Our planet has suffered five mass extinctions, the last of which occurred about 66 million years ago, when a giant asteroid believed to have landed near the Yucatán Peninsula set off a chain reaction that wiped out the dinosaurs and roughly three-quarters of the other species on earth. A few years ago, in a book called “The Sixth Extinction,” the writer Elizabeth Kolbert warned of a devastating sequel, with plant and animal species on land and sea already disappearing at a ferocious clip, their habitats destroyed or diminished by human activities.

This time, she made clear, the asteroid is us — and we will pay heavily for our folly.

Humanity’s culpability in what many scientists believe to be a planetary emergency has now been reaffirmed by a detailed and depressing report compiled by hundreds of international experts and based on thousands of scientific studies. A summary was released last Monday in Paris, and the full 1,500-page report will be available later in the year. Its findings are grim. “Biodiversity” — a word encompassing all living flora and fauna — “is declining faster than at any time in human history,” it says, estimating that “around 1 million species already face extinction, many within decades,” unless the world takes transformative action to save natural systems. The at-risk population includes a half-million land-based species and one-third of marine mammals and corals.

Most of the causes of this carnage seem familiar: logging, poaching, overfishing by large industrial fleets, pollution, invasive species, the spread of roads and cities to accommodate an exploding global population, now seven billion and rising. If there is one alpha culprit, it is the clearing of forests and wetlands for farms to feed all those people (and, perversely, to help them get to work: The destruction of Indonesia’s valuable rain forests, and their replacement with palm oil plantations, has been driven in part by Europe’s boundless appetite for biodiesel fuels.)

Add to all this a relatively new threat: Global warming, driven largely by the burning of fossil fuels, is expected to compound the damage. “While climate change has not been the dominant driver of biodiversity loss to date in most parts of the world, it is projected to become as or more important,” said Sir Robert Watson, chairman of the biodiversity panel and former chairman of the United Nations Intergovernmental Panel on Climate Change, whose most recent alarming report on global warming has given that issue new currency in American politics. Rising seas and increased extreme weather events propelled in part by climate change — fire, floods, droughts — have already harmed many species. The most obvious victim is the world’s coral reefs, which have suffered grievously from ocean waters that have grown warmer and more acidic as a result of all the carbon dioxide they’ve been asked to absorb.