Nations must agree an action plan to curb climate change or face an "unprecedented reversal" in human development, a UN assessment warns.
The window of opportunity for action is closing, the report warns
The UN Human Development Report said the poorest nations risked a downward spiral of malnutrition, water scarcity and ecological damage.
It called for global CO2 levels to be cut by 80% from 1990 levels by 2050.
The UN Development Programme (UNDP) report coincides with next week's key UN climate negotiations in Indonesia.
The lead author of the report - Fighting Climate Change: Human Solidarity in a Divided World - is Kevin Watkins. He said he hoped the report would act as an incentive for the conference's delegates.
"We are issuing a call to action, not providing a counsel of despair," he said. "Working together with resolve, we can win the battle against climate change."
The two-week gathering on the island of Bali is set to debate what shape the global agreement to curb greenhouse gas emissions should take after 2012, which is when the current Kyoto Protocol ends.
UNDP administrator Kemal Dervis said although climate change was a global challenge, developing nations faced the greatest risk.
"It is the poor, a constituency with no responsibility for the ecological debt we are running up, who face the most immediate and severe human costs."
'Twin track' plan
The report outlined a number of risks that threatened human development if the world failed to act, including:
- the breakdown of agricultural systems, especially in sub-Saharan Africa, leaving up to 600 million more people facing malnutrition
- an additional 1.8 billion people at risk from water scarcity by 2080
- up to a third of a billion people living in coastal regions being displaced by tropical storms and flooding
- hundreds of millions of people at increased risk from emerging diseases, such as malaria
The authors called on nations to adopt a "twin track" approach with measures to mitigate future warming while helping at risk nations to adapt to human-induced climatic shifts.
They said developed nations had to show leadership by cutting emissions by at least 80% from 1990 levels by 2050 to ensure global warming did not exceed 2C (3.6F).
A mixed approach would deliver the necessary cuts, including carbon taxes, cap-and-trade schemes and funding low-carbon technology transfer projects, they added.
But rich nations also had to put climate change adaptation at the centre of international poverty reduction programmes.
Even with stringent mitigation, the report said past emissions meant that continued warming was locked in to the global climate system for the first half of the century.
While rich nations were able to afford to "climate-proof" their economies and population, poor nations were woefully under-funded.
According to the report, international funding for adaptation in developing nations to date amounted to $26m (£13m), roughly the same amount as the UK spends on its flood defences in a week.
"Nobody wants to understate the very real long-term ecological challenges that climate change will bring to rich countries," Mr Watkins explained.
"But the near-term vulnerabilities are not concentrated in lower Manhattan and London, but in flood-prone areas of Bangladesh and drought-prone parts of sub-Saharan Africa.
"Allowing the window of opportunity to close would represent a moral and political failure without precedent in human history."