Time is running out to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit), the UN's climate experts warned Sunday, saying current carbon emissions were a potential path to disaster.

Crowning a landmark review, the Intergovernmental Panel on Climate Change (IPCC) said emissions of three key greenhouse gases were at their highest since more than 800,000 years ago, when mammoths and mastodons roamed the earth.

Earth, it warned, is on a likely trajectory for at least 4 C warming over pre-industrial times by 2100 -- a recipe for worsening drought, flood, rising seas and species extinctions.

Many could face hunger, homelessness and conflict in the scramble for precious resources.

UN chief Ban Ki-moon, who in September hosted a special summit on climate change, said: "Human influence on the climate system is clear, and clearly growing."

"We must act quickly and decisively if we want to avoid increasingly disruptive outcomes," Ban said, describing it as a "myth" that tackling carbon emissions was costly.

US Secretary of State John Kerry said the report was a fresh warning -- "another canary in the coal mine" -- while France, which is to host a UN climate conference in December 2015, called for "immediate, all-round mobilisation".

French President Francois Hollande called climate change one of the "big challenges" for the global community and pledged to act for the "wellbeing ... of the planet".

IPCC chief Rajendra Pachauri said there remained "little time before the window of opportunity to stay within 2 C of warming closes."

"To keep a good chance of staying below 2 C, and at manageable costs, our emissions should drop by 40 to 70 percent globally between 2010 and 2050, falling to zero or below by 2100."

Seizing on the report, green groups demanded the phaseout of coal, oil and gas that are driving the carbon problem.

"Renewables and the smart use of energy are the quickest and cleanest ways to cut emissions," said Greenpeace's Kaisa Kosonen.

"Any technology that 'handles' emissions rather than replaces fossil fuels is like smoking crack to solve an alcohol addiction."

The first overview by the Nobel-winning organisation since 2007 -- comes ahead of UN talks in Lima next month to pave the way to a 2015 pact in Paris to limit warming to a safer 2 C.

But the negotiations have been hung up for years over which countries should shoulder the cost for reducing carbon emissions, derived from the world's cornerstone energy sources today.

The report said switching to cleaner sources, reducing energy efficiency and implementing other emission-mitigating measures would be far cheaper than the cost of climate damage.

The bill today for doing this is affordable, but delaying beyond 2030 would cause the cost -- and the climate peril -- to spiral for future generations.

"Ambitious" carbon curbs would shave just 0.06 percentage points annually from global consumption this century, targeted to grow by 1.6-3.0 percent annually, the IPCC said.

Under the lowest of four emissions scenarios, global average temperatures over this century are likely to rise by 0.3-1.7 C (0.5-3.1 degrees Fahrenheit), leading to between 26-55 cm in sea-level rise.

Under the highest scenario, warming would be 2.6-4.8 C, causing sea-level rise of 45-82 cm.

- 4 C world -

The report warned bleakly that on current trends, "warming is more likely than not to exceed 4 C" over pre-industrial levels by 2100.

Without further action to tame emissions, "warming by the end of the 21st century will lead to very high risk of severe, widespread and irreversible impacts globally," it said.

It pointed to:

-- worse food security, with impacts on grain harvests and fish catches

-- accelerating species extinction and damage to ecosystems on which mankind depends

-- migration caused by climate-related economic damage and loss of land from rising seas and storm surges

-- greater water stress, especially in sub-tropical regions, but also a greater risk of flooding in northerly latitudes and the equatorial Pacific

-- risk of conflict over resources as well as health dangers from heatwaves and spread of mosquito-borne disease

The IPCC was set up in 1988 to provide governments with neutral advice about global warming, its impacts and the options for tackling it.

Sunday's synthesis report encapsulated three previous volumes published over the last 13 months.

They were written by more than 800 experts, whose work then went through arduous review and commentary by outside specialists.

The IPCC's fourth report helped open the way to the 2009 Copenhagen Summit, where attempts to forge a global treaty nearly ended in a fiasco.

"Maybe, looking back, at the time world leaders might not have been fully ready to engage themselves," Ban said.

"They were more focusing on national priorities rather than global priorities. Since then, we have been building up on what we had discussed... Now we are going to Lima, Peru, and our final destination will be Paris by the end of next year."

Climate: Main points from landmark UN review
Copenhagen (AFP) Nov 02, 2014 - Following are the main points from the summary of a vast overview on global warming by the UN's Intergovernmental Panel on Climate Change (IPCC).

SITUATION TODAY

"Human influence on the climate system is clear, and recent anthropogenic (manmade) emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impact on human and natural systems. Warming of the climate system is unequivocal."

Since the start of the Industrial Revolution in 1750, around 2,000 billion tonnes of carbon dioxide (CO2) have been emitted. Half of manmade emissions have occurred in the last 40 years.

Levels of carbon dioxide (CO2), methane and nitrous oxide are the highest in at least 800,000 years.

From 1880-2012, the global average surface temperature rose by 0.85 degrees Celsius (1.5 degrees Fahrenheit), while the global mean sea level rose by 19 centimetres (7.6 inches) from 1901-2010.

Each of the last three decades has been successively warmer than any preceding decade since 1850. In the northern hemisphere, 1983-2012 was likely the warmest period of the last 1,400 years.

Heatwaves are more common in large parts of Europe, Asia and Australia, heavy rainfall has likely occurred more often and with greater intensity in North America and Europe.

The Greenland and Antarctic ice sheets have lost mass over the last two decades and the extent of Arctic sea ice has shrunk. Ocean acidity has risen by 26 percent since the start of the industrial era.

THE FUTURE

The report uses four scenarios called Representative Concentration Pathways (RCP) based on greenhouse gas levels.

Under RCP 2.6, the lowest scenario, global average temperatures over this century are likely to rise by 0.3-1.7 C (0.5-3.1 degrees Fahrenheit), leading to between 26-55 cm in sea-level rise.

Under RCP 8.5, the highest scenario, warming would be 2.6-4.8 C, causing sea-level rise of 45-82 cm.

Without additional measures to curb greenhouse gases "warming is more likely than not to exceed 4C" by 2100 over pre-industrial levels, the report says.

On present emissions trends, "warming by the end of the 21st century will lead to very high risk of severe, widespread and irreversible impacts globally."

Risks include:

-- worse food security, with impacts on wheat, rice and maize harvests and fish catches

-- accelerating species extinction and damage to ecosystems on which mankind depends

-- migration caused by climate-related economic damage and loss of land from rising seas and storm surges

-- greater water stress, especially in sub-tropical regions, but also a greater risk of flooding in northerly latitudes and the equatorial Pacific.

-- risk of conflict over scarce resources and worsening health caused by heatwaves and spread of mosquito-borne disease

If CO2 emissions continue over the long term, ocean acidification and sea-level rise will continue for centuries to come. "Abrupt and irreversible ice loss from the Antarctic ice sheet is possible, but current evidence and understanding is insufficient to make a quantitative assessment," the report says.

THE 2 C TARGET

UN members have vowed to agree an emissions-curbing pact next year to limit warming to 2 C over pre-industrial levels.

For a "likely" chance (at least 66 percent or more) of achieving 2 C, atmospheric CO2 should not exceed about 450 parts per million by 2100. This means humans can emit only around another 1,000 billion tonnes of CO2.

The report points to one pathway to reach 2 C: cut annual greenhouse gas emissions by 40-70 percent by 2050 compared with 2010, and to near zero or even below by 2100.

Reaching 450ppm would put a small brake of about 0.06 percentage points annually on growth in consumption, a measure of spending activity, which is otherwise projected to increase 1.6-3.0 percent per year over the century.

Delaying action beyond 2030 will hand on a greater bill and heftier risk to those in the second half of the century.

POLICY OPTIONS

"No single option is sufficient by itself," the report says, advocating a mix of measures.

Achieving the 2 C target will require a massive change in energy habits, led by a switch out of high-polluting fossil fuels.

By 2030, investment in low-carbon energy and energy efficiency in transport, industry and buildings will need to rise by several hundred billion dollars per year.

It says a carbon price "in principle... can achieve mitigation in a cost-effective way" but also notes pitfalls if the policy is poorly designed, such as cap-and-trade systems whose ceilings are too high.

Countries can shore up defences by reducing water waste and encouraging recycling, preventing settlement in climate-prone areas and conserving wetlands and mangroves, which are shields against climate stress. Preventing deforestation and encouraging afforestation are also a carbon buffer.

The IPCC highlights a potential role for carbon capture -- only used so far on a pilot scale -- to sequester CO2 from fossil-fuel power plants.

But it dismisses "geo-engineering" proposals to reflect sunlight back into space by sowing the atmosphere with reflective particles. The cooling idea is beset by risk and ethical and governance questions, it says.

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SOURCE: Intergovernmental Panel on Climate Change (IPCC), Fifth Assessment Report. Summary for Policymakers from Synthesis Report