Mankind is still facing the existential threat of runaway consumption of limited resources by a rapidly growing population, they warn. And "scientists, media influencers and lay citizens" aren't doing enough to fight against it, according to the letter.
If the world doesn't act soon, there be catastrophic biodiversity loss and untold amounts of human misery, they warn.
Only the hole in the ozone layer has improved since the first letter was written, and the letter urges humanity to use that as an example of what can happen when it acts decisively. But every single other threat has just got worse, they write, and there is not long left before those changes can never be reversed.
There are some causes for hope, the letter suggests. But humanity isn't doing nearly enough to make the most of them and soon won't be able to reverse its fate.
"Soon it will be too late to shift course away from our failing trajectory, and time is running out," the letter warns. "We must recognize, in our day-to-day lives and in our governing institutions, that Earth with all its life is our only home."
A host of environmental calamities are highlighted in the warning notice, including catastrophic climate change, deforestation, mass species extinction, ocean "dead zones", and lack of access to fresh water.
Writing in the online international journal BioScience, the scientists led by top US ecologist Professor William Ripple, from Oregon State University, said: "Humanity is now being given a second notice ... We are jeopardising our future by not reining in our intense but geographically and demographically uneven material consumption and by not perceiving continued rapid population growth as a primary driver behind many ecological and even societal threats.
"By failing to adequately limit population growth, reassess the role of an economy rooted in growth, reduce greenhouse gases, incentivise renewable energy, protect habitat, restore ecosystems, curb pollution, halt defaunation, and constrain invasive alien species, humanity is not taking the urgent steps needed to safeguard our imperilled biosphere."
In their original warning, scientists including most of the world's Nobel Laureates argued that human impacts on the natural world were likely to lead to "vast human misery".
The new notice, written as an open-letter "viewpoint" article, won the support of 15,364 scientists from 184 countries who agreed to offer their names as signatories.
The authors drew on data from government agencies, non-profit organisations and individual researchers to set out their case that environmental impacts were likely to inflict "substantial and irreversible harm" to the Earth.
Prof Ripple said: "Those who signed this second warning aren't just raising a false alarm. They are acknowledging the obvious signs that we are heading down an unsustainable path.
"We are hoping that our paper will ignite a widespread public debate about the global environment and climate."
Progress had been made in some areas - such as cutting ozone-depleting chemicals, and increasing energy generated from renewable sources - but this was far outweighed by the damaging trends, said the scientists.
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