Seawater is a little alkaline and during historic times has had a pH of about 8.2. More recently, increased amounts of carbon dioxide in the atmosphere have resulted in more of it dissolving in the ocean forming carbonic acid and has raised this pH level to 8.1. The pH is expected to reach 7.7 by the year 2100, an increase of 320% in acidity in a century. One important element for the formation of skeletal material in marine animals is calcium but it is easily precipitated out in the form of calcium carbonate as the sea becomes more acid. This is likely to have profound effects on certain planktonic marine organisms because their ability to form shells will be reduced. These include snail-like molluscs known as pteropods, single-celled algae called coccolithophorids and foraminifera. All of these are important parts of the food chain and a reduction in their numbers will have significant results. In tropical areas, corals are likely to be very much affected by a lack of calcium with knock-on effects for other reef residents.