Alarm over acidifying oceans
25th September 2003
Climate change may be veering out of control before we understand the consequences, say scientists studying the world's oceans.
If carbon dioxide emissions keep rising, surface waters could become more acidic than they have been for 300 million years - except perhaps during global catastrophes. And this warning follows a report that the biological productivity of the oceans has fallen by six per cent since the 1980s.
"We are changing the chemistry of the ocean and we don't know what it's going to do," says Ken Caldeira, a climate specialist at Lawrence Livermore National Laboratory in California.
As the amount of CO2 in the atmosphere rises, more of the gas reacts with seawater to produce bicarbonate and hydrogen ions, increasing the acidity of the surface layer of water. Ocean pH was 8.3 after the last ice age and 8.2 before CO2 emissions took off in the industrial era. It is now 8.1.
To work out what might happen in the future, Caldeira and his colleague Michael Wickett assumed the "business as usual" scenario, in which CO2 emissions rise with population and economic growth throughout this century, then decline as fossil fuels are exhausted.
In this scenario, atmospheric CO2 levels peak around the year 2300 at 1900 parts per million (ppm), five times as high as today. The researchers calculate that because the ocean will soak up some of this CO2, its surface pH will drop to 7.4 by 2300 and stay that low for hundreds of years (Nature, vol 425, p 365).
Atmospheric CO2 has risen well above 2000 ppm several times in the past 300 million years. Caldeira says this never pushed ocean pH below 7.5 because carbonate rocks on the seafloor act as a natural buffer, limiting seawater's acidity. But that process takes 10,000 years or so - enough time to neutralise acid deposited by geological processes, but not to deal with the more rapid changes caused by human activity or natural catastrophes such as asteroid impacts.
It is not clear what such a dramatic change in acidity would do to ocean life. But acidity tends to dissolve carbonate, so the most vulnerable creatures will be those with calcium carbonate shells or exoskeletons, such as corals and some algae.
Experiments with double the present CO2 level in the giant, self-contained greenhouse Biosphere 2 showed that the rate of calcium carbonate formation in such animals fell by 40 per cent.
Meanwhile, satellite measurements of chlorophyll levels in the open ocean show that primary productivity - the amount of new biomass being produced from carbon dioxide by photosynthesis - has dropped sharply in the past couple of decades (Geophysical Research Letters, DOI: 10.1029/ 2003GL016889).
A team led by Watson Gregg of the NASA Goddard Space Flight Center in Greenbelt, Maryland, compared data from two instruments: the Coastal Zone Color Scanner, which worked from 1979 to 1986, and the Sea-viewing Wide Field of View Sensor (SeaWiFS), which has been running since 1997.
Across the globe, the researchers found ocean productivity has dropped by an average of six per cent since the 1980s. There were regional variations, however, and Gregg says there is probably a range of causes.
In northern waters, sea surface temperatures have risen, reducing mixing between layers and decreasing the supply of nutrients to the surface. This may have cut productivity. Meanwhile, extra nutrients deposited from dust clouds may have contributed to higher productivity in equatorial waters.
The drop could just be part of a natural cycle, but Gregg says we know so little about the factors controlling ocean productivity that it is impossible to be sure. He and others warn that by failing to control CO2 levels, we are making a huge leap in the dark.
"We are taking the reins of the geochemical cycles of the Earth," says David Archer, an expert in global carbon cycles from the University of Chicago. "It's really frightening."
See also Ocean plant life slows down and absorbs less carbon - 17th September 2003