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The recent increase in the number of major Atlantic hurricanes may just be a return to the norm after a period of unusually low storm frequency, say researchers.

Johan Nyberg of the Geological Survey of Sweden and colleagues used marine sediment cores of coral samples from the northeast Caribbean to build a proxy record of wind shear and sea-surface temperatures since 1730, and from this they estimated hurricane activity since that time.

High wind shear – the difference in speed and direction between low winds that blow close to the Earth’s surface and winds that are higher up in the atmosphere – causes a decrease in rainfall, which generates denser coral. This is because less rain runs off the land into the sea, raising the water’s salinity and affecting the way coral polyps build their skeletons.