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How big was the iceberg that split off from the Larsen C ice shelf in western Antarctica this week? Pretty damn big. As big as Prince Edward Island, according to a description in this publication. The Taiwanese described it as one-sixth the size of Taiwan. In Spain, it was the size of 10 Madrids (the website Quartz compiled a handy chart of the comparisons).

Technically, the iceberg "calved" from the ice shelf, and though I'm no farm girl, I find it hard to believe that calves come that big. All the comparisons of the iceberg's size were useful, but what I really wanted was a guide to the size of panic I should be feeling over the progress of climate change. On a scale of Ingmar Bergman (dread, but no immediate threat) to Cloverfield (imminent destruction by rampaging monster), how worried should I be about this wayward iceberg containing twice the volume of Lake Erie?

Apparently, somewhere in the middle. Psycho, let's say. Yes, I'm joking about a very serious subject, but the point is that it's extremely hard to communicate effectively the threat posed by climate change. If it were easier, we would all be pulling in the same direction and a man who thinks climate change is a hoax perpetrated by China would not have been elected president of the United States.

According to the Union of Concerned Scientists, the new iceberg – which some activists want to name Exxon Knew – won't raise sea levels because it was already floating, so the effect is similar to "ice cubes melting in a cup of water." And it's not uncommon for icebergs to be created this way in Antarctica. That's a relief, right? The bigger picture is a lot more terrifying.

National Geographic's current issue contains a special report on Antarctica, a measured but still heart-stopping survey of the current science and the consequences of warming air and seas. "The Antarctic Peninsula is a hot spot," it says. "The Larsen A and B Ice Shelves have broken up. Larsen C is cracking now." Make that has cracked. In the brief space between the article being written and the magazine hitting coffee tables, an iceberg the size of 10 Madrids sailed off to its watery destiny.

If the Western Antarctic Ice Sheet breaks up and melts "as researchers increasingly believe it might, it would raise sea levels by roughly 10 feet, drowning coasts around the world." The article quotes Helen Fricker of the Scripps Institution of Oceanography in San Diego saying, "I think it's time for us scientists to stop being so cautious" when talking about the risks.

That, of course, is the million-dollar question: How do you adequately communicate the risk while not frightening people so much that their brains shut off and they turn to Instagram to find a nice, soothing picture of a labradoodle? Is it time to sound the emergency klaxon – even if it will cause many people to clap their hands over their ears?

New York magazine tested those waters this week with an article titled The Uninhabitable Earth. "No matter how well-informed you are, you are surely not alarmed enough," writes David Wallace-Wells, before embarking on a worst-case-scenario tour of a baking, suffocating, plague-ridden Earth mostly unfit for human habitation. This is not a prediction but rather a warning, the author says, of what could conceivably happen "absent aggressive action" to curb our gassy bad habits.

The article generated an enormous amount of coverage, both on social media ("we're so screwed!") and among climate scientists about whether, in fact, alarmism is the most useful communications method, or even a particularly good one. In the ensuing hubbub, Mr. Wallace-Wells defended his freak-out rhetorical strategy in an interview with Gothamist: "It seemed like a much bigger concern that people weren't worried enough, than that they would get too worried."

Fortunately, someone has already written a very good book on the subject of climate change, psychological denial and effective messaging. In his 2014 book Don't Even Think About It: Why Our Brains Are Wired to Ignore Climate Change, George Marshall looks at a variety of research into the way humans react to fear and threats, especially those that are far off and hazy, and concludes that "people have only a limited capacity for staying on red alert."

In fact, alarming messages can often be paralyzing and counterproductive. "When people feel threatened and isolated," Mr. Marshall writes, "they can adopt a range of strategies to diminish their sense of internal fear: denial, uncertainty, playing down the threat, fatalism, and anger toward the communicator."

Instead, a more effective way to mobilize people may be what he calls "a narrative of positive change," which essentially means that telling compelling stories about how people can come together in pursuit of a more just, equal and not-quite-so-sweltering planet. Such stories require heroic journeys, emotional connections, recognizable symbols. An iceberg the size of Prince Edward Island is probably a good place to start.