Batten down the hatches! Researchers attempting to recreate a worldwide hot spell that began about 5 million years ago have found that such warm periods spawn more frequent hurricane-sized storms, which in turn can increase global temperatures and help prolong the warming by thousands or even millions of years. The findings could help scientists better understand our current climate and build new models to predict future trends.
The early Pliocene period, which began about 5 million years ago, made present-day global climate seem positively chilly. Although atmospheric carbon dioxide concentrations were similar to today—350 to 400 parts per million then versus 387 ppm now—temperatures were as much as 4˚C warmer. Today, the phenomenon known as El Niño periodically spawns a zone of very warm water stretching across the mid-Pacific Ocean and causes a jump in severe weather patterns, particularly in North America. But back in the Pliocene, El Niño eventually settled in to become a more or less constant phenomenon, perpetuating the lousy weather.
Although scientists still don't fully understand what gave rise to these conditions, a team has found a possible engine to keep them going. The researchers suspected that tropical cyclones--the generic name for hurricanes and typhoons--could have helped prolong the warming period by stirring up waters in the upper ocean. And that's exactly what they found when they ran a tropical cyclone model using temperature, atmospheric, and ocean-circulation data that simulated the Pliocene climate. The frequency of the storms roughly doubled over today's rate. And as the team reports tomorrow in Nature, the more-numerous storms quickly became climate drivers, significantly warming the waters of the eastern Pacific ocean, which fueled atmospheric warming--and even more cyclones.
Climate dynamicist and lead author Alexey Fedorov of Yale University describes the phenomenon as cyclones contributing "to a permanent El Niño in the tropics" and a warming that persisted for 2 million years. But from this research it's clear, he says, that cyclonic storms helped to drive the Pliocene warming, and the pattern could emerge again in a newly warming world. The early Pliocene heat wave ended about 3 million years ago, ushering in the current cycle of ice ages. Why the planet cooled still remains a mystery, Fedorov says.
Fedorov cautions that the research doesn't necessarily link the Pliocene scenario to what might happen in the future. For one thing, he says, the severity and number of tropical cyclones in the simulations differ from the projections contained in the current climate models. "Unless we understand the causes of these differences," he says, "we will not be sure whether current projections are correct."
The findings "add support to the idea that tropical cyclones can actively contribute to the global climate system," says climate scientist Ryan Sriver of Pennsylvania State University, University Park. "They are not simply passive responders to climate change," he says.