雅思阅读备考材料：全球变暖问题

　　H. Another worry is that Arctic warming will influence ocean circulation in ways that are not fully understood. One link in the chain is the salinity of seawater， which is decreasing in the north Atlantic thanks to an increase in glacial melt waters. “Because fresh water and salt water have different densities， this ‘freshening’ of the ocean could change circulation patterns.” said Dr. Thomson， a British climate expert. “The most celebrated risk is to the mid-Atlantic Conveyor Belt， a current which brings warm water from the tropics to north-western Europe， and which is responsible for that region’s unusually mild winters，” he added. Some of the ACIA’s experts are fretting over evidence of reduced density and salinity in waters near the Arctic that could adversely affect this current.

　　I. The biggest popular worry， though， is that melting Arctic ice could lead to a dramatic rise in sea level. Here， a few caveats are needed. For a start， much of the ice in the Arctic is floating in the sea already. Archimedes’s principle shows that the melting of this ice will make no immediate difference to the sea’s level， although it would change its albedo. Second， if land ice， such as that covering Greenland， does melt in large quantities， the process will take centuries. And third， although the experts are indeed worried that global warming might cause the oceans to rise， the main way they believe this will happen is by thermal expansion of the water itself.

　　J. Nevertheless， there is some cause for nervousness. As the ACIA researchers document， there are signs that the massive Greenland ice sheet might be melting more rapidly than was thought a few years ago. Cracks in the sheet appear to be allowing melt water to trickle to its base， explains Michael Oppenheimer， a climatologist at Princeton University who was not one of the report’s authors. That water may act as a lubricant， speeding up the sheet’s movement into the sea. If the entire sheet melted， the sea might rise by 6-7 meters. But when will this kind of disastrous ice disintegration really happen? While acknowledging it this century is still an unlikely outcome， Dr. Oppenheimer argues that the evidence of the past few years suggests it is more likely to happen over the next few centuries if the world does not reduce emissions of greenhouse gases. He worries that an accelerating Arctic warming trend may yet push the ice melt beyond an “irreversible on / off switch”.

　　K. That is scary stuff， but some scientists remain unimpressed. Patrick Michaels， a climatologist at the University of Virginia， complains about the ACIA’s data selection， which he believes may have produced evidence of “spurious warming”. He also points out， in a new book， that even if Arctic temperatures are rising， that need not lead directly to the ice melting. As he puts it， “Under global warming， Greenland’s ice indeed might grow， especially if the warming occurs mostly in winter. After all， warming the air ten degrees when the temperature is dozens of degrees below freezing is likely to increase snowfall， since warmer air is generally moister and precipitates more water.”

　　L. Nils-Axel Morner， a Swedish climate expert based at Stockholm University， points out that observed rises in sea levels have not matched the IPCC’s forecasts. Since this week’s report relies on many such IPCC assumptions， he concludes it must be wrong. Others acknowledge that there is a warming trend in the Arctic， but insist that the cause is natural variability and not the burning of fossil fuels. Such folk point to the extraordinarily volatile history of Arctic temperatures. These varied， often suddenly， long before sport-utility vehicles were invented. However， some evidence also shows that the past few millennia have been a period of unusual stability in the Arctic. It is just possible that the current period of warming could tip the delicate Arctic climate system out of balance， and so drag the rest of the planet with it.

　　M. Not everybody wants to hear a story like that. But what people truly believe is happening can be seen in their actions better than in their words. One of the report’s most confident predictions is that the breakup of Arctic ice will open the region to long-distance shipping and， ironically， to drilling for oil and gas. It is surely no coincidence， then， that the Danish government， which controls Greenland， has just declared its intention to claim the mineral rights under the North Pole. It， at least， clearly believes that the Arctic ocean may soon be ice-free.