1The Earth might have been sent into an ice age by the break-up of a supercontinent 750 mi

1 The Earth might have been sent into an ice age by the break-up of a supercontinent 750 million years ago, creating a global snowball.

2 The break-up probably caused an increase in rainfall and weathering of rock, say climatologists. This would have sucked greenhouse gases out of the atmosphere and caused a run-away cooling effect. The mechanism could explain how the entire planet becomes encased in a mass of ice, as many researchers think it has done in the past.

3 The theory that the Earth was once completely frozen emerged in the 1960s, when scientists realized that global freezing could happen if the polar ice sheets grew above a certain threshold size. Because bright ice reflects sunlight and heat back into space, growing ice sheets cause further cooling. This feedback loop could tip the climate system into a deep freeze.

4 The planet could eventually thaw as carbon dioxide from volcanoes poking through the ice warm it.

5 In the late 1980s, Joe Kirschvink of the California Institute of Technology nicknamed this state 'Snowball Earth'. Around the same time, geologists began to uncover hints in the geological record that this freeze-thaw process might have happened at least once in the distant past—at the end of the Proterozoic eon, 600 to 800 million years ago.

6 But it was unclear what could have tipped the world into that state in the first place. Now Yannick Donnadieu of the Laboratoire des Sciences du Climat et de L'Environnement in Gif sur Yvette, France, and co-workers provide an explanation in this week's Nature.

7 Donnadieu and colleagues have run computer simulations of global climate change 750 million years ago during the break-up of Rodinia, a supercontinent in which nearly all of the present-day continents were welded together around the South Pole. As the vast land mass fragmented into smaller pieces, driven by the engine of continental drift, they found that evaporation from smaller seas between the isolated continents increased the rainfall over land areas.

8 The increased rainfall in turn speeded the weathering of any exposed rock. As rock is worn away by water, chemical reactions take place in which carbon dioxide from the air becomes bound up in carbonate minerals. The more rain there is, the more of this greenhouse gas is extracted from the air.

9 The team also note that the break-up of Rodinia was prompted in part by the eruption of great plains of volcanic rock. The fresh rock from a volcano is more reactive than old rock, and so it weathers more quickly, sucking up even more carbon dioxide.

10 In the researchers' computer models, the combined effects of higher rainfall and quick-weathering rock reduced the levels of carbon dioxide below the threshold needed to trigger a Snowball Earth.

11 Proving that a Snowball Earth once existed, and understanding how it came about, will help researchers understand more about our planet's climate and the evolution of life. Some scientists think that the thaw after a deep freeze could have stimulated the appearance of the first multicellular organisms, by providing lots of empty space into which new life could expand. There was indeed a proliferation of such life in the late Proterozoic, though no one can yet prove why.

12 The theory may also help predict whether a Snowball Earth will happen again. But there is no need to panic. Estimates say the planet will not form. a new supercontinent for another 250 million years.

"Greenhouse gases" in Para. 2 refer to ______.

A．oxygen

B．hydrogen

C．carbon dioxide

D．air