The Earth's oceans are being forced to absorb more and more planet-warming carbon dioxide (CO2) emissions, causing them to acidify at rates not seen in the last 300 million years, says a new report released for policymakers today at the UN climate talks in Warsaw.
The oceans supply the oxygen in every second breath we take, and also absorb at least 33 percent of the CO2 human beings produce. Acidification not only limits the capacity of oceans to absorb CO2 but has the potential to affect food security. Within decades, large parts of the polar oceans will become corrosive, particularly affecting marine organisms unprotected by shells, such as squids.
The report is a summary of the state of scientific knowledge on ocean acidification, based on the latest research presented at the Third Symposium on the Ocean in a High-CO2 World, held in Monterey, California, in September 2012. Scientists from across the globe involved in the research urged countries in Warsaw to work towards limiting the global temperature increase to less than two degrees Celsius by the end of this century.
Stefan Rahmstorf, Head of Earth System Analysis at Potsdam University, Germany, writes in a recent web-post that the "increase in the amount of heat in the oceans amounts to 17 x 1022 Joules over the last 30 years. That is so much energy it is equivalent to exploding a Hiroshima bomb every second in the ocean for thirty years".
The oceans are also becoming more acidic. The acidity or alkalinity of aqueous solutions is measured by the pH scale, and the lower the number, the more acidic the solution. As CO2 dissolves in seawater, its pH decreases - a process known as acidification.
This can reduce the availability of calcium - which is alkaline - for plankton and shelled species, threatening their survival. Much of the marine food chain depends on these organisms for food, so the entire ecosystem could be affected.
Fisheries support the livelihoods of 540 million people, or eight percent of the world's population, says Wendy Broadgate, Deputy Director, Natural Sciences, for the International Geosphere-Biosphere Programme, one of report’s sponsors. "However, very little is known about the direct effects of ocean acidification on fish that are the target of commercial and subsistence fishing, which results in high uncertainties in predicting changes in fisheries in the future." Scientists continue to research this aspect, she added.
But they do know more about the impact of these changes on molluscs such as squid, oysters and cuttlefish, which "appear to be one of the most sensitive groups of organisms studied under ocean acidification regimes", says Broadgate. The scientists estimate that by 2100, declines in mollusc production could lead to annual global economic losses of more US$130 billion (at 2010 price levels) if man-made emissions remain as they are.
She notes that "oyster larvae in hatcheries in the northeast Pacific Ocean region are very sensitive to ocean acidification and are already affected by low pH waters, and they are shutting off their seawater intake when… [it] has lower pH."
One of the major concerns is whether the ocean has reached a limit beyond which it would be unable to absorb more heat. Richard Feely, a senior scientist at the US government's National Oceanic and Atmospheric Administration (NOAA), says computer-generated climatic models "suggest a decreasing trend in the fraction of anthropogenic [by human activity] CO2 emissions taken up by the oceans over this century, but the observations have not been able to confirm this trend yet because of the uncertainties in the measurements as well as a lack of complete coverage of the oceans with measurements. Consequently, this is a topic for which more research is required."
There is hope if countries aim to lower emissions so that the global increase in temperature by the turn of century remains less than two degrees Celsius.
Broadgate says, "Significant emissions reductions could ensure 50 percent of surface waters remain favourable for coral reef growth and corrosive conditions in most of the Southern Ocean can be avoided."
Shellfish fisheries and aquaculture in some areas may be able to cope by adjusting their management practices to avoid ocean acidification impacts, she added. "For example, seawater monitoring around shellfish hatcheries can identify when to limit the intake of seawater with a lower pH, hatcheries can be relocated, or managers can select larval stages or strains that are more resilient to ocean acidification for breeding."
At a local level, the report suggests few mitigation measures other than developing sustainable fisheries management practices such as regulating catches to reduce overfishing. The writers urge governments to protect their coasts and the habitats of fishes by maintaining protected areas that will help make them resilient. The scientists also ask for monitoring and regulating "localised sources of acidification" such as the runoff from fertilisers, and reducing sulphur dioxide and nitrous oxide emissions from coal-fired plants and ship exhausts.