Pope Francis laughs alongside US President Barack Obama upon arrival at Andrews Air Force Base in Maryland, September 22, 2015, on the start of a 3-day trip to Washington. AFP PHOTO / SAUL LOEBSAUL LOEB/AFP/Getty Images

These days, climate change momentum is everywhere. It’s in U.S. politics, as the Clean Power Plan promises to significantly curb U.S. emissions by 2030. It’s in the clean energy industry, as a solar and wind boom couldn’t possibly arrive at a more opportune time. It’s in the international arena, as the nations of the world appear finally on the verge of a carbon agreement this December in Paris.

And most of all it’s in hearts and minds, as Pope Francis, arriving in the U.S. as a climate rockstar the likes of which we’ve really never seen before, tells a billion Catholics and then some that climate change is a moral issue — and gives the Paris talks even more momentum.

Those who have followed the climate issue over the years — generally a pretty depressing task — aren’t used to so much good news all at once. So it can be hard to know what it means, and what it doesn’t. Could it ultimately signify that it’s time to feel optimistic that the world is, finally, on the verge of being “saved”?

Certainly that’s what some are arguing. In New York Magazine, Jonathan Chait recently wrote a powerful article on the topic; “This is the year humans finally got serious about saving themselves from themselves,” the subtitle reads. Chait cites many of the trends above, observing that, “Even if all the Paris talks do is simply eliminate the risk of the all-too-thinkable worst-case scenario, it would constitute a monumental achievement in the history of human civilization, like the development of modern medicine.”

There’s a crucial distinction, though, between finally beginning to address climate change as a global society — something that the Pope is clearly helping push the world to do — and taking major climate-related risks entirely off the table.

There’s little doubt that this year, thanks to growing consciousness and policy momentum — epitomized by Pope Francis, but also major developments like last year’s emissions deal between the U.S. and China — we are poised to finally begin addressing the climate problem, through the United Nations process. Unlike in 2009 with the letdown in Copenhagen, there are many reasons to expect that Paris will indeed lead to a successful agreement.

But steady now: The problem is that even with a Paris agreement, there would be a very long way to go. And it isn’t really clear yet how much damage has already been done — or how much will be done before we get there.

To see why, let’s consider first the growth of clean energy — without which the climate problem will never go away. Chait is right to argue that it has been stunning, featuring not only plunging costs for wind and solar but also an ever-growing percentage of energy that comes from carbon-free sources.

However, let’s be clear: In the U.S., wind and solar still only contribute a relatively small fraction of electricity. Wind provides 5 percent and solar, considerably less than that. With these two energy sources, it is important not to confuse rapid growth rates — which clearly exist — with the idea that they’re powering large chunks of the land. The truth is that even if a real, momentous change is indeed afoot right now, Americans are still going to be using a substantial amount of carbon intensive energy for a long time.

So are countries across the world.

Meanwhile, on the climate front, a U.N. agreement this year is not expected, on its own, to set the world on a course to limit warming to 2 degrees Celsius above pre-industrial levels. Certainly the national pledges leading into the meeting don’t add up to that, according to the International Energy Agency.

And even as staying below the 2 degree threshold remains far from assured, more and more scientists are expressing the opinion that 2 degrees is not a safe limit on warming — especially not in light of how temperatures and carbon dioxide levels pretty close to our own have prompted very large rises in sea level during past eras of Earth’s history. That’s why so many nations of the world want to keep the limit below 1.5 degrees C, rather than 2 degrees — even though 1.5 degrees is a nearly unattainable target.

So what’s the climate’s real safe zone? Nobody can say for sure. That’s why the experiment humans are running is so unsettling — and why what happens after Paris will wind up being so important.

For even assuming that Paris is a success and clean energy continues to grow like gangbusters, emissions won’t suddenly cease tomorrow, and there’s a fair amount of warming (and sea level rise) already baked into the system. And we honestly don’t know whether or not, by the time temperatures have peaked and are on the way down again, human emissions will have already in effect flipped any climate change “wild cards,” as my colleague Joby Warrick recently put it, involving the circulation of the oceans, the planet’s increasingly worrisome ice sheets, or Arctic permafrost (to name a few).

Indeed, climate researcher Michael Mann recently argued that climate change risk has a “fat tail” — “the likelihood of very large impacts is greater than we would expect under typical statistical assumptions,” he observed.

“Uncertainty is not our friend when it comes to the prospects for dangerous climate change,” Mann continued.

So, the outlook now is definitely for a great deal more political (climate policy) and technological (clean energy industry) progress. Pope Francis is the largest global figurehead of this, and he arrives with the right message at a time when the world is primed to hear it.

But even assuming prompt action in Paris this year, it’s still hard to know whether this means the planet will stay in a relative climate safe zone, and avoid any triggers or abrupt changes.

The case for climate optimism, then, is simply this — action really does seem to be starting to happen. The momentum is indeed real. The case for realism, though, is that it’s still not clear how fast emissions will come down, or whether it will be fast enough.