More than 15,000 scientists meanwhile warned that carbon emissions, human population growth, and consumption-driven lifestyles were poisoning the planet and depleting its resources.
"We are jeopardising our future," they wrote in a comment entitled "World Scientists' Warning to Humanity: A Second Notice," echoing a similar open letter from 1992.
It is "especially troubling" that the world continues on a path toward "potentially catastrophic climate change due to rising greenhouse gases from burning fossil fuels," they said.
"We have unleashed a mass extinction event, the sixth in roughly 540 million years."
Another group of scientists cautioned that rising global temperatures were bringing Earth ever closer to dangerous thresholds that could accelerate global warming beyond our capacity to rein it in.
"In the last two years, evidence has accumulated that we are now on a collision course with tipping points in the Earth system," Johan Rockstrom, executive director of the Stockholm Resilience Centre.
Some scientists, for example, have concluded that the planet's surface has already warmed enough -- 1.1 degrees Celsius (2.0 degrees Fahrenheit) on average -- in the last 150 years to lock in the disintegration of the West Antarctic ice sheet, which holds enough frozen water to lift global oceans by six or seven metres.
It may take 1,000 years, but -- if they are right -- the ice sheet will melt no matter how quickly humanity draws down the greenhouse gases that continue to drive global warming. Rockstrom and colleagues identified a dozen such natural processes that could tip into abrupt and irreversible change.
An increase of 1-3 C, for example, would likely provoke the loss of Arctic summer sea ice, warm-water coral reefs, and mountain glaciers. A degree or two more would see large swathes of the Amazon rainforest turn into savanna, and slow a deep-sea current that regulates weather on both sides of the northern Atlantic.
The International Union for Conservation of Nature (IUCN), meanwhile, released a report Monday showing that climate change now imperils one in four natural World Heritage sites, including coral reefs, glaciers, and wetlands -- nearly double the number from just three years ago.