Since 1995, Gray and others have been saying that conditions are ripe for a return to more active hurricane seasons, a periodic shift that could last two or three decades. He pointed to known historical cycles to buttress his argument that nature is at a turning point. In between, from the early 1930s through the late 1960s, more active seasons were the rule. While many factors control the number of hurricanes in a given season, Gray has said the long-term trend is ruled by a conveyor belt of sorts that moves Atlantic Ocean water northward, from near the Caribbean to an area east of Greenland. There, the current sinks deep, moves southward, then flows into the southern Atlantic Ocean and beyond.
Recent measurements show that the water in the conveyor belt contains more salt than normal.
This increases the water's density, researchers explain, causing it to sink to greater depths. This in turn increases the flow of deep water southward, which forces an increase in the flow of warm, tropical surface water northward. Thus, the northern Atlantic becomes warmer than normal, providing the heat needed to make hurricanes into powerful killers. The settling of this air tends to choke hurricanes in their infancy. Details from the 1999 season Hurricane Floyd threatened Florida in September as a strong Category 4 hurricane with 155 m.p.h.