Hurricane Harvey was the first major hurricane to make landfall in the United States since 2005, but in some ways, it was long overdue. For decades now, tropical storms have been getting a boost from a powerful but still mysterious long-term cycle in North Atlantic sea surface temperatures, which appears to be holding steady in its warm, storm-spawning phase. This cycle, called the Atlantic Multidecadal Oscillation (AMO), swings between warm and cool phases every 20 to 60 years, shifting North Atlantic temperatures by a degree or so and setting the backdrop for hurricane season. Since about 1995, the AMO has been in a warm state, but researchers aren't sure where it's headed next. The AMO has traditionally been attributed to natural shifts in ocean currents, and some think it's on the cusp of shifting back toward a cool, quiescent phase. But others propose that human activities—a combination of declining air pollution and greenhouse warming—might prolong the current warm period, keeping hurricane activity high
