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Here's the sobering truth: Around half the species on Earth today could disappear by middle of the century, unless we humans can tackle climate change and slow our population growth.

That's a view shared by leading biologists and ecologists, many of whom are gathering in the Vatican this week for a wonky but optimistic-sounding conference: "How To Save the Natural World on Which We Depend."

Scientists estimate that by mid-century, as many as 30 to 50 percent of all species could face extinction.

"The living fabric of the world ... is slipping through our fingers without our showing much sign of caring," the Pontifical Academy of Sciences, which organized the conference, said on its website.

The Catholic Church has made ecological issues a top concern under Pope Francis.

The pontiff's 2015 encyclical, called Laudato Si, urges the world's 1.2 billion Roman Catholics -- and everyone else -- to protect the environment and spare communities from climate change, water and food scarcity, and toxic pollution.

In a section on biodiversity, Pope Francis writes, "Because all creatures are connected, each must be cherished with love and respect, for all of us as living creatures are dependent on one another."

Starting Monday, scientists, scholars and Catholic leaders will focus on the threats to not only well-known species like polar bears and elephants, but other, less famous varieties of animal and plant life as well. Humans need biological diversity to ensure we still have abundant food supplies, disease-curing medicines, breathable air and drinkable water, among other vital benefits.

The conference will focus on the so-called "sixth extinction," which our planet is likely experiencing right now.

During Earth's 4.5-billion-year history, five major extinction events have wiped out nearly all the species on the planet, the geological record shows. The last die-off happened around 65 million years ago, when dinosaurs disappeared. Asteroid strikes, volcanic eruptions and natural climate shifts were likely to blame for those past events.

The planet may now be heading for a sixth mass die-off, this time because of humans.

Before today, about one to five species a year would become extinct due to natural causes. Scientists estimate we're now lowering species at 1,000 to 10,000 times the normal rate, Harvard Medical School researchers found in a 2008 report.

Burning fossil fuels for energy, clear-cutting forests for agriculture, filling in wetlands to build cities, dumping pollution in the ocean -- all these activities are making Earth less hospitable to microscopic organisms and majestic beasts alike.

Our soaring population, set to reach 11.2 billion people by 2100, only adds more planetary stress.

Estimates for extinction rates this century are far from certain and vary, though most are still troubling.

A 2015 study by University of Connecticut professor Mark Urban suggested up to one in six species -- or 16 percent -- could become extinct in 2100.

"The extinctions we face pose and even greater threat to civilization than climate change, for the simple reason they are irreversible," Peter Raven, a biologist at the Missouri Botanical Garden, told the Observer ahead of the Vatican conference.

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