Since then, nearly all major threats to the environment have grown more dire, particularly the booming world population, which has added two billion people since 1992, a 35 percent increase, according to the update.

Other key threats are global warming and the ever-mounting carbon emissions driven by fossil fuel use, as well as unsustainable farming practices, deforestation, lack of fresh water, loss of marine life and growing ocean dead zones.

"Humanity is now being given a second notice, as illustrated by these alarming trends," said the letter.

Mass extinction under way

"We are jeopardizing 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," it added.

Scientists noted 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."

Animals are suffering as a result of human activities, and are disappearing at an unprecedented pace.