by William Logan

Created on: February 11, 2010

The Earth's atmosphere is changing, and across the almost 140 million square miles where the oceans and atmosphere meet, the atmosphere is changing the oceans. The burning of fossil fuels is rapidly increasing the concentrations of atmospheric carbon dioxide, and this, in turn, is altering the fundamental chemistry of the ocean, warming it, acidifying it, and altering the salinity. The changes threaten biodiversity, affecting species across the food chain. It will affect ocean levels, change important currents, and will take tens of thousands of years to reverse. To make matters worse, it appears that the damage will still occur if the level of CO2 in the atmosphere goes no higher than 450 ppm, the upper limit target set by the IPCC that is looking more and more difficult to achieve.

The level of carbon dioxide in the atmosphere has increased from about 280 parts per million before the industrial age to about 390 parts per million now. As the airborne concentrations rise the amount absorbed by the ocean rises with it, and through a series of straightforward chemical reactions with carbonates in the water, produces carbonic acid, lowering the pH. The oceans natural buffering system, the introduction of calcium and CO3 from the dissolving of rocks on land, simply can't keep up. The historical norm for ocean pH is about 8.1. It has already dropped to around 8.0. The pH scale is logarithmic, so this represents a 25% increase in the level of hydrogen ions, and hence acidity. Most estimates of the oceanic pH, if the atmosphere reaches 450 ppm, hover around 7.7, a drastic and potentially lethal change. Similar changes in temperature and pH in the past have been implicated in mass extinction events, including the Permian extinction, where 95% of all ocean species disappeared. Currently, one hundred and twenty million people depend on the ocean for their livelihood and one in five depend on the ocean for their main source of protein.

The reaction of more acidic water with dissolved carbonate ions decreases the availability of calcium carbonate. This leaves any species that builds a shell or exoskeleton out of calcite or aragonite vulnerable. Most importantly, it puts the small phytoplankton at the bottom of the food chain at risk, threatening the food supply of the larger species. The great coral reefs find it more difficult to synthesize the carbonates they need to create their elegant structures, eliminating vital habitat for other species.

Increased acidity though,

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