A landmark United Nations (UN) report on climate change highlighting the importance of limiting global warming to 1.5 degrees Celsius was released in South Korea on Monday after a week-long meeting of the 195-nation Intergovernmental Panel on Climate Change (IPCC).

More than 90 scientists wrote the report, which is based on more than 6,000 peer reviews.

A “Summary for Policymakers” of the 400-page tome underscores how quickly global warming has outstripped humanity's attempts to tame it, and outlines stark options — all requiring a makeover of the world economy — for avoiding the worst ravages of climate change.

“Things that scientists have been saying would happen further in the future are happening now,” Jennifer Morgan, executive director of Greenpeace International, told AFP.

Before the Paris Agreement was inked in 2015, nearly a decade of scientific research rested on the assumption that 2C was the guardrail for a climate-safe world.

The IPCC report, however, shows that global warming impacts have come sooner and hit harder than predicted. It warns that warming is on track toward an unliveable 3C or 4C rise and avoiding global chaos will require a major transformation.

1.5C vs 2C: the stakes are high

A raft of recent research shows that 2C is not the guardrail it was previously assumed to be.

“Climate impacts are exponentially more dramatic when we go from 1.5C to 2C,” said Henri Waisman, a scientist at the Institute for Sustainable Development and International Relations, and a coordinating lead author of the IPCC report.

Capping global warming at 1.5C above pre-industrial levels will require “rapid, far-reaching and unprecedented changes in all aspects of society”, the IPCC said.

Earth's average surface temperature has already gone up 1C — enough to unleash a crescendo of deadly extreme weather — and is on track to rise another 2-3C in the absence of a sharp and sustained reduction in carbon pollution.

Meeting the tougher-to-reach 1.5C goal “could result in around 420 million fewer people being frequently exposed to extreme heat waves, and about 65 million fewer people being exposed to exceptional heat waves,” the report says.

The deadly heat waves that hit India and Pakistan in 2015 could become practically yearly events if the world becomes hotter.

What used to be once-a-century heatwaves in the northern hemisphere, for example, will become 50 per cent more likely in many regions with an extra half-degree of warming.

Some tropical fisheries are likely to collapse somewhere between the 1.5C and 2C benchmark. Staple food crops will decline in yield and nutritional value by an extra 10-15pc. Coral reefs will mostly perish. The rate of species loss will accelerate “substantially”.

Most worrying of all, perhaps, are temperature thresholds between 1.5C and 2C that could push Arctic sea ice, methane-laden permafrost, and melting polar ice sheets with enough frozen water to lift oceans by a dozen metres, past a point of no return.

At current levels of greenhouse gas emissions, we could pass the 1.5C marker as early as 2030, and no later than mid-century, the report finds with “high confidence”.

To have at least a 50-50 chance of staying under 1.5C without overshooting the mark, the world must, by 2050, become “carbon neutral”.

“That means every tonne of CO2 we put into the atmosphere will have to be balanced by a tonne of CO2 taken out,” said lead coordinating author Myles Allen, head of the University of Oxford's Climate Research Programme.

The 30-page executive summary of the IPCC report details humanity's “carbon budget”, i.e., the amount of CO2 we can emit and still stay under the 1.5C ceiling.

For a two-thirds chance of success, that is about 420 billion tonnes, an allowance that would — according to current trends — be used up in 10 years.

Emissions of carbon dioxide — the main greenhouse gas — should peak no later than 2020 and curve sharply downward from there, according to scenarios in the report.

Easier said than done: humanity dumped a record amount of CO2 into the atmosphere last year.

The share of electricity generated by renewables — mainly hydro, solar and wind — would have to jump by mid-century from about 20-70pc. The share of coal, meanwhile, would need to drop from 40pc to low single digits.

Limiting global warming to 1.5C will require investing about $2.4 trillion in the global energy system every year between 2016 and 2035, or about 2.5pc of world GDP.

This price tag, however, must be weighed against the even steeper cost of inaction, the report says.

A contrasting “pay later” scenario compensates for a high-consumption lifestyles and continued use of fossil fuels with a temporary breaching of the 1.5C ceiling.

It depends heavily on the use of biofuels. But the scheme would need to plant an area twice the size of India in biofuel crops, and assumes that some 1,200 billion tonnes of CO2 ─ 30 years' worth of emissions at current rates ─ can be safely locked away underground.

“Is it fair for the next generation to pay to take the CO2 out of the atmosphere that we are now putting into it?” asked Allen. “We have to start having that debate.”

'Slim window of opportunity to avoid unthinkable damage'

The stakes are especially high for small island states, developing nations in the tropics, and countries with densely-populated delta regions already suffering from rising seas.

Read: Climate change a ‘threat multiplier’ for Pakistan, says researcher

“We have only the slimmest of opportunities remaining to avoid unthinkable damage to the climate system that supports life as we know it,” said Amjad Abdulla, chief negotiator at the UN climate talks for the Alliance of Small Island States.

“We have done our job, we have now passed on the message,” Jim Skea, a professor at Imperial College London's Centre for Environmental Policy and an IPCC co-chair, said at a press conference. “Now it is over to governments, it's their responsibility to act on it.”

'We are not heading in the right direction'

Special Representative on Water Affairs for the Netherlands, Henk Ovink, while talking to reporters in Hague about the alarming UN report, stressed that countries must work urgently to solve water issues caused by climate change.

“We need a radical change,” said Ovink. “There is this urgent need to change, the IPCC report is right, we are not heading in the right direction,” he told a small group of international journalists.

Ovink — who travels the world spreading the know-how gleaned by the low-lying Netherlands in its millennium-long battle against the seas and is billed as the world's only “water ambassador”— said the report shows “we all need to do more".

Recent events such as the disastrous flooding caused by Hurricane Florence in the United States, Japanese typhoons and the Indonesian tsunami showed that “the world isn't ready for these challenges, and is responding after these crises, not before".

He said one important area was fostering “collaboration” — within and between countries, agencies and the UN — citing the example of the Netherlands where “water democracy is nearly 1,000 years old".

But while US President Donald Trump's decision to pull out of the Paris climate pact caused international dismay, Ovink said that there was no need to be unduly pessimistic.

“I don't despair only because of Trump,” said Ovink, who previously served on a task force on hurricane rebuilding created by Trump's predecessor, president Barack Obama.

“The world is not dependent on one nation. It wasn't easy before Trump.”

The way forward

IPCC authors say the 1.5C goal is technically and economically feasible, but depends on political leadership to become reality.

Read more: Are politicians really serious about the climate?

The report lays out four 1.5C scenarios that shadow current and future policy debates on the best way to ramp up the fight against climate change.

One pathway, for example, relies heavily on a deep reduction in energy demand, while another assumes major changes in consumption habits, such as eating less meat and abandoning cars with internal combustion engines.

Two others depend on sucking massive amounts of CO2 out of the air, either though large-scale reforestation, use of bio-fuels, or direct carbon capture.