In this Wednesday, Dec. 16, 2009 photo, steam and smoke rises from a coal burning power plant in Gelsenkirchen, Germany. (AP Photo/Martin Meissner)

This story has been updated.

The world is busy trying to achieve a global warming goal that some think we’re not on course to achieve, and that some scientists worry may not be enough to prevent some troubling scenarios — like significant sea level rise.

That goal is limiting warming to 2 degrees Celsius above pre-industrial levels. But there’s another view, embraced by many countries (which tend not to be the big emitters), suggesting that a safer target involves limiting global warming to just 1.5 degrees C.

Yet that target would be still tougher to achieve, finds a new paper in Nature Climate Change that takes a thorough look at what 1.5 degrees would actually mean. The research, based on several models used by the U.N.’s Intergovernmental Panel on Climate Change, suggests that keeping global warming under 1.5 C would probably require that we overshoot the goal and then reverse course to reach it again through so-called “negative emissions” technologies, which would effectively pull carbon dioxide back out of the air again.

“We know that the 2 degree target is quite challenging to achieve,” says Gunnar Luderer of the Potsdam Institute for Climate Impacts Research, one of the paper’s authors. “In terms of technologies, it basically takes a full scale decarbonization by the end of century … if you look at the one-and-a-half-degree target, it’s even more challenging, more extreme in several aspects.”

Indeed, the study finds that while it’s technically possible, “the window for achieving this goal is small and rapidly closing.”

The first challenge to achieving a 1.5 degree C world involves how much you have to constrain the planet’s remaining carbon emissions. Acccording to Luderer — who is here relying on the work of the U.N.’s Intergovernmental Panel on Climate Change — the world can emit between 630 and 1180 gigatons of carbon dioxide and still have a good chance of staying below 2 degrees Celsius. (A gigaton is a billion metric tons.)

But to stay under 1.5 degrees, we’d have to emit much less, the new study finds.

“For the 1.5 degree scenario, we found that the remaining budget for the century is on the order of 200 to 400 gigatons, so a lot less, rather a third of what we have for the 2 degree target,” says Luderer.

Last year the world emitted about 32 gigatons, according to the International Energy Agency. So a budget of 200 to 400 gigatons — the study intends this as a cumulative total by the year 2100 — either gives us a narrow window to sharply slash emissions, or requires that we overshoot and then somehow pull large amounts of carbon out of the atmosphere later in the century.

Measured in terms of carbon dioxide in the atmosphere, meanwhile, staying under 1.5 degrees C would mean concentrations of 420 to 440 parts per million by 2100, versus 455 to 480 parts per million for a “likely” chance of staying under 2 degrees C, the study says. Currently, atmospheric concentrations are just in the process of firmly crossing the 400 parts per million threshold.

Slashing emissions so dramatically in a few decades probably will not happen, of course — which leads to the second major punchline of the study. This is that you will still have to temporarily go past the 1.5 degree threshold, and then employ so-called “negative emissions” technologies to subtract some net carbon dioxide from the atmosphere, and get back down to 1.5 degrees C by 2100.

“You have to have these technologies to do negative emissions, and that is directly related to the property that really, staying below one and a half degrees at all times is not possible,” says Luderer. “There will be a temporary overshoot of the one and a half degrees.”

What are those technologies?

In essence, they combine planting a massive amount of trees with something called bioenergy with carbon capture and storage, or BECCS. In this technology, trees or other plants would be used to make fuels or electricity, but the carbon they contain would be captured in the process — and then, when the plants grow back again, the net result would be negative emissions.

It certainly sounds nifty, but “important barriers to its widespread implementation still exist,” the study notes.

Taking all of this in, I finally asked Luderer whether he was saying that it’s impossible to stay under 1.5 degrees.

“It’s feasible from a pure technological point of view,” he said. “But given the sluggish progress in the international climate policy arena, it is going to be extremely difficult.”

Of course, the likeliest scenario is that the world will keep muddling through, examining more science and adjusting its targets, and that clean energy and even “negative emissions” technologies will improve — meaning that in the end, we still have to wait and see just which target we can actually achieve, and how the planet responds.