Diplomats gathering in the French capital next week face a tall task in trying to unite 190-plus countries behind a plan to fight climate change. Yet a success in Paris may not, by itself, be enough to prevent dangerous warming of the planet in the decades to come, a new study warns. 

The report in the journal Science suggests that the Earth can avoid the most disruptive impacts of climate change only if the Paris agreement is a first step, followed by more ambitious cuts in greenhouse-gas pollution in future years.

In fact, without further emissions reductions, there is "virtually no chance" that global temperatures will stay below the threshold that many scientists say is safe - a maximum increase of 2 degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial averages, the authors said.

The release of the report comes four days before the start of the Paris talks, when world leaders will try to cement an international accord to stop the sharp rise in atmospheric levels of carbon dioxide and other heat-trapping gases from fossil-fuel burning. More than 150 countries have announced pledges to reduce or scale back emissions through the year 2030.

U.S. officials have acknowledged that pledges aren't enough to ensure that the temperatures remain below the 2-degree mark. In the Science study, a team of 15 researchers sought to calculate the odds for keeping the Earth's temperatures below that threshold, using different scenarios ranging from no further pollution cuts to aggressive and continuing steps to ratchet back on emissions.

In an optimistic case - assuming countries honor their Paris commitments and continue to make further pollution cuts - the chances of holding the temperature rise to below 2 degrees improves, but only to about 30 percent, the study found. Real success depends on a "robust process that allows pledges to be progressively tightened over time," said the researchers, a group of U.S. government scientists and academics from the University of Maryland and other institutions in the United States and Austria.

"It is important to note that this round of [pledges] that countries are submitting only extend through 2025 or 2030," said Allen Fawcett, a government economist and the lead author of the study. While success in Paris will "reduce the probabilities of extreme warming," the treaty must be followed by "further post-2030 reductions that would improve the odds" of meeting the 2-degree goal, he said.

The researchers found that, in order to ensure a strong likelihood of keeping temperature rise in a moderate range, nations will eventually have to achieve a "net-zero" or even "negative" emissions growth, meaning future emissions are offset by capturing the excess carbon through re-forestation or through new technologies that prevent greenhouse gases from entering the atmosphere.

The idea is to deploy carbon-capture technologies on a scale "greater than, or equal to the amount emitted from all other sources," said Gokul Iyer, the study's lead scientist at the Joint Global Change Research Institute, a collaboration between the Department of Energy's Pacific Northwest National Laboratory and the University of Maryland

"These scenarios include rapid emissions reductions beyond 2030 phasing out fossil fuels, and many also include negative global emissions in the second half of the century," he said.