Much of the change in climate change is happening to the ocean. It's not just the extra heat hiding within the waves. The seven seas also absorb a big share of the carbon dioxide released by burning the fossilized sunshine known as coal, natural gas and oil. All those billions and billions of CO2 molecules interact with the brine to make it ever so slightly more acidic over time and, as more and more CO2 gets absorbed, the oceans become more acidic.

Now scientists have delivered the most comprehensive maps of this acid phenomena, a global picture of the oceans in 2005 against which future scientists can track just how much more acidic the oceans have become.

The maps are an attempt to bring to visual life a problem that is just as invisible as the excess CO2 piling up in the atmosphere for the past couple of centuries. People cannot see, taste or feel the subtle shift in the seawater and it has taken years of measurement around the world to gather enough data for this new global picture. Calls for such measurements had been made since at least 1956. Charles David Keeling heeded the call back then, producing the Keeling Curve, which tracks rising CO2 levels to this day. But a similar set of measurements for the oceans has been lacking—until now.

Geochemist Taro Takahashi of Columbia University has spent four decades measuring these changes, which amount to a generally basic ocean growing 30 percent more acidic—a change in pH from 8.2 to 8.1. That's the result of absorbing roughly 25 percent of annual CO2 pollution, which now amounts to 36 billion metric tons in total.

In other words, the oceans currently take in roughly 9 billion metric tons of CO2 each year—a number that is growing. Cumulatively the oceans have absorbed at least 150 billion metric tons of CO2. Despite the size of the increase, it is still just one part in 1,000 of the CO2 already in the ocean.

The new maps highlight the fact that the world ocean is not uniform. The vast belts of seawater that lie in the tropics and temperate zones vary the least in pH while Arctic and Antarctic waters seesaw as plankton blooms, sucking CO2 out of the water, before the cold waters of winter again absorb yet more CO2 and become more acidic. In fact, in the northern winter the Bering Sea becomes the most acidic ocean on Earth, reaching a pH of as low as 7.7. That won't remain an anomalously low number for long, however, as that is the pH the entire ocean may experience if present trends continue through 2100.

Another anomaly is the Indian Ocean, which is about 10 percent more acidic than the Atlantic or Pacific. The exact reason why is a scientific mystery but perhaps it is because it is the most isolated sea, dominated by monsoon rains and river drainage rather than ocean currents mixing the waters, Takahashi suggests.

Globally, if current trends continue, then corals and other shell-building microscopic sea animals and plants may find it harder to build and sustain their shells. Already, the shells of foraminifera (microscopic amoeba-like animals) are shrinking as a result of more acidic waters in the Southern Ocean around Antarctica. And shell-building life in Arctic oceans may become chemically impossible, forcing shell-building sea snails and sea slugs that thrive there now to migrate to warmer waters, if that's even possible.

Fossil fuel burning is now making oceans more acidic at a rate unseen in at least 300 million years so it will be important to continue to follow up on these measurements. Perhaps there can soon be a "Takahashi Map" to complement the Keeling Curve. Because as marine chemist Scott Doney of Woods Hole Oceanographic Institution wrote in Scientific American in March 2006: "Although the effects may be hidden from people's view, dramatic alterations in the marine environment appear to be inevitable."