CNN columnist John D. Sutter is spending the rest of the year reporting on a tiny number -- 2 degrees -- that may have a huge impact on the future of the planet. He'd like your help. Subscribe to the "2 degrees" newsletter or follow him on Facebook , Twitter and Instagram . You can shape his coverage.

Humans never have lived on a planet that's 2 degrees Celsius (3.6 Fahrenheit) warmer than it was before we started burning fossil fuels, in the late 1800s, and climate experts say we risk fundamentally changing life on this planet if we do cross that 2-degree mark.

"This is gambling with the planet," said Gernot Wagner , the lead senior economist at the Environmental Defense Fund and co-author of the book "Climate Shock."

Think super droughts, rising seas and mass extinctions.

Yet for all of its importance, I don't think the 2-degree threshold is as famous as it should be. I've heard it referred to as the "north star" for climate negotiations. Meaning: This one little number carries huge importance as a way to focus the world's attention.

It's so significant that it's going to be the subject of my reporting for the rest of the year. I'm calling that effort "2 degrees," and I need your help to make it work.

Until 5 p.m. ET Monday, you can vote on the first story I'll report for the series.

Tell CNN's John Sutter which of these climate change stories you like best - and he'll report on the winner. This poll closes Monday, April 27, at 5pm ET. #2degrees Posted by CNN on Tuesday, April 21, 2015

All of those story ideas came from you, by the way.

They focus on what a 2-degree world might look like.

CNN kicked off this effort with a Facebook chat last week. We asked for your questions about climate change and about the 2-degree threshold, specifically.

I don't have all the answers right now. We'll continue to explore the importance of this number together. But below you'll find quick responses to seven basic questions about this crucial number. Many of them come straight from you, the readers. And I tossed in a couple of my own.

If you'd like to follow this project as it evolves, I'd encourage you to sign up for the "2 degrees" newsletter . And feel free to ask more questions in the comments section below. They'll shape the way I spend the rest of the year reporting on this super-critical number.

1. Where did the idea for 2 degrees come from?

One guy, it turns out. William Nordhaus , an economist at Yale.

The estimate was "crude, but it was a reasonable first start," he told me.

"If there were global temperatures more than 2 or 3 degrees above the current average temperature, this would take the climate outside of the range of observations which have been made over the last several hundred thousand years," he wrote in "The Climate Casino."

A growing body of research now supports the idea.

2. How did 2 degrees become the international standard?

Science has continued to raise red flags about 2 degrees of warming. And that work has led policy experts to conclude that a 2-degree world is something none of us should want.

"You need a judgment call for these things," said Carlo Jaeger, chair of the Global Climate Forum, who has written on the history of 2 degrees Celsius. "And this 2-degree thing was a judgment call that happened at the interface of science and policy."

The United States was among the signatories

3. What would the world look like at 2 degrees?

I'm going to spend the month of May exploring this question, so look for more on this. But here are some striking facts about what scientists expect a post-2-degree world to look like. These are pulled from reports by the National Research Council , the Intergovernmental Panel on Climate Change and the World Bank



• Hurricanes are expected to become 2% to 8% more intense.

• A range of species will be at risk for extinction, particularly amphibians. The IPCC estimates

• The Arctic is expected keep melting, losing 30% of its annual average sea ice.

• Certain crop yields in the United States, India and Africa are expected to decrease 10% to 30%.

• The availability of freshwater is expected to decline by 20%. • Wildfires in the United States are expected to increase 400% to 800% in size.• Hurricanes are expected to become 2% to 8% more intense.• A range of species will be at risk for extinction, particularly amphibians. The IPCC estimates 20% to 30% of animals and plants species will be at "increasingly high risk of extinction" at or near the 2-degree mark.• The Arctic is expected keep melting, losing 30% of its annual average sea ice.• Certain crop yields in the United States, India and Africa are expected to decrease 10% to 30%.• The availability of freshwater is expected to decline by 20%.

So ... not good.

And numbers don't convey the emotional toll.

"I'm from New Mexico," said Nordhaus, the economist who proposed the 2-degree threshold. "I love it there, and I know it's going to be a completely different climate. The trout fishing probably won't be as good. The hiking won't be as good. These forests may look completely different, or burn down. I love to ski. It's one of my things I love most. And that's obviously affected by warming. I love the ocean, and the New England coastline, and it's in peril. That's just for starters."

4. What happens at 2.1 degrees?

No one knows, exactly. Think of 2 degrees like a sort of speed limit -- or a zone of increased risk. Katharine Hayhoe, a climate scientist at Texas Tech, told me that burning fossil fuels is like smoking. How many cigarettes give you cancer? No one knows, exactly. But the more you smoke, the more you up your risk. And 2 degrees, policy experts agree, is certainly risky territory.

Plus, everyone hates a fraction.

Targeting 1 degree of warming is "ridiculous because you can't do it," said Nordhaus, the economist. "Three sounds too high. And you can't have a fraction because it's too complicated.

"So two is kind of an obvious number."

5. How much has the climate warmed already?

The climate already has warmed 0.85 degrees since the Industrial Revolution . And we reasonably can expect to reach 1.5 degrees simply based on the pollution we're already putting into the atmosphere, even with "very ambitious mitigation action" to reduce carbon emissions, according to a 2014 report from the World Bank. Some of that warming is "locked-in to the Earth's atmospheric system," that organization says. The impacts of climate change already are being felt.

6. Is it possible to stay below 2 degrees?

Yes, but it won't be easy.

"If you want to stay below 2 degrees, you have to reduce emissions at an amazing speed -- to an incredible degree," said Jaeger from the Global Climate Forum.

Here's the best guess for what that "amazing speed" might need to look like: Cutting greenhouse gas emissions by some 80% to 90% by 2050, said Jennifer Morgan, global director of the climate program at the World Resources Institute. "It depends on how much risk you want to accept," she said. Some activists, including those from 350.org and Avaaz, which together organized the largest climate change rally in history last year in New York , want to accept less risk than that.

"Our position is 100% clean energy," said David Sievers, a senior campaigner for Avaaz.

7. What happens if we don't take action?

Some writers have called for the world to abandon the 2-degree target, saying it's too ambitious, or even naive. But we need a yardstick to measure progress -- and we need that "north star" to help us set goals that actually would be weighty enough to make a dent in this problem.

If you think 2 degrees sounds bad, 5 degrees is far, far worse.

The IPCC expects a 5-degree world to be characterized by "major extinctions around the globe" and a "reconfiguration of coastlines worldwide." Just beyond that, at 6 degrees, we're looking a "catastro-f***" that would be almost "infinitely costly," said Wagner, the Environmental Defense Fund economist. "It's akin to killing the planet, basically. Or society on the planet."

This much should be clear: Something has to change.

If we shoot for 2 degrees and end up at 3, that's still better (or less awful) than 5 or 6. What's important is that we maintain a sense of urgency, and keep sight of the goal.

Photos: Effects of global warming around the world Melting polar ice caps – The consequences of climate change go far beyond warming temperatures, which scientists say are melting the polar ice caps and raising sea levels. Click through the gallery for a look at 10 other key effects of climate change, some of which may surprise you. Hide Caption 1 of 11 Photos: Effects of global warming around the world Drought – In the coming decades climate change will unleash megadroughts lasting 10 years or more, according to a new report by scholars at Cornell University, the University of Arizona and the U.S. Geological Survey. We're seeing hints of this already in many arid parts of the world and even in California, which has been rationing water amid record drought. In this 2012 photo, a man places his hand on parched soil in the Greater Upper Nile region of northeastern South Sudan. Hide Caption 2 of 11 Photos: Effects of global warming around the world Wildfires – There's not a direct link between climate change and wildfires, exactly. But many scientists believe the increase in wildfires in the Western United States is partly the result of tinder-dry forests parched by warming temperatures. This photo shows a wildfire as it approaches the shore of Bass Lake, California, in mid-September. Hide Caption 3 of 11 Photos: Effects of global warming around the world Coral reefs – Scientists say the oceans' temperatures have risen by more than 1 degree Fahrenheit over the last century. It doesn't sound like much, but it's been enough to affect the fragile ecosystems of coral reefs, which have been bleaching and dying off in recent decades. This photo shows dead coral off the coast of St. Martin's Island in Bangladesh. Hide Caption 4 of 11 Photos: Effects of global warming around the world Food prices – food prices. Here, in 2010, workers on combines harvest soybeans in northern Brazil. Global food experts have warned that climate change could double grain prices by 2050. A U.N. panel found in March that climate change -- mostly drought -- is already affecting the global agricultural supply and will likely drive upfood prices. Here, in 2010, workers on combines harvest soybeans in northern Brazil. Global food experts have warned that climate change could double grain prices by 2050. Hide Caption 5 of 11 Photos: Effects of global warming around the world Pollen allergies – Are you sneezing more often these days? Climate change may be to blame for that, too. Recent studies show that rising temperatures and carbon dioxide levels promote the growth of weedy plant species that produce allergenic pollen. The worst place in the United States for spring allergies in 2014, according to the Asthma and Allergy Foundation of America? Louisville, Kentucky Hide Caption 6 of 11 Photos: Effects of global warming around the world Deforestation – forests. Invasive species such as the bark beetle, which thrive in warmer temperatures, have attacked trees across the North American west, from Mexico to the Yukon. Climate change has not been kind to the world'sforests. Invasive species such as the bark beetle, which thrive in warmer temperatures, have attacked trees across the North American west, from Mexico to the Yukon. University of Colorado researchers have found that some populations of mountain pine beetles now produce two generations per year, dramatically boosting the bugs' threat to lodgepole and ponderosa pines. In this 2009 photo, dead spruces of the Yukon's Alsek River valley attest to the devastation wrought by the beetles. Hide Caption 7 of 11 Photos: Effects of global warming around the world Mountain glaciers – The snows capping majestic Mount Kilimanjaro, Africa's highest peak, once inspired Ernest Hemingway. Now they're in danger of melting away altogether. Studies suggest that if the mountain's snowcap continues to evaporate at its current rate, it could be gone in 15 years. Here, a Kilimanjaro glacier is viewed from Uhuru Peak in December 2010. Hide Caption 8 of 11 Photos: Effects of global warming around the world Endangered species – Polar bears may be the poster child for climate change's effect on animals. But scientists say climate change is wreaking havoc on many other species -- including birds and reptiles -- that are sensitive to fluctuations in temperatures. One, this golden toad of Costa Rica and other Central American countries, has already gone extinct. Hide Caption 9 of 11 Photos: Effects of global warming around the world Animal migration – It's not your imagination: Some animals -- mostly birds -- are migrating earlier and earlier every year because of warming global temperatures. Scholars from the University of East Anglia found that Icelandic black-tailed godwits have advanced their migration by two weeks over the past two decades. Researchers also have found that many species are migrating to higher elevations as temperatures climb. Hide Caption 10 of 11 Photos: Effects of global warming around the world Extreme weather – The planet could see as many as 20 more hurricanes and tropical storms each year by the end of the century because of climate change, according to a 2013 study published in the Proceedings of the National Academy of Sciences. This image shows Superstorm Sandy bearing down on the New Jersey coast in 2012. Hide Caption 11 of 11

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Email questions to: climate (at) cnn (dot) com.