Barbel Honisch Diving for PlanktonThe world’s oceans are growing more acidic as carbon emissions from the modern world are absorbed by the sea. A new film, “A Sea Change,” explores what this changing chemistry means for fish and the one billion people who rely on them for food. This first-ever documentary about ocean acidification is told through the eyes of a retired history teacher who reads about the problem in a piece in The New Yorker and is inspired to find out more. His quest takes him to Alaska, California, Washington and Norway to talk with oceanographers, climatologists and others.Bärbel Hönisch, an expert on ocean acidification at Columbia University’s Lamont-Doherty Earth Observatory, will speak after a screening of the film this Thursday. In a paper published earlier this year, Hönisch found that seawater is more acidic now than it has been for at least two million years, and that carbon dioxide levels are also currently at their highest since then. She is able to reconstruct the past acidity of oceans and how much carbon dioxide was in the air by measuring boron isotopes in the shells of tiny plankton and coral.The film will be aired outdoors, near the offices of Solar One, an environmental advocacy group located at 23rd Street and FDR Drive. The film, directed by Barbara Ettinger, is part of a free, seven-night program called the “Solar-Powered Film Series” hosted by Solar One and Green Edge Collaborative.Details: Thursday, Sept. 17, 2009, 7 pm to 10 pm, (screening begins at sunset, at 7:30 pm or so), 23rd Street & the East River.