Warming oceans globally will also allow for more thermal expansion of the waters themselves—the distance between liquid water molecules rises as the water grows warmer. That will raise sea levels further than the current roughly three millimeters per year.
Those warmer ocean waters are already lapping at the icy shores of Greenland, speeding the melt of outlet glaciers for the massive ice sheet. Combined with weather anomalies, like a heat wave that hit central Greenland this July and temporarily melted nearly the entire ice sheet surface, this could presage a more precipitous meltdown in the North. "Extreme melting from past years is preconditioning this year's melt," says ice melt researcher Marco Tedesco of the City College of New York, by melting away any accumulated snowfall from the winter sooner. "It's like putting money in a bank account. If you start spending more money than you put in, you go negative. That is what is happening on the ice sheet."
If Greenland were to melt entirely—which is still a distant prospect according to most glaciologists' estimates—the ice sheet contains enough water to raise sea level by six meters globally. "How many people live within six meter sea level rise of the coast?" Barber asks. "The answer is: too many."
Not all is lost
The seasonal loss of all "Arctic sea ice is one of those tipping points and unfortunately we're going to pass that tipping point," said climate scientist James Hansen, director of the NASA Goddard Institute for Space Studies in New York City, at the same Greenpeace event. "I think we're going to lose that sea ice. The good news is: this tipping point is reversible." Should local conditions change, for whatever reason, however, it is possible the ice could regrow.
After all, the ice spreads anew each cold, dark Arctic winter. Some scientists and environmentalists have even suggested it might be time to attempt geoengineering of one form or another to restore the Arctic's cooler temperatures. "We need to look at the possibility of [solar radiation management], which some people call geoengineering," which could be an option to control or reverse the Arctic meltdown, argues environmentalist Rafe Pomerance, former Deputy Assistant Secretary of State for Environment and Development. "Effectiveness and downsides and what the risks are, we need to know all that." Cutting back on emissions of greenhouse gases other than carbon dioxide—such as methane or black carbon—might also have a bigger impact in the Arctic than elsewhere, given the role that soot plays in melting ice.
There are potential positives to the loss of sea ice to consider as well. Open ocean might permit more carbon-absorbing plankton to bloom, much as happens in the Southern Ocean around Antarctica. "At this time, the Arctic Ocean is a biological desert," notes ecologist Louis Fortier of Laval University in Quebec City. If the plankton blooms, the tiny photosynthesizers pull carbon dioxide out of the air and can serve as the bottom of a food chain that could create new and productive fisheries. Plus, if the plankton die without being eaten or decomposed, they could bury CO2 with them as the tiny corpses fall to the seafloor. In fact, artificially fertilizing such plankton blooms has been tried as a geoengineering technique in the Southern Ocean, with some success.