简答题:根据下面资料，回答46-50题
(46) A major Antarctic glacier is at risk of disintegrating irreversibly if it passes a key tipping point, which could trigger the collapse of the entire West Antarctic ice sheet—and we can't say when it might happen. Pine Island glacier is one of two glaciers flowing into Pine Island Bay, part of the Amundsen Sea off West Antarctica. The other is Thwaites glacier. Both have retreated rapidly due to climate change, contributing to rising sea levels.
Worse, the two glaciers are the weak point of the West Antarctic ice sheet, which sits on bed rock below sea level. (47) A dramatic glacier retreat could let water get under the ice and thus collapse the entire ice sheet， leading to more than 3 meters of sea level rise，over centuries. The main reason the Pine Island glacier is retreating is a current of warmer water that now periodically flows under its floating tip, melting it from below. (48) Rosier and his colleagues simulated the glacier's behavior as the oceanwater at its tip slowly warmed；they found that it passed through not one but three tipping points.The first two both led to rapid ice loss, even if the ocean was later cooled；the third caused the glacier to collapse entirely. This couldn't be stopped by cooling the ocean.
"Tipping point three is sort of game over," says Ted Scambos at the University of Colorado in Boulder. In the model, the third tipping point occurred when the ocean water had warmed by 1.2℃. A 2014 study found that the Amundsen Sea is warming by 0.1 to 0.3℃ per decade. However, Rosier says the 1.2℃ threshold is only "a rough ball park". "It's highly likely that things might happen over a quicker period of time," he says. (49) Rapid ocean warming could even trigger a cascade, in which the first tipping point caused enough melting to unleash the second, and then the third.The real challenge is that the Amundsen Sea isn't warming as if a thermostat were being turned up, says Scambos.
(50) Instead, warmer water that used to be kept out by currents and winds is entering in pulses, controlled by shifts in winds across the Pacific Ocean. Could we see the tipping point coming? In the model, the researchers were able to spot warning signs：Pine Island glacier became slow to respond to perturbations in ocean temperature. But they needed 300 years of data for this to work out how the glacier behaved earlier in its history by studying sediment cores from the ocean floor just off Antarctica, says Scambos.
第（49）题答案