The world's CO2 emissions fell in 2015. But don't celebrate just yet.

The chart below has inspired more than a few breathless headlines this week:

(Global Carbon Project)

See that dip at the end? A new study in Nature Climate Change reports that worldwide carbon dioxide emissions likely dropped in 2015, something that usually doesn't happen unless there's a global economic crisis.

That's definitely noteworthy. And there are some interesting underlying reasons for the drop, mainly regarding shifts in China's energy habits, which we'll get to in a sec.

The trouble is that a number of media outlets, including the New York Times, have gone even further, suggesting that global carbon emissions might finally be peaking for good in 2015. That seems unlikely. And don't take my word for it: One of the study's authors, Corinne Le Quéré of the University of East Anglia, has flat-out said it's very unlikely.

So before anyone uncorks the champagne, it's worth exploring why we're (probably) not at peak emissions just yet. It's a good way to see what a vast undertaking it will be to decarbonize the world's energy supply and halt global warming.

Why CO2 emissions fell worldwide in 2015 (hint: China)

It's true that something remarkable happened in 2015. Historically, worldwide CO2 emissions have gone up each and every year unless there's a major crisis that puts a dent in global economic growth. So emissions dipped temporarily in the early 1990s after the collapse of the Soviet Union, and they fell temporarily in 2008 with the financial crisis. But once growth returns, they usually go up, up, up.

This year appears to be the first time this century that didn't happen. The global economy grew at healthy 3.1 percent clip this year, but emissions fell nonetheless.

If we zoom in for a country-by-country breakdown, we can see more clearly what's happening here:

(Global Carbon Project)

Emissions are clearly declining in a lot of wealthy, developed countries around the world. They've hit a peak in overall fossil fuel consumption and are now making the transition to cleaner forms of energy. In the United States, we've been replacing coal plants with lower-carbon natural gas plants and adding more wind and solar. In the European Union, CO2 emissions have been declining since 1990.

In poorer, developing nations, by contrast, emissions continue to grow rapidly. China, India, South Africa, Turkey, Indonesia, etc. are all ramping up their use of coal, oil, and natural gas to increase electricity use, power factories, put more vehicles on the road, and generally get richer. They're trying to catch up with the Canadas and Japans of the world. (Also note that rich countries have been outsourcing their factories, and hence emissions, to poorer countries.)

But the biggest story of all is China, the world's No. 1 emitter. Ever since the 1990s, China has been growing at a spectacular rate, mainly fueled by coal. That, in turn, has helped drive up global emissions.

Until now. In 2015, as best anyone can tell, China's emissions appear to have stalled out and then dropped — which, in turn, helped drive the decline in global emissions. China is so huge that its every hiccup causes the world to rumble. Which raises the follow-up question...

What the heck is going on in China?

Let's zoom in for a breakdown of China's energy use. The country's energy statistics are notoriously unreliable, so we do need to be cautious here. But the big story here seems to be coal:

(Global Carbon Project)

China's coal consumption appears to have tapered off in 2014 and then dropped in 2015, which was arguably the main reason for the dip in emissions worldwide.

Now here's where things get murky: It's quite difficult to figure out how much of the drop in China's coal use (if it's real) was due to temporary factors and how much was due to permanent structural changes that will last for years to come. I wrote about this at length here.

On the temporary side: China has had some unusually wet years of late, which have allowed it to squeeze more electricity out of its hydroelectric dams and reduce output from its coal power plants. Also, China has been going through a major economic slowdown this year, after years of breakneck growth. That's also put a dent in coal consumption, though it raises the possibility of a rebound in the years ahead.

At the same time, however, China is taking other measures to facilitate a gradual shift away from coal. Utilities are adding more nuclear, wind, and solar capacity. The country is also trying to transition from factory-centric growth to service-centric growth, which would likely entail slower rates of coal consumption growth in the future.

If you put this all together, what you'd expect is that China's CO2 emissions will likely rebound in the years ahead and keep growing — but at a considerably lower rate than they have over the last two decades. Indeed, that's the official view from Beijing, which expects emissions to keep growing more slowly in the years ahead until peaking sometime around 2030.

Why we (probably) haven't hit peak global emissions yet

No one has a crystal ball, but right now, many experts are predicting that global emissions won't peak until around 2030 or so, at least on our current course. Indeed, most analyses of the climate pledges currently being submitted to the UN suggest as much:

(Javier Zarracina/Vox)

There are a few reasons why most forecasters think emissions will keep rising for the next 15 years or so (at least):

First, China. As noted above, it's entirely plausible that China's drop in emissions this year was a blip, an artifact of better-than-average hydro output and the economic slump. Going forward, the country may not go through the same coal-fueled industrial binge that it did in the 1990s and 2000s, but there's still plenty of room for further fossil-fuel growth. Chinese car ownership is still well below European levels and even per capita household electricity use remains one-fourth of Germany's. That all helps explain why the central government doesn't expect emissions to peak until 2030 or so.

Then there's India. India is still very poor, with emissions per capita just one-third that of Europe. The country still has 300 million people without electricity, and the government has ambitious plans to greatly expand coal consumption in the decades ahead. Fossil-fuel use is all but certain to skyrocket. The big question for India is how much of that expected demand can be supplanted by cleaner sources. The country has major plans to expand solar power, but solar can't (yet) compete with coal on either price or reliability.

Then there are all those other developing countries. Indonesia. South Africa. Mexico. Turkey. They're also expected to see continued emissions growth in the decades ahead, and they collectively have plans for hundreds of new coal plants.

The betting line is that this emissions growth in poorer developing countries will continue to outweigh the cuts made by the United States, Europe, and other rich countries for the next decade or two. If we want to reach a global CO2 peak sooner, then either those rich countries will have to make even deeper cuts or those poorer countries will have to switch over to clean energy even faster than they're doing.

And, of course, if we want to halt global warming, then it's not enough for annual CO2 emissions simply to peak. Emissions then have to start declining sharply, eventually going down all the way to zero.

Carbon intensity has to fall 6% each year if we want to avoid serious global warming. We're at ~1%.

(PriceWaterhouseCoopers)

Or here's a simpler way to think about the decarbonization challenge, courtesy of a recent report from PriceWaterhouseCoopers.

The graph above shows carbon intensity, which is the amount of carbon-dioxide the world needs to emit to generate a dollar of economic activity. The number needs to go down sharply over time if we want to enjoy economic growth without corresponding emissions growth.

The good news is that global carbon intensity has been declining over time, by about 1.3 percent per year since 2000. We've been getting more energy-efficient and adding more clean-energy capacity. Today, we're able to generate more economic activity with fewer and fewer fossil fuels.

The bad news is that carbon intensity isn't dropping fast enough. If we want emissions to peak by 2030 and keep global warming to around 3°C — which is currently what countries are aiming to do in their UN climate pledges — then carbon intensity has to decline by 3 percent per year. Only a few countries have managed to sustain that pace since the beginning of the millennium. (The United Kingdom is one.)

And if we want to stay within the carbon budget thought necessary to avoid 2°C of global warming? Then carbon intensity has to drop by a jaw-dropping 6.3 percent each year. Very, very few countries have managed to sustain that pace for any length of time. It's equivalent to what France pulled off back in the 1980s when it was massively scaling up its nuclear program. To stay below 2°C, every country in the world would have to make similar strides, and sustain it for decades.

That's the scale of the challenge. And the question of whether we can decarbonize that rapidly is way more relevant for climate change than fussing over the precise year that emissions peak.

Further reading

-- The Nature Climate Change paper on global CO2 output has a lot of good numbers to break down what it would take to hit peak emissions. Recommended for anyone who wants to dig in.

-- The Global Carbon Project's presentation on emissions trends is also highly recommended. Many of the charts in this post come from their 2015 report.

-- China's CO2 emissions have been plummeting lately. What's going on?

-- There are 2,100 coal plants being planned worldwide — enough to cook the planet

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