New York (CNN)The United Nations on Friday will mark Earth Day in a historic way.
The organization will ask leaders from countries around the world to sign a landmark climate change accord that came out of negotiations at the COP21 climate summit last December in Paris.
"This would be a landmark in international law, as the number of signatories of the Paris Agreement would then surpass the previous record of 119 signatures for an opening day signing for an international agreement, set by the Law of the Sea in Montego Bay in 1982," the U.N. Framework Convention on Climate Change, which organizes climate talks, said in a news release.
The Paris Agreement is perhaps the world's biggest leap forward in climate change policy in history. It sets the ambitious goal of limiting warming to "well below" 2 degrees Celsius above preindustrial levels. That more or less requires the entire world to get off fossil fuels this century.
But the agreement still needs to be signed and ratified to become international law. And, harder than that, its promises must be translated into real-world action.
Here are answers to a few common questions about the Paris Agreement and what's next. I'll be at the signing ceremony at the United Nations in New York on Friday. Follow me on Twitter for updates.
What's happening exactly on Earth Day?
Expect a bunch of heads of state and diplomats to physically sign this document, which has been translated into six languages and includes pages for countries to sign.
This is the next step toward the Paris Agreement becoming international law.
Wait, didn't everyone agree on this already?
Yes, kind of. At the COP21 U.N. climate summit in Paris, 195 countries agreed to adopt the Paris Agreement. On December 12, they negotiated the text of the document and agreed to adopt it without objection. That was a historic moment and is seen as the major decision point.
Other such conferences have unraveled. But the agreement isn't law yet. It must be signed and ratified by at least 55 countries representing at least 55% of global greenhouse gas emissions. When countries sign the agreement on Friday, they are signaling their intent to ratify the document as soon as possible.
So Friday's signing ceremony is mostly symbolic, but it will indicate whether enthusiasm for the deal has continued in the months since COP21. Some 155 countries, from Somalia to Singapore, are expected to sign on Friday.
Why is the Paris Agreement such a big deal?
Humans are causing global warming primarily by burning fossil fuels like coal, gas and oil, and by chopping down rainforests. If we don't stop doing that stuff we're going to ruin the future. We risk making the planet unlivable for future generations. Warming threatens to drown coastal cities, push animals to extinction, decrease crop yields in certain regions and encourage the spread of disease.
The United Nations had been trying for decades to get countries to agree on a framework for fighting climate change, which is a unique problem in that it requires the cooperation of polluters around the world. The Paris Agreement sets in motion a process for steep emissions cuts -- and it establishes the important goal of limiting warming to only 1.5 or 2 degrees Celsius above preindustrial levels.
Symbolically, the agreement shows something that never has been apparent before: The world is united on this issue. We finally are starting to recognize we have a moral responsibility to act.
Is this all diplomacy, or are countries doing anything?
The Paris Agreement sets in place a process for countries to cut emissions, report on their progress and be held responsible to each other. It's seen as a critical first step -- and a revolutionary turning point. But "the big challenge is we have to turn these words that are on paper into actual action on the ground," said Flynn.
There are signs that's happening already -- and it's being called the "Paris effect," a reference to the fact that countries now know they're going to be held accountable for emissions cuts and are now trying to figure out how to get to zero emissions sometime this century. Since December, for example, India approved six solar projects; Vietnam has said it will stop building coal plants; China announced a five-year plan that set stricter climate targets; the United States halted new coal leases on federal land; JP Morgan said it would stop financing new coal mines; and Saudi Arabia said it won't be an oil state within 20 years.
Countries will be asked to submit updates on how they plan to cut emissions in 2018.
And then they will be asked to come up with new plans in 2020.
Are the U.S. and China going to sign the accord?
Yes. Both countries have said they will sign the document on Friday. That's important because China is the world's biggest annual greenhouse gas emitter. The United States comes in second when you look at annual pollution; counting all emissions, the country has done more than any other to cause this problem.
It's also worth noting that the United States and China stood in the way of previous climate change treaties. They have been key proponents of the Paris Agreement, however, which is cause for hope.
What has to happen for countries to ratify the Paris Agreement?
After countries sign the Paris Agreement they still have to ratify if. Each country has its own national process for doing so. The Obama administration likely will try to ratify the agreement by executive action, without the consent of U.S. Congress, where many Republican lawmakers are skeptical of climate science. (Climate change is real and humans are causing it; 97% of climate scientists agree on this point). Some countries, mostly small island states, which fear they won't exist if the terms of the Paris Agreement aren't met, because melting ice could cause seas to rise and flood waters to overtake their land, are prepared to ratify the agreement on Friday. Other countries have processes that may last many months. The European Union, for example, is likely to ratify the agreement only after each of its member states has done so.
How soon could the Paris Agreement become law?
The deadline for countries to sign the Paris Agreement is April 21, 2017. That's one year from Friday. Again, that indicates their intent to ratify the agreement. They still have to do so. The text of the agreement says it should become law in 2020, said Cassie Flynn, climate change adviser for the U.N. Development Program. It's possible it could become official in 2018, she told me, which would be ahead of schedule.
Have any countries said they won't sign this thing?
Not yet, thankfully. A Malaysia think tank has called for developing countries to stall on signing the agreement to try to pressure richer countries to give more money to help them transition to clean energy and to deal with the consequences of warming that's already happening.
What happens if the United States elects a climate skeptic?
The United States is expected to try to ratify the Paris Agreement before President Barack Obama leaves office in January. Hillary Clinton, the Democratic front-runner to succeed him, supports the Paris Agreement.
Donald Trump and Ted Cruz, the leading Republicans, doubt climate change is real and could try to pull the country out of this international process. If the U.S. already has ratified this treaty, however, it would be difficult, and the formal process could take a few years. Short of that, a Republican president could try to ignore the Paris Agreement without actually pulling out of it.
Among the issues still to be determined is what happens to countries who default on their commitments to cut pollution, said Flynn, the U.N. climate change adviser.
Trump and Cruz are both known for their maverick styles, but some observers say it would be politically costly on the international stage for either of them to abandon this climate accord.
"This time, you (would) have to blow off the whole rest of the world," said Jeffrey Sachs, director of the Earth Institute at Columbia University and a senior adviser to the U.N. "And I don't think (the United States would) find another partner to do that. You'd have to just be the renegade state."