A stark warning by UN scientists of menacing climate change prompted demands Monday for urgent action to curb greenhouse-gas emissions even as a global pact remained elusive.

Scientists, politicians, envoys and green groups united in calls for faster, more drastic action to avoid the worst-case scenarios of conflict, drought and massive displacement highlighted in the expert report.

"The path of tomorrow is undoubtedly determined by our choices today," United Nations climate chief Christiana Figueres said after the UN's Intergovernmental Panel on Climate Change (IPCC) warned of a "severe, pervasive and irreversible impact" if nothing is done.

"This report requires and requests that everyone accelerate and scale up efforts towards a low-carbon world and manage the risks of climate change in order to spare the planet and its people," she said.

Activist groups said governments now have all the proof they need that inaction will lead to disaster, as well as the scientific basis on which to plan an appropriate response.

Many voiced hope that the dire warning would give impetus to negotiations for a new, global pact on curbing carbon emissions from burning fossil fuels.

"The gap between the science and what governments are doing remains huge," said Sandeep Chamling Rai of green group WWF.

"Now it is up to people to hold their governments to account, to get them to act purposefully and immediately."

The Alliance of Small Island States said the new alert came as no surprise for nations already grappling with sea-level rise, droughts and record-breaking storms.

"We hope that it helps convince the international community, particularly those most responsible for climate change, to address the crisis with greater urgency and not at some abstract date in the future but immediately."

After a nearly 22-year effort under the auspices of the UN Framework Convention on Climate Change (UNFCCC), negotiators are seeking to sign a new, global pact in Paris next year, to take effect in 2020.

The aim is to contain warming to 2.0 degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels, though even this level would require adaptive measures for a changed climate.

Countries remain deeply divided on what the deal will look like, what it must contain, how binding its provisions should be and how they will be enforced.

- No denial, says US -

US Secretary of State John Kerry said the political system must "wake up" to the threat.

"Denial of the science is malpractice," he said in a statement. "There are those who say we can't afford to act. But waiting is truly unaffordable. The costs of inaction are catastrophic."

The IPCC warned that untamed greenhouse gas emissions may cost trillions of dollars in damage to property and ecosystems, and in bills for shoring up climate defences.

For Bjorn Lomborg of the Copenhagen Consensus Centre think-tank, "the best solution is to dramatically ramp up funding for research and development of effective green technology."

Today's renewable technology, such as solar and wind energy, required subsidies of at least $100 billion (73 billion euros) per year to become viable.

"Innovation can help us developing technologies to provide green energy more cheaply than fossil fuels, and then everyone will adopt them," said Lomborg.

In the run-up to the conclusion of this year's global climate negotiations in Lima, Peru, in December, UN chief Ban Ki-moon will host heads of state and government in New York on September 23 "to mobilise action and ambition on climate change".

European Climate Commissioner Connie Hedegaard said the world should oppose "hitting the snooze button" on the issue.

"Europe is preparing an ambitious reduction target for 2030 to be adopted later this year. I appeal to all major emitters to do the same thing," she said in a statement.

Tom Mitchell, a climate expert at Britain's Overseas Development Institute (ODI), cautioned against hopes the Paris meeting will be the definitive solution.

"The real challenge is in what happens after 2015," he said in a phone interview with AFP in Tokyo.

"If countries say they are going to reduce emissions, what is the architecture for holding countries to account, and ensuring that they are contributing their fair share in the first place?"

Climate change: regional impact
Yokohama, Japan (AFP) March 31, 2014 - This is how climate change may affect the world's regions this century, as forecast in a major report published by UN scientists Monday.

The report is part of the fifth overview on global warming by the Intergovernmental Panel on Climate Change (IPCC) since 1988.

The document identifies each region's key challenges; options for addressing them; and level of risk from warming of either 2 C (3.6 F) or 4 C (7.2 F) by 2100 compared to pre-industrial levels. This risk is calculated on the basis of present policies for adapting to climate change.

AFRICA

Challenge: Water stress

Risk: High at 2 C, very high at 4 C

Options: Smarter use of water resources

Challenge: Food shortages

Risk: Very high at 2 C and 4 C

Options: Stress-tolerant crops, help for small farmers

Challenge: Mosquito- and water-borne diseases

Risk: Very high at 2 C and 4 C

Option: Outbreak early-warning systems, improved sanitation

EUROPE

Challenge: Flooding in river basis and on coasts

Risk: Medium at 2 C, very high at 4 C

Options: Improve flood protection

Challenge: Water stress in dry regions

Risk: High at 2 C, very high at 4 C

Options: Reduce water waste, including through irrigation

Challenge: Heatwaves and air pollution affecting health

Risk: High at 2 C, very high at 4 C

Options: Reduce emissions to improve air quality and adapt homes and workplaces for heatwaves.

ASIA

Challenge: Flood damage to homes and infrastructure

Risk: High at 2 C, very high at 4 C

Options: More resilient buildings and "selective relocation"

Challenge: Deaths from extreme heat

Risk: Very high at 2 C and 4 C

Options: Strengthen health systems, improve city planning to reduce urban heat buildup

Challenge: Malnutrition caused by drought

Risk: Medium at 2 C, high at 4 C

Options: Beef up vigilance on food supplies, improve disaster preparedness

AUSTRALASIA

Challenge: Damage to coral reefs and, in Australia, animal and plant species loss

Risk: High at 2 C, very high at 4 C

Options: Reduce pressures on ecosystems from pollution, tourism and introduced species

Challenge: Flooding, and coastal infrastructure lost to rising seas

Risk: Medium at 2 C, very high at 4 C

Options: Smarter land use to reduce exposure to floods and coastal erosion.

NORTH AMERICA

Challenge: Wildfires for ecosystems and homes

Risk: Very high at 2 C and 4 C

Options: Improve fire prevention measures

Challenge: Deaths from heatwaves

Risk: High at 2 C, very high at 4 C

Options: Encourage residential air conditioning, build cooling centres for the vulnerable

Challenge: Property and infrastructure damage from extreme rainstorms

Risk: High at 2 C, very high at 4 C

Options: Install drainage systems that allow water runoff to recharge groundwater resources, easing flood risk

CENTRAL AND SOUTH AMERICA

Challenge: Water stress in semi-arid areas that depend on glaciers for water supply

Risk: Very high at 2 C and 4 C

Options: Improve water supply and land use

Challenge: Flooding in urban areas from extreme rainfall

Risk: Very high at 2 C and 4 C

Options: Improve urban flood management, early-warning systems and weather alerts

Challenge: Decreased food production and food quality

Risk: Very high at 2 C and 4 C

Options: Develop drought-resistant crop strains

POLAR REGIONS

Challenge: Risk to ecosystems from changes to permafrost, snow and ice

Risk: High at 2 C , very high at 4C

Options: Enhanced monitoring of risk, hunt different species if possible

Challenge: Food insecurity and lack of reliable and safe drinking water

Risk: Very high at 2 C and 4 C

Options: Improve monitoring systems, shift resources, settle elsewhere

Challenge: Impact on Arctic communities if climate change happens very fast

Risk: Very high at 2 C and 4 C

Options: Improve communications, education and training, encourage co-management of ecosystem

SMALL ISLANDS

Challenge: Loss of homes, farmland, infrastructure and livelihoods from rising seas and storms

Risk: High at 2 C, very high at 4 C

Options: Boost coastal buffers and improve management of water and soil resources

Challenge: Loss of low-lying land in coastal areas from a combination of rising seas and storm surges

Risk: Very high at 2 C and 4 C

Options: Not constructing new buildings in areas at risk

OCEANS

Challenge: Decline in fish catches at low latitudes

Risk: Medium at 2 C, high at 4 C

Options: Flexible management reactive to stock variability, expanding aquaculture

Challenge: Biodiversity loss from heat-damaged coral reefs

Risk: Very high at 2 C and 4 C

Options: Reduce other human-induced stresses like pollution, tourism and fishing

Challenge: Damage to coastal ecosystems such as mangroves and sea grass from soil runoff from heavy rain and coastal erosion

Risk: High at 2 C, very high at 4 C

Options: Reduce soil runoff caused by deforestation

SOURCE: "Climate Change 2014: Impacts, Adaptation, and Vulnerability " (Summary for Policymakers)