杏吧原创

AFTER years of looking to the Sun to explain the natural cycles of the climate, scientists say they have found a major new influence 鈥� the Moon. American researchers believe the Earth鈥檚 inert satellite not only triggered a little ice age 500 years ago, but is on course to warm the world for several hundred years to come, even without the greenhouse effect.

The Moon resets the planet鈥檚 thermostat through its influence on the tides, says Charles Keeling of the Scripps Institution of Oceanography at the University of California, San Diego. He says lunar oscillations explain an intriguing 1500 to 1800-year cycle in the world鈥檚 temperature uncovered three years ago by Gerard Bond of the Lamont-Doherty Earth Observatory at Columbia University in Palisades, New York. This puts the Earth in the middle of a warming phase that began at the end of the little ice age, and will carry on until the 24th century.

Bond unearthed this rhythmic fluctuation between warm and cold eras by analysing sediment cores from the Atlantic Ocean. He has now traced the cycle back more than 100 000 years to the last major ice age. 鈥淚t seems to be the pacemaker of rapid climate change,鈥� he says. But he had no explanation for this cycle.

But Keeling and his colleague Timothy Whorf think they know what鈥檚 going on. They say that the changing alignment of the Earth, Moon and Sun causes gradual changes in the strength of tides on the same timescale as Bond鈥檚 climate cycle. Strong tides, Keeling says, 鈥渋ncrease the vertical mixing of water in the oceans, drawing cold ocean water from the depths to the surface, where it cools the atmosphere above鈥�. Weak tides reduce this mixing 鈥� keeping the cold water at the bottom of the sea out of contact with the atmosphere, allowing the world to warm. 鈥淐harles鈥檚 idea is very interesting 鈥� absolutely a possible explanation,鈥� says Bond.

Keeling鈥檚 calculations show that tides reached their maximum strength around 1425 in the depths of the little ice age when the River Thames frequently froze over in London. Since then the tides have weakened and will not peak again until around 3100. Although other forces are at work, he says, the 1800-year cycle has been 鈥渢he principal driver of climate change鈥� for the past 10 000 years.

Keeling, who first demonstrated the accumulation of greenhouse gases in the atmosphere forty years ago, says his findings do not contradict the idea that these gases are warming the world. Instead, they make it all the more important that the world redoubles its efforts to control greenhouse gases.