The evidence that society is "on the edge of a threshhold of a future with unprecedented environmental risks" is unequivocal, they conclude.
Their declaration will be presented to government delegations here.
In the main negotiations, nations are reportedly closing in on an agreement.
The Brazilian government, which is now chairing the talks, wants negotiators to finish work on the draft text by the end of Monday.
Heads of government from about 130 countries, and ministers from others, will begin their session on Wednesday and would sign off the text by the end of the week.
While the majority of people here expect a deal to be done, there are warnings from various organisations that it will not go nearly far enough towards sorting out the environment and development issues facing humanity.
Without explicitly criticising the draft agreement, the panel's declaration makes it clear that they agree.
"The combined effects of climate change, resource scarcity, loss of biodiversity and ecosystem resilience at a time of increased demand, poses a real threat to humanity's welfare," they write.
"There is an unacceptable risk that human pressures on the planet, should they continue on a business as usual trajectory, will trigger abrupt and irreversible changes with catastrophic outcomes for human societies and life as we know it." More