Whether humanity has pushed the planet into a "mass extinction" event may be a matter of definition, but by any measure the rate at which life-forms are disappearing is deeply alarming, scientists say.

"In each of the five previous mass extinctions, we lost about 75 percent of species," said Robert Watson, head of the UN science panel on biodiversity, which unveiled a grim assessment of the state of Nature on Monday.

Over the last several centuries Earth has lost about two percent, and so -- by that criterion -- remains far below the threshold, he told AFP.

But if one looks instead at the rate at which species are dying off, the picture becomes bleaker.

Currently the pace of extinctions is up to several hundred times greater than the average over the last ten million years, the new report concluded. At that rate, we could hit the 75 percent mark within a couple of hundred years.

Here are Earth's biggest die-offs over the last half-billion years, each showing up in the fossil record at the boundary between geological periods.

- Ordovician extinction -

When: about 445 million years ago

Species lost: 60-70 percent

Likely cause: Short but intense ice age

Most life at this time was in the oceans. It is thought that the rapid, planet-wide formation of glaciers froze much of the world's water, causing sea levels to fall sharply. Marine organisms such as sponges and algae, along with primitive snails, clams, cephalopods and jawless fish called ostracoderms, all suffered as a consequence.

- Devonian extinction -

When: about 375-360 million years ago

Species lost: up to 75 percent

Likely cause: oxygen depletion in the ocean

Again, ocean organisms were hardest hit. Fluctuations in sea level, climate change, and asteroid strikes are all suspects. One theory holds that the massive expansion of plant life on land released compounds that caused oxygen depletion in shallow waters. Armoured, bottom-dwelling marine creatures called trilobites were among the many victims, though some species survived.

- Permian extinction -

When: about 252 million years ago

Species lost: 95 percent

Possible causes: asteroid impact, volcanic activity

The mother of all extinctions, the "Great Dying" devastated ocean and land life alike, and is the only event to have nearly wiped out insects as well. Some scientists say the die-off occurred over millions of years, while others argue it was highly concentrated in a 200,000-year period.

In the sea, trilobites that had survived the last two wipeouts finally succumbed, along with some sharks and bony fishes. On land, massive reptiles known as moschops met their demise. Asteroid impacts, methane release and sea level fluctuations have all been blamed.

- Triassic extinction -

When: about 200 million years ago

Species lost: 70-80 percent

Likely causes: multiple, still debated

The mysterious Triassic die-out eliminated a vast menagerie of large land animals, including most archosaurs, a diverse group that gave rise to dinosaurs, and whose living relatives today are birds and crocodiles. Most big amphibians were also eliminated.

One theory points to massive lava eruptions during the breakup of the super-continent Pangea, which might have released huge amounts of carbon dioxide, causing runaway global warming. Other scientists suspect asteroid strikes are to blame, but matching craters have yet to be found.

- Cretaceous extinction -

When: about 66 million years ago

Species lost: 75 percent

Likely cause: asteroid strike

A space rock impact is suspect No. 1 for the extinction event that wiped out the world's non-avian dinosaurs, from T-Rex to the three-horned Triceratops. A huge crater off Mexico's Yucatan Peninsula supports the asteroid hypothesis.

But most mammals, turtles, crocodiles and frogs survived, along with birds as well as most sea life, including sharks, starfish and sea urchins. With dinosaurs out of the way, mammals flourished, eventually giving rise to the species -- Homo sapiens -- that has sparked fear of the sixth mass extinction.

Lost world: UN report shows Nature at death's door
Paris (AFP) May 6, 2019 - A landmark UN report on the state of Nature released Monday shows how humanity has wreaked havoc on the environment, undercutting Earth's essential life-support systems.

A Summary for Policymakers approved by 132 nations and the underlying 1,800-page scientific report lay bare a planet ravaged by rampant consumption and pollution, where a million species are at risk of extinction.

Here are the report's key findings, which read like a charge sheet against history's most destructive creatures: ourselves.

- Extinction -

The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) report, the first of its kind in 15 years, predicts a harrowing future for plants and animals.

One million species face the risk of extinction -- many within decades.

Species are going extinct up to several hundred times quicker than during the last 10 million years, and half a million plants and animals currently have "insufficient habitat for long term survival".

This mass extinction will have a direct and lasting impact on human life, the report warns.

- Consumption -

As our population swells, so does mankind's consumption. The report depicts a world ravaged by an insatiable demand for resources.

Crop production has surged 300 percent since 1970, meaning one third of all land is now used to make food -- an industry that uses 75 percent of all fresh water on Earth.

At least one quarter of all man-made emissions come from agriculture, the vast majority from meat production.

What's worse, half of all new agricultural land is taken from forests, the lungs of the planet that suck greenhouse gases from the air.

There is currently less than 70 percent of the forest cover Earth had before the Industrial Revolution.

Experts found that 93 percent of marine fish stocks are either overfished or fished to the limit of sustainability; one third of all fishing worldwide is said to be illegal or unreported.

In total, we extract around 60 billion tonnes of natural resources from the Earth every year -- a rise of 80 percent in a matter of decades.

- Pollution -

And we are leaving our mark in other ways.

We dump up to 400 million tonnes of heavy metals, toxic sludge and other waste into oceans and rivers each year.

There are roughly 17,000 mines operating worldwide, and at least 6,500 oil and gas installations, kept viable by $345 billion in fossil fuel subsidies.

The underlying report, compiled from more than 15,000 academic papers and research publications, estimates that 75 percent of land, 40 percent of oceans and 50 percent of rivers "manifest severe impacts of degradation" from human activity.

Our plastic production has increased 10-fold since 1990 and the use of fertilisers -- which poison ecosystems and wreck soil's CO2-absorption rates -- has doubled in just 13 years.

- Inequality -

The report's story of Earth is also one of rife inequality, with richer nations consuming vastly more per capita than poorer ones battling to retain their natural resources.

Indeed, per capita demand for materials is four times greater in high- than in low-income economies.

In Europe and North America, humans now consume several times the recommended intake of meat, sugar and fat for optimal health, while 40 percent of the world's people lack access even to clean drinking water.

Eleven percent of humans are undernourished; more than 820 million face food insecurity in Africa and Asia alone.

The inequality gap is huge and widening: GDP per head is already 50 times larger in wealthy nations than in poor ones.

- Climate -

The authors stress that whatever losses humans inflict on Nature will in turn be inflicted upon us.

More than two billion people still rely on wood as their main energy source, and up to half of all medicines come from plants and animals.

What's more, the world's oceans and forests absorb more than half of our greenhouse gas emissions, which have doubled since 1980 contributing to a 0.7 Celsius global temperature rise.

In October a sister report said only drastic cuts in fossil fuel use could avert runaway global warming.

The IPBES summary said that five percent of Earth's species are at risk of extinction if the temperature rises just 2C -- still within the targets of the Paris climate deal.

Business as usual is predicted to warm Earth 4.3C by 2100. Were that to happen, the authors warned, as many as one in six of all species could be wiped out.

SOURCES: Peer-reviewed studies, National Geographic, Encyclopaedia Britannica.

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How to fix Nature and avoid human misery: UN report
Paris (AFP) May 3, 2019
Revamping global food production, retooling the financial sector, moving beyond GDP as a measure of progress and other "transformative changes" are needed to save Nature and ourselves, a major UN biodiversity report is set to conclude. Delegates from 130 nations wrap up week-long negotiations in Paris Saturday on the executive summary of a 1,800-page tome authored by 400 scientists, the first UN global assessment of the state of Nature - and its impact on humanity - in 15 years. The bombshell ... read more