单项选择题

One of the most vivid symbols of global warming is the torrents of melt water that drain from the lakes that form each summer on Greenland’s ice sheet.
Recent studies have shown that this water, which flows deep into the ice through natural drainpipes called moulins, allows the ice to slide faster over bedrock toward the ocean. And the faster the ice flows, the faster sea levels rise. But a Dutch study using 17 years of satellite measurements in western Greenland suggests that the movement associated with the meltwater is not as rapid as had been feared. The acceleration appears to be a transient summer phenomenon, the researchers said, with the yearly movement actually dropping slightly in some places.
"The positive-feedback ,mechanism between melt rate and ice velocity," says the report, published Friday in the journal Science, "appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades."
Greenland is still losing more ice through melting than it gains through snowfall, other measurements show.
The study was led by Roderik S. W. van de Wal of the Institute for Marine and Atmospheric Research of the University of Utrecht. He said the overall slowdown might be because of changing plumbing deep inside the ice. The study builds on earlier work also showing a limited overall change in ice flow from the surface melting.
The authors and independent experts familiar with the research stressed that the findings did not preclude the possibility that more widespread surface melting could eventually destabilize big areas of Greenland, the world’s second largest ice storehouse. Richard B. Alley, a glaciologist at Pennsylvania State University, said that big lakes were likely to form as areas of melting spread inland, and that this could unlock new ice regions to start sliding more.
But Dr. Alley and other experts said the new study showed that it was unlikely that Greenland’s ice had already become destabilized in ways that could cause a surge in sea levels.
The third paragraph can be best understood that ______.