There is a five-percent chance of limiting average global warming to under two degrees Celsius (3.6 degrees Fahrenheit), the target set in the 2015 climate-rescue Paris Agreement, researchers said on Monday.
And chances of meeting the lower, aspirational 1.5 C goal, also listed in the 196-nation pact, were a mere one percent, they wrote in the journal Nature Climate Change.
A US-based expert team used projections for population growth to estimate future production, and related carbon emissions from burning fossil fuels.
Based on these data, "the likely range of global temperature increase is 2 C to 4.9 C, with median 3.2 C and a five percent chance that it will be less than 2 C," they wrote.
Their calculations were not based on worst-case-scenario predictions of unabated energy use, the team said, and provided for ongoing provision for efforts to curb fossil fuel use.
However, it does not provide for the possibility of a sudden, massive shift to renewable energy.
"Achieving the goal of less than 1.5 C warming will require carbon intensity to decline much faster than in the recent past," wrote the team.
The world's nations concluded years of acrimonious negotiations two years ago to conclude the Paris Agreement on climate change.
They undertook to hold global warming to "well below" 2 C over pre-Industrial Revolution levels, and to strive for 1.5 C in a bid to stave off disastrous sea level rise, droughts, storms and other climate effects.
Experts have long warned that even 2 C would be a tall order.
The UN's climate science panel recommends a 40-70-percent cut in greenhouse gas emissions from fossil fuels by 2050 from 2010 levels.
The Paris Agreement is less precise, with signatories aiming for emissions to peak "as soon as possible".
The UN estimates that the global population will increase from about 7.5 billion today to 11.2 billion by 2100, putting additional pressure on energy resources.