That huge iceberg should freak you out. Here's why

Some scientists think this has a lot to do with global warming.

I spent most of Thursday on the phone with scientists, talking to them about the huge iceberg off Antarctica and what it means. Here are my five takeaways.

There is no disagreement among climate scientists about whether humans are warming the Earth by burning fossil fuels and polluting the atmosphere with greenhouse gases. We are. And we see the consequences.

But there is some dispute about whether there is enough evidence to tie the breakoff of this particular piece of ice to global warming.

In a widely quoted statement, Martin O'Leary, a Swansea University glaciologist who was part of the team studying Larsen C, said that the iceberg calving was "a natural event" and that "we're not aware of any link to human-induced climate change."

Not everyone agrees with that assessment, however.

"They're looking at it through a microscope" rather than seeing macro trends, including the fact that oceans around Antarctica are warming, helping thin the ice, said Kevin Trenberth, a distinguished senior scientist at the US National Center for Atmospheric Research.

"To me, it's an unequivocal signature of the impact of climate change on Larsen C," said Eric Rignot, a glaciologist at NASA's Jet Propulsion Laboratory and the University of California, Irvine. "This is not a natural cycle. This is the response of the system to a warmer climate from the top and from the bottom. Nothing else can cause this."

Rignot said colleagues who say otherwise are burying their heads "in the ice."

The difference in opinion stems, in part, from a perceived lack of data. Compared with other parts of the world, Antarctica is cold, weird, remote and hard to study. Some scientists say they don't have the super-long-term datasets they would need to prove that man-made warming affected this particular ice sheet.

Conversely, they can't disprove global warming's contribution, either.

"I myself don't see clear evidence convincing me that this is climate change-related," said Christopher Shuman, a research scientist at NASA's Goddard Space Flight Center and the University of Maryland, Baltimore County. "I think we need to wait and see. We need to watch carefully and wait for the signs."

If the Larsen C ice shelf continues to collapse, he said, we'll know that climate change had something to do with this week's events. If not, his theory will be confirmed, meaning the iceberg is part of a natural cycle of calving and regeneration.

Nearby ice sheets with similar names -- Larsen A and B -- broke down for reasons that are related to climate change, Shuman said. The cause of Larsen C's break this week is less clear, he said, because it's winter in the Antarctic now; there was no evidence of meltwater on the surface of the iceberg, and there aren't enough data about temperature trends in that area, both in the water and in the air.

Rignot, the scientist in California, said the fact that this collapse happened during the dead of winter in the Southern Hemisphere makes it all the more remarkable.

He sees the broader collapse of the Larsen C ice shelf as inevitable.

The question for him is when: likely 10, 20, maybe 30 years, he said.

Further complicating things, climate change is reshaping Earth's southernmost continent in a variety of ways. Air temperatures have risen over the Antarctic Peninsula, where Larsen C is located, said David Vaughan, director of science at the British Antarctic Survey, but not as much in other locations.

Ocean temperatures are up worldwide too, including near Antarctica, he said. "The amount of heat that's going into the ocean is really huge," Trenberth said.

That's helping thaw some of the ice from above and below. Ice that touches the water is thought to be particularly vulnerable to the effects of global warming. But, again, Antarctica is big and cold and weird. It's not melting as quickly as ice in the Arctic, where temperatures are rising about twice as fast as the global average.

It's clear that some ice sheets in Antarctica are melting because of climate change and that some glaciers are getting thinner, researchers told me. The question is whether there are sufficient data to say that this particular iceberg was broken off because of greenhouse gas pollution.

"Do I believe that's a climate change impact? Probably. Not 100%," said Vaughan, of the British Antarctic Survey. "But the reason the warm water comes onto the continental shelf at all is because of the winds that blow across the southern ocean, and we believe the winds have gotten more intense because of climate change.

"This is not straightforward," he added. "It's simply not as straightforward as 'the temperature rises and the ice melts.' It's a whole chain of quite subtle and very consecutive processes that cause the ice to melt and the sea levels to rise."

So, again, it's complicated.

But step back from the microscope: Greenhouse gas pollution is contributing to warming and melting.

To better understand the ice in Antarctica, imagine a cocktail glass with ice and liquid in it. What happens to the level of the drink as the ice melts?

"When my ice melts, the cocktail doesn't overflow," said Rignot, the glaciologist. The surface stays about level.

This type of ice -- the floating-in-water kind -- is what broke off into the ocean this week in Antarctica. Scientists call it an "ice shelf." As opposed to a glaciers or ice sheets, which are found on land, floating ice shelves don't raise global sea levels appreciably when they break off into the ocean and melt.

Again, think of the cocktail glass.

But -- and this is an important "but" -- those floating ice shelves often have a "buttressing effect" on inland ice masses that will raise global sea levels if they melt, said Trenberth, the climate scientist at the National Center for Atmospheric Research. As that fringe of ice goes, it can help to start destabilizing the middle.

So while scientists are right to say the Larsen C iceberg won't change global sea levels in a literal and immediate sense, the big picture may be less rosy and simplistic.

"Floating ice doesn't change sea level at all," Trenberth said, "but the consequences of this may well end up increasing sea level quite substantially in the long term."

Already, global sea levels are up about 8 centimeters since 1992, largely because of global warming, he said. And they're rising at a rate of 3.4 millimeters per year.

Those numbers probably sound small. They are small. Who cares about a few millimeters of difference in the tides?

But this reveals yet another failing in the way most of us are talking about the melting of the world's ice: We must force ourselves to think on bigger timescales.

We should think about New York, Miami, Shanghai and other coastal cities threatened by rising seas in our lifetimes. I can't write about this issue without remembering my trip to the Marshall Islands, a tiny island nation in the Pacific that stands to lose all of its territory -- and language, culture and history -- if temperatures rise just 2 degrees Celsius, which seems increasingly likely.

But we must also consider future generations. What kind of world are we dumping on them?

When you think in terms of decades or centuries, the most vulnerable part of the West Antarctic Ice Sheet could raise global sea levels as much as 3 meters, Rignot told me. And the larger East Antarctic Ice Sheet, in the very long term, could lift tides 19 meters, he said. That's more than 62 feet, or a building that's at least six stories tall.

"The actions we're taking now actually have consequences 50 years from now, 100 years from now -- and 200 years from now," Trenberth said.

When we see trillion-ton ice chunks falling off a continent, that's well worth remembering.