"If not checked," wrote the scientists, led by particle physicist and Union of Concerned Scientists co-founder Henry Kendall, "many of our current practices put at serious risk the future that we wish for human society and the plant and animal kingdoms, and may so alter the living world that it will be unable to sustain life in the manner that we know."
But things were only going to get worse.
To mark the letter's 25th anniversary, researchers have issued a bracing follow-up. In a communique published Monday in the journal BioScience, more than 15,000 scientists from 184 countries assess the world's latest responses to various environmental threats. Once again, they find us sorely wanting.
"Humanity has failed to make sufficient progress in generally solving these foreseen environmental challenges, and alarmingly, most of them are getting far worse," they write.
This letter, spearheaded by Oregon State University ecologist William Ripple, serves as a "second notice," the authors say: "Soon it will be too late to shift course away from our failing trajectory."
Global climate change sits atop the new letter's list of planetary threats. Global average temperatures have risen by more than half a degree Celsius since 1992, and annual carbon dioxide emissions have increased by 62 percent.
But it's far from the only problem people face. Access to fresh water has declined, as has the amount of forestland and the number of wild-caught fish (a marker of the health of global fisheries). The number of ocean dead zones has increased. The human population grew by a whopping 2 billion, while the populations of all other mammals, reptiles, amphibians and fish have declined by nearly 30 percent.
"The rapid global decline in ozone depleting substances shows that we can make positive change when we act decisively," the letter says.
The authors offer 13 suggestions for reining in our impact on the planet, including establishing nature reserves, reducing food waste, developing green technologies and establishing economic incentives to shift patterns of consumption.
To this end, Ripple and his colleagues have formed a new organization, the Alliance of World Scientists, aimed at providing a science-based perspective on issues affecting the well-being of people and the planet.
"Scientists are in the business of analyzing data and looking at the long-term consequences," Ripple said in a release. "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."
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