We ignored the warnings, and now it's too late: global heating has arrived with a vengeance and will see Earth's average temperature reach 1.5 degrees Celsius above preindustrial levels around 2030, a decade earlier than projected only three years ago, according to a landmark UN assessment published on Monday.

The Intergovernmental Panel on Climate Change (IPCC) bombshell -- landing 90 days before a key climate summit desperate to keep 1.5C in play -- says the threshold will be breached around 2050, no matter how aggressively humanity draws down carbon pollution.

Years in the making, the sobering report approved by 195 nations shines a harsh spotlight on governments dithering in the face of mounting evidence that climate change is an existential threat.

Nature itself has underscored their negligence.

With only 1.1C of warming so far, an unbroken cascade of deadly, unprecedented weather disasters bulked up by climate change has swept the world this summer, from asphalt-melting heatwaves in Canada, to rainstorms turning city streets in China and Germany into rivers, to untameable wildfires sweeping Greece and California.

"This report is a reality check," said Valerie Masson-Delmotte, who co-led hundreds of scientists in reviewing a mountain of published climate science.

"It has been clear for decades that the Earth's climate is changing, and the role of human influence on the climate system is undisputed."

Indeed, all but a tiny fraction of warming so far is "unequivocally caused by human activities," the IPCC concluded for the first time in its three-decade history.

The world must brace itself for worse -- potentially much worse -- to come, the report made clear.

- Invisible threshold -

Even if the 1.5C target humanity is now poised to overshoot is miraculously achieved, it would still generate heatwaves, rainfall, drought and other extreme weather "unprecedented in the observational record", it concluded.

At slightly higher levels of global heating, what is today once-a-century coastal flooding will happen every year by 2100, fuelled by storms gorged with extra moisture and rising seas.

"This report should send a shiver down the spine of everyone who reads it," said Dave Reay, director of the Edinburgh Climate Change Institute at the University of Edinburgh, who was not among the authors.

Another looming danger is "tipping points", invisible thresholds -- triggered by rising temperatures -- for irreversible changes in Earth's climate system.

Disintegrating ice sheets holding enough water to raise seas a dozen metres; the melting of permafrost laden with double the carbon in the atmosphere; the transition of the Amazon from tropical forest to savannah -- these potential catastrophes "cannot be ruled out," the report cautions.

"An important reason for taking urgent action on emissions is to decrease the chances that one or more of these world-changing events will be triggered over the 21st century," said Andrew Watson, Royal Society Research Professor at the University of Exeter.

Our natural allies in the fight against climate change, meanwhile, are suffering battle fatigue.

Since about 1960, forests, soil and oceans have steadily absorbed 56 percent of all the CO2 humanity has chucked into the atmosphere -- even as those emissions have increased by half.

- Sliver of hope -

But these carbon sinks are becoming saturated, according to the IPCC, and the percentage of human-induced carbon they soak up is likely to decline as the century unfolds.

"The alarm bells are deafening, and the evidence is irrefutable," UN Secretary-General Antonio Guterres said.

"Greenhouse gas emissions from fossil fuel burning and deforestation are choking our planet and putting billions of people at immediate risk."

The report offers a sliver of hope for keeping the 1.5C goal alive.

The IPCC projected the increase in global surface temperature for five emissions scenarios -- ranging from wildly optimistic to outright reckless -- and identifies best estimates for 20-year periods with mid-points of 2030, 2050 and 2090.

By mid-century, the 1.5C threshold will be breached across the board -- by a 10th of a degree along the most ambitious pathway, and by nearly a full degree at the opposite extreme.

But under the most optimistic storyline, Earth's surface will have cooled a notch to 1.4C by century's end.

The other long-term trajectories, however, do not look promising.

Temperature increases by 2090 range from a hugely challenging 1.8C to a catastrophic 4.4C.

The report's authors were at pains to emphasise that the 1.5C goal is not all-or-nothing.

- 'Every bit of warming matters' -

"It is important politically, but it is not a cliff edge where everything will suddenly become very catastrophic," said lead author Amanda Maycock, director of the Institute for Climate and Atmospheric Sciences at the University of Leeds.

Ed Hawkins, a professor of climate science at the University of Reading and a lead author, said that "every bit of warming matters."

"The consequences get worse and worse as we get warmer and warmer. Every tonne of CO2 matters."

Part 2 of the IPCC assessment -- on impacts -- is slated for publication in February. Part 3, to be released in March, focuses on ways to reduce carbon in the atmosphere.

The focus now will shift to the political arena, where a non-stop series of ministerial and summit meetings, including a critical G20 in October, will lead up to the COP26 UN climate conference in Glasgow, hosted by Britain.

"I hope today's IPCC report will be a wake-up call for the world to take action now," said British leader Boris Johnson.

Countries do not see eye-to-eye on many basic issues, beginning with the 1.5C goal.

China, India, Brazil, South Africa and Russia are lukewarm on it, US special presidential envoy for climate John Kerry told the New Yorker last week. Rich countries, meanwhile, have badly missed a deadline to provide funding for developing nations to green their economies and adapt to climate change already in the pipeline.

"The new IPCC report is not a drill but the final warning that the bubble of empty promises is about to burst," said Saleemul Huq, director of the International Centre for Climate Change and Development in Dhaka.

"It's suicidal, and economically irrational to keep procrastinating."

No good news here: Key IPCC findings on climate change
Paris (AFP) Aug 9, 2021 - The UN Intergovernmental Panel on Climate Change's first major scientific assessment since 2014, released Monday, shows unequivocally that global warming is unfolding more quickly than feared and that humanity is almost entirely to blame.

Here is a rundown of some of its key findings from the IPCC Working Group 1 report on physical science:

Goodbye 1.5C, hello overshoot

Earth's average surface temperature is projected to hit 1.5 or 1.6 degrees Celsius above preindustrial levels around 2030 in all five of the greenhouse gas emissions scenarios -- ranging from highly optimistic to reckless -- considered by the report. That's a full decade earlier than the IPCC predicted just three years ago.

By mid-century, the 1.5C threshold will have been breached across the board, by a tenth of a degree along the most ambitious pathway, and by nearly a full degree at the opposite extreme.

There is a silver lining: in the most ambitious if-we-do-everything-right scenario, global temperatures -- after "overshooting" the 1.5C target -- fall back to 1.4C by 2100.

Natural climate allies weakening

Since about 1960, forests, soil and oceans have absorbed 56 percent of all the CO2 humanity has chucked into the atmosphere -- even as those emissions have increased by half. Without nature's help, Earth would already be a much hotter and less hospitable place.

But these allies in our fight against global heating -- known in this role as carbon sinks -- are showing signs of becoming saturated, and the percentage of human-induced carbon they soak up is likely to decline as the century unfolds.

Yes, climate change is to blame

The report highlights the stunning progress of a new field, attribution science, in quantifying the extent to which human-induced global heating increases the intensity and/or likelihood of a specific extreme weather event such as a heatwave, a hurricane or a wildfire.

Within weeks, for example, scientists established that the record-shattering heatwave that devastated British Columbia in June would have been "virtually impossible" without the influence of climate change.

More generally, the 2021 IPCC report includes many more findings reached with "high confidence" than before.

Sea rising higher, more quickly

Global oceans have risen about 20 centimetres (eight inches) since 1900, and the rate of increase has nearly tripled in the last decade. Crumbling and melting ice sheets atop Antarctica and especially Greenland have replaced glacier melt as the main driver.

If global warming is capped at 2C, the ocean watermark will go up about half a metre over the 21st century. It will continue rising to nearly two metres by 2300 -- twice the amount predicted by the IPCC in 2019.

Because of uncertainty over ice sheets, scientists cannot rule out a total rise of two metres by 2100 in a worst-case emissions scenario.

Dire warnings from the deep past

Major advances in palaeoclimatology -- the science of natural climate in Earth's past -- have delivered sobering warnings.

For example, the last time the planet's atmosphere was as warm as today, about 125,000 years ago, global sea levels were likely 5-10 metres higher -- a level that would put many major coastal cities under water.

Three million years ago, when atmospheric CO2 concentrations matched today's levels and temperatures were 2.5C to 4C higher, sea levels were up to 25 metres higher.

Methane in the spotlight

The report includes more data than ever before on methane (CH4), the second most important greenhouse gas after CO2, and warns that failure to curb emissions could undermine Paris Agreement goals.

Human-induced sources are roughly divided between leaks from natural gas production, coal mining and landfills on one side, and livestock and manure handling on the other.

CH4 lingers in the atmosphere only a fraction as long as CO2, but is far more efficient at trapping heat. CH4 levels are their highest in at least 800,000 years.

A focus on regional differences

Although all parts of the planet -- from the oceans to the land to the air we breathe -- are warming, some areas are heating faster than others. In the Arctic, for example, the average temperature of the coldest days is projected to increase at about triple the rate of global warming across the planet as a whole.

Sea levels are rising everywhere, but will likely increase up to 20 percent above the global average along many coastlines.

Tipping points = abrupt change

The IPCC warns against abrupt, "low likelihood, high impact" shifts in the climate system that, when irreversible, are called tipping points. Disintegrating ice sheets holding enough water to raise seas a dozen metres; the melting of permafrost laden with billions of tons of carbon; the transition of the Amazon from tropical forest to savannah -- are all examples.

"Abrupt responses and tipping points of the climate system... cannot be ruled out," the report says.

Global ocean 'conveyor belt'

The Atlantic Meridional Overturning Circulation (AMOC) -- a large system of ocean currents that regulates the global transfer of heat from the tropics into the northern hemisphere -- is slowing down, a trend "very likely" to continue throughout the 21st century.

Scientists have only "medium confidence" that the AMOC will not stall altogether, as it has in the past. If it did, European winters would become much harsher, monsoon seasons would likely be disrupted, and sea levels in the north Atlantic basin could rise substantially.

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