Greta Thunberg addressed a blazer-clad crowd of financiers, bankers, investors, and whomever else was in attendance at the Davos World Economic Forum on Tuesday.

The 17-year-old climate activist didn't stray from her blunt message.

"In chapter two on page 108 in the S.R. 1.5 IPCC Report that came out in 2018, it says that if we are to have a 67 percent chance of limiting the global average temperature rise to 1.5 degrees Celsius, we had on January 1st, 2018 420 gigatons of CO2 left to emit in that budget," she said.

Thunberg is telling the grown-ups that, unless radical and immediate action is taken to curb civilization's carbon emissions, the planet will (sometime around 2035) warm by over 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above temperatures in the late 1800s. And according to the landmark U.N. report referenced by Thunberg, this means humanity would experience bad and ever-worsening consequences of a warming planet. How bad? Well, we've seen what just over 1 degree C of warming can do to Australia.

"Since last summer I've been repeating these numbers over and over again in almost every speech," Thunberg said, noting the failure of powerful people and institutions to broadcast these figures.

"I know you don’t want to report about this. I know you don’t want to talk about this. But I assure you I will continue to repeat these numbers until you do."

Thunberg's central number is 1.5 Celsius. She's even repeated it in front of a rapt crowd of international diplomats at the U.N. Now in 2020, Thunberg hasn't let up. And according to experts in communication, it's a smart way to convince people of the need to slash society's still-rising carbon emissions.

"I think her messaging is dead-on," said Jennifer Marlon, a research scientist at the Yale Program on Climate Change Communication.

“I know you don’t want to report about this.

I know you don’t want to talk about this.

But I assure you I will continue to repeat these numbers until you do.” https://t.co/IfyIVd7AZv — Greta Thunberg (@GretaThunberg) January 21, 2020

Why repeating 1.5 C is smart

The 1.5 Celsius figure comes from the over 600-page "Global Warming at 1.5 C" U.N. report, which brims with statistics and analysis most people haven't read, and probably won't ever read. But "1.5 C" summarizes why the report, authored by 91 scientists citing over 6,000 studies, matters.

Repeating the figure, even if it's somewhat wonky and contains a decimal, makes the number familiar and meaningful to people, similar to a succinct slogan (ex: "Just Do It").

"She's taking climate change, which is often very mathematical and statistical, and messaging it with a slogan," said Mike Allen, a professor in the Department of Communication at the University of Wisconsin-Milwaukee. "It reduces a complex issue."

"When something is repeated, it's more likely to be picked up," agreed Jeffrey Jarman, an expert on rhetoric from Wichita State University. "Having these kernels of knowledge out there gives people something to hold on to.

"1.5 degrees [C] is an easy number," he added.

"Repeating is powerful."

Though, Thunberg's message isn't just about repetition. She's combining the number with talking points conveying the ethical need to avoid damaging levels of planetary warming — something people her age are bound to inherit.

"What makes her message so powerful is that she doesn't just talk about the scientific conclusions [of 1.5 C], but couples them with a moral message of the imperative of climate action," said John Cook, a research assistant professor at the Center for Climate Change Communication at George Mason University. "And the moral certainty she speaks with as a teenager inheriting the climate impacts our generation created is the heart of her power."

"She’s connecting emotionally to people — and that’s important," said Allen.

Thunberg is correct to point out that the extreme weather events wrought by a planet barreling towards 1.5 C are already impacting humanity and the environment.

"There are images of burnt koala bears," noted Yale's Marlon, referencing Australia's unprecedented fires. "It's becoming really intense."

Distilling a climate message with a digestible number has another big boon: A vibrant, singular number can withstand the deluge of information, misinformation, and disinformation that floods our digital lives. "Repetition is key, especially in this intense media environment," said Marlon. "It does feel like we're being inundated."

"Repeating is powerful," said Wichita State's Jarman.

How big is the CO₂ bucket for 1.5°C?



Well, the bucket is about to overflow in a few short years, unless we:

1. Turn off the tap (urgently)

2. Put a hole in the bottom to remove CO₂ (negative emissions)#COP25 #CarbonBudget @FutureEarth @gcarbonproject https://t.co/ycYcuFSPdF pic.twitter.com/o7JwZt7GlX — Glen Peters (@Peters_Glen) December 13, 2019

Effective climate messaging in action. Image: Michael Probst / AP / Shutterstock

Though, repeating messages can also effectively sow false or misleading information about climate change. Frank Luntz, a former Republican strategist who once urged Republicans to cast doubt climate change, has emphasized the strength of the repeated message.

"You say it again, and you say it again, and you say it again, and you say it again, and you say it again, and then again and again and again and again, and about the time that you're absolutely sick of saying it is about the time that your target audience has heard it for the first time," Lutz told PBS in 2004.

Repeating 1.5 C, and its associated messaging, certainly won't work for everyone. There is some segment of the population, which Cook estimates at around 10 percent, who are fully-entrenched climate denialists — even if long-respected research agencies like NASA and NOAA continually present evidence of the climate's radical, relentless change.

"People who are climate-deniers dispute from the outset," said Jarman. Repeating a climate message to these folks won't change their views, he said. Rather, it'll probably only agitate them.

But there is a large and growing population of people who are already convinced climate change is occurring and is a threat to future humanity. Thunberg's repetitive messaging motivates those folks, said Cook.

"Climate communication doesn't always have to be about persuading the undecided [or deniers]," said Cook. "It's often more useful to activate the already convinced. And that's what she's done, creating a huge social movement."

Indeed, Thunberg has inspired massive climate strikes around the planet.

Yet, however potent her messaging has been, society still has a daunting, unprecedented task ahead. Earth's temperature will only stop warming when climate emissions drop to zero — but carbon emissions hit a historic high in 2019, and are still rising. Right now, it's extremely unlikely humanity will curb warning at 1.5 C, or even 2 C.

That's sour news, but you can't argue with hard numbers. According to a recent U.N. report, global civilization must cut carbon emissions by 7.6 percent every year for the next decade to meet 1.5 C targets. That's unprecedented.

Curbing warming at 1.5 — or somewhere even near 1.5 C — will be a historic task requiring historic ambition. But that's what's needed to avoid the ever-worsening consequences of a heating globe.

"We do still need the numbers and the basic facts," said Marlon. "It's not the whole conversation, but it's got to be part of it."

"I'm glad Greta's keeping those numbers front and center," she said.