請依下文回答第 46 題至第 50 題：Those who doubt the power of human beings to change Earth’s climate should look to the Arctic, and shiver.There is no need to pore over records of temperatures and atmospheric carbon-dioxide concentrations. The process isstarkly visible in the shrinkage of the ice that covers the Arctic ocean. In the past 30 years, the minimum coverage ofsummer ice has fallen by half; its volume has fallen by three-quarters. On current trends, the Arctic ocean will belargely ice-free in summer by 2040.Climate-change sceptics will shrug. Some may even celebrate: an ice-free Arctic ocean promises a shortcut forshipping between the Pacific coast of Asia and the Atlantic coasts of Europe and the Americas, and the possibility ofprospecting for perhaps a fifth of the planet’s undiscovered supplies of oil and natural gas. Such reactions areprofoundly misguided. Never mind that the low price of oil and gas means searching for them in the Arctic is no longerworthwhile. Or that the much-vaunted sea passages are likely to carry only a trickle of trade. The right response is fear.The Arctic is not merely a bellwether of matters climatic, but an actor in them.The current period of global warming that Earth is undergoing is caused by certain gases in the atmosphere,notably carbon dioxide. These admit heat, in the form of sunlight, but block its radiation back into space, in the form oflonger-wave-length infra-red. That traps heat in the air, the water and the land. More carbon dioxide equals morewarming--a simple equation. Except it is not simple. A number of feedback loops complicate matters. Some dampenwarming down; some speed it up. Two in the Arctic may speed it up quite a lot.One is that seawater is much darker than ice. It absorbs heat rather than reflecting it back into space. That meltsmore ice, which leaves more seawater exposed, which melts more ice. And so on. This helps explain why the Arctic iswarming faster than the rest of the planet. The deal on climate change made in Paris in 2015 is meant to stop Earth’ssurface temperature rising by more than 2°C above pre-industrial levels. In the unlikely event that it is fullyimplemented, winter temperatures over the Arctic ocean will still warm by between 5°C and 9°C compared with their1986-2005 average.The second feedback loop concerns not the water but the land. In the Arctic much of this is permafrost. Thatfrozen soil locks up a lot of organic material. If the permafrost melts its organic contents can escape as a result of fireor decay, in the form of carbon dioxide or methane (which is a more potent greenhouse gas than CO2). This will speedup global warming directly--and the soot from the fires, when it settles on the ice, will darken it and thus speed itsmelting still more.
46 Which of the following is the main idea of the second paragraph?
(A) To raise doubts about the optimism that climate-change disbelievers express.
(B) To introduce potential commerce that an ice-free Arctic ocean may bring.
(C) To indicate incomplete knowledge people have of the Arctic’s economy.
(D) To differentiate the views of those who fear climate-change from those who don’t.