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An epoch of ancient time known as the early Pliocene 3-5 million years ago holds special fascination for climate scientists, because levels of carbon dioxide in the atmosphere were about as high as they are in modern climate.

What puzzles researchers is why, even though conditions such as sunlight, geography, and CO2 concentrations were much like today, Earth's climate was different — temperatures were 7 degrees Fahrenheit warmer, there were no polar ice caps, sea level was 82 feet higher, and the Pacific Ocean was locked in a permanent El Niño.

New modeling work by Yale University researcher Alexey Federov and colleagues proposes a radical explanation for the wide band of especially warm water along the equator — the oceans were virtually alive with hurricanes, which act to vertically mix the tropical ocean and transport heat toward the poles.  The researchers describe a positive feedback mechanism between this vertical mixing and hurricane creation that does not exist in models of modern climate. These depictions that compare current hurricane patterns (top) with the scourge of hurricanes north and south of the equator during the Pliocene Epoch are courtesy of the journal Nature, where Federov's report is published.  His  general circulation model shows dramatically different atmospheric circulation patterns and develops the following hurricane scenario:

Reduced vertical wind shear combined with warmer SSTs lead to a widespread increase in tropical cyclones. There are now two broad bands of cyclone activity both north and south of the Equator extending from the western to the eastern Pacific. Because of the warm pool’s expansion, the lifespan of an average tropical cyclone is now 2–3 days longer. There are many more hurricanes in the South Pacific (and even a few in the South Atlantic). The overall number of hurricanes almost doubles… With reduced SST contrasts, the seasonal dependence of tropical cyclone activity becomes less pronounced, with cyclones occurring throughout the seasons.

The scenario may make the shape of the Pliocene climate more plausible, but it challenges a set of widely held assumptions about modern climate change, today's El Niño and hurricanes. Other models, for instance, support the idea that a tropical ocean will see less mixing rather than more as the climate warms, that hurricanes will become stronger but fewer in the years ahead.  Also, current thinking is that El Niño itself creates high-level winds that tear off the tops of building tropical cyclones, again leading to seasons with fewer hurricanes.

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