in climate variability and change is aided by the recent development of paleoclimate proxies for seawater carbonate chemistry. New time series resolve the changes in ocean pH (ocean acidification) that have occurred over time scales ranging from centuries to millions of years. The slow changes are not surprising given the rates of surface and deep ocean circulation and also the chemical exchanges that occur across three reservoirs (atmosphere, ocean, and seafloor sediment). These natural cycles are directly relevant to future change because the carbon dioxide now entering the atmosphere will eventually make its way through these reservoirs. Three aspects of the natural variability are relevant to understanding future change. On century-scales, ocean circulation processes affect uptake and release of carbon from the atmosphere. Upon these natural variations the anthropogenic change is superimposed. On thousand-year scales, changes in ocean chemistry track the changes in carbon dioxide in the atmosphere observed in ice cores, confirming the ocean's role in climate change, and revealing the sensitivity of the climate to carbon dioxide forcing. On million year scales, large and abrupt changes in ocean pH provide important clues to the climate and ocean changes that lie ahead.