YOU’VE HEARD THIS one before: The Earth is complex and constantly changing, so how can scientists possibly know that burning fossil fuels will do so much harm to the planet? This argument has never been persuasive. It is no mystery that adding heat-trapping gases to the atmosphere will trap more heat. That some uncertainties remain does not necessarily favor the doubters: Things could be worse than expected — not just better.

Two new papers on how meltwater flows on the surface of Antarctica’s vast icy expanse drive this essential point home. There is an astonishing amount of water frozen on top of the southern continent, hemmed in by floating ice shelves abutting the Antarctic land mass. For now, that is: A major ice shelf disintegrated in 2002, and scientists just reported an ominous new crack in another close by. Losing ice shelves encourages the ice further back to melt and drain into the ocean, raising the seas to dangerous levels. A major threat to these ice shelves is meltwater that pools on the surface, widening cracks and encouraging them to break up. Scientists have known about this threat for years, yet they still do not know much about Antarctica’s “plumbing.”

A team from Columbia University and the University of Sheffield that examined decades of satellite monitoring and aerial photographs found vast networks of meltwater-fed streams and lakes across Antarctica. The streams can flow for up to 75 miles before reaching melt ponds or the sea. Melt ponds, meanwhile, can be massive — up to 50 miles long. If this system delivers increasing amounts of water to the wrong parts of delicate ice shelves, it could severely damage them.

As the temperature rises, more meltwater will flow into this hydrological system, and the scientists warn that the region might enter a devastating feedback loop. As more ice melts around the continent, more rocks and other nonwhite features of the landscape are exposed. These darker features absorb more of the sun’s heat. This encourages melting. The resulting meltwater could then encourage ice shelves to decline, which could encourage further thinning farther back on the continent, and therefore further exposure of heat-absorbing rocks.

Even so, it is not clear every ice shelf is in critical danger. In another paper, the scientists discussed a drainage system in one part of Antarctica that diverted meltwater directly into the ocean, apparently without undermining sensitive parts of the ice shelf over which it flowed. Rather than undermining the stability of the ice, the flow appears to be bolstering it.

It will take years more research for scientists to better account for Antarctic meltwater in climate models. But it would be foolhardy to assume that it will all harmlessly drain into the ocean. Better to take the warning: Dramatically higher sea levels may come sooner than we think.