The oceans are becoming more acidic faster than they have in the past 300 million years, a period that includes four mass extinctions, researchers have found.

Then, as is happening now, increases in carbon dioxide in the atmosphere warmed the planet and made the oceans more acidic. These changes are associated with major shifts in climate and mass extinctions.

But while past increases in the atmosphere's carbon dioxide levels resulted from volcanoes and other natural causes, today that spike is due to human activities, the scientists note.