"The world is at a critical crossroads," UN Secretary-General Ban Ki-moon said in a message read to the academic gathering.
While nations have committed to limiting average global warming to two degrees Celsius over pre-Industrial Revolution levels, research shows the world is heading for double or more that on current greenhouse gas emission trends, he said.
And based on emissions curbs pledged so far, the combined effort will likely "not be sufficient to meet the 2 degree target," said Ban.
The four-day international science conference opened today in the French capital, which will also host the November 30-December 11 UN climate conference for 195 nations to finalise a new, global climate pact.
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Dubbed "Our common future under climate change", the gathering of academics from nearly 100 countries will review the most up-to-date science on climate challenges and solutions to feed into the Paris pact.
"It is not scientists' role to tell governments what to do in December, but to illuminate the choices - each with different levels of cost and risk, as well as opportunities to help build robust economies and vibrant communities," said Chris Field, chairman of the conference's scientific committee.
Emissions will have to drop 40-70 per cent between 2010 and 2050, and to zero by 2100, it said.
"The emissions gap - the difference between energy reductions pledged by parties and what is needed to stay within two degrees - has been increasing," said Ban.
So too has the gap between funding and capacity needed, and what has been committed, for countries to adapt to climate changes that can no longer be avoided.
"Science can help fill the holes in knowledge and technology."
World Meteorological Organization (WMO) head Michel Jarraud painted a picture of rising sea levels, acidic oceans and ever more frequent extreme weather events in a warmer world.