Spewing too much carbon dioxide into the atmosphere could shut down the major ocean current that ferries warm water to the North Atlantic, new climate simulations suggest. Even with a more realistic scenario, the applicability to the real world will be hampered by a lack of direct long-term observations of the Atlantic circulation, says Gerald Meehl, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. Observations help improve simulations, but such data for the Atlantic current don’t go back “for more than a decade or two,” he says. It carries warm water from the South Atlantic northward along the ocean surface into the North Atlantic.