If atmospheric carbon dioxide levels are allowed to reach 500 parts per million (ppm) then nearly the entire ocean will be out of compliance with the US Environmental Protection Agency’s (EPA) quality criteria by the middle of this century, according to an international team of scientists. Atmospheric carbon dioxide levels could reach 500 ppm in just a few decades if emissions of the gas proceed unchecked.
The EPA’s water quality criteria indicate what changes to ocean chemistry might be acceptable. The guidelines are meant to help develop US state laws and regulations. “This is the only place that I know of that puts the US government on record as implying that specific atmospheric levels may be environmentally dangerous,” team leader Ken Caldeira of Stanford University, US, told environmentalresearchweb.
About one third of the carbon dioxide produced by burning fossil fuels is absorbed by the world’s oceans, explained Caldeira. When carbon dioxide gas dissolves in the oceans, it makes carbonic acid, which can damage coral reefs and other calcifying organisms such as some phytoplankton and zooplankton species. These organisms are some of the most critical players at the bottom of the food chain.
In sufficiently high concentrations, carbonic acid can corrode the calcium carbonate shells of shellfish and even interfere with the ocean’s oxygen supply. What’s more, the effects of increasing carbon dioxide concentrations on the carbonate system in seawater are not reversible on human timescales and it might take thousands of years before the system can return to pre-industrial era conditions.
Most climate scientists say that if efforts are not made to limit carbon dioxide emissions now, the atmospheric concentration of this greenhouse gas will increase to 760 ppm by the end of the century, reaching 500 ppm by 2050. By contrast, today’s concentration stands at 380 ppm and pre-industrial figures stood at 280 ppm.
According to the EPA, if ocean chemistry is to be safeguarded, the pH of the euphotic zone in oceans should not change by more than 0.2 units outside the naturally occurring variation. The euphotic zone goes to a depth of about 200 m – it’s a region where light can still reach and photosynthesis can occur. For oceanic pH levels to stay within these limits, atmospheric carbon dioxide emissions need to remain below 500 ppm, explained Caldeira. He suggests that a combination of increased efficiency, conservation and carbon-neutral energy sources such as wind, solar and nuclear power could help keep emissions below the 500 ppm level.
“Our result may contribute to legal arguments for considering carbon dioxide as a pollutant – or industrial waste – that cannot be simply disposed of in the atmosphere without violating water quality standards,” he states.
The researchers obtained their results through chemical calculations, comparing 30 years of history with observations. Such calculations are highly reliable, according to Caldeira.
The team is now busy comparing how predicted ocean chemistry at the locations where coral reefs are found compares with the chemistry at these sites before the industrial revolution.
The work was published as a commentary in Geophysical Research Letters.
environmentalresearchweb, 4 December 2007. Article.
