Scientists are increasingly worried that a vast system of ocean circulation, which delivers warmth to northern Europe and impacts climate globally, is at risk of collapse. Mounting evidence suggests it may be nearing a tipping point, though the research is far from certain. As the world careens past our hoped-for target of 1.5 degrees Celsius warming, scientists are growing increasingly alarmed that we may be nearing a dramatic, long-feared “tipping point” — a moment when the main ocean current in the Atlantic Ocean becomes destined to shut down, clamping off the primary source of warmth for northern Europe and playing havoc with the global climate. Such a scenario has been a concern for many decades, but the issue is now heating up. “I have personally researched this for 35 years,” says Stefan Rahmstorf, a physical oceanographer at the Potsdam Institute for Climate Impact Research in Germany.…