When Marcus Eriksen takes expeditions to Wyoming to find dinosaur remains, he's digging up more than fossils. The dinosaurs he finds — which date back 65 million years — are pieces of history that teach us the reality of extinction, particularly the one he and other scientists believe is happening right now.

A recent study published in the Proceedings of the National Academy of Sciences says that populations of nearly 9,000 species have declined significantly since 1900, indicating that we are in the midst of a "sixth mass extinction" in our planet's history.

Mass extinction events occur when at least half of all species die out in a relatively short time. Currently, almost 200 species have gone extinct in the last 100 years. And in the last 400 years, there has been a 50-fold increase in extinctions, said Eriksen, who was not part of the study.

"What they all have in common is that once species are gone, they're gone for any concept of time that would ever be meaningful to humans," said Eriksen, research director and co-founder of the 5 Gyres Institute.

The most recent and well-known one — the Cretaceous-Tertiary Extinction — occurred 65 million years ago and wiped out dinosaurs along with some three-quarters of the plant and animal species on Earth.

Some of the previous mass extinction events occurred suddenly, while others occurred over several lifetimes. But they were mostly caused by environmental factors — volcanoes, oceanic overturn, geomagnetic reversals, climate change or asteroids.

Scientists say this one is being caused by mankind. Humans' overfishing, overpopulation and deforestation are all tied to habitat loss, leading to species' extinction.

"I think it's very clearly human caused," Eriksen said.

Alison Teal, an explorer and filmmaker who joined Eriksen on his most recent dinosaur dig, said she's seen coral reefs disappear before her eyes in her Hawaii fishing village.

"The smallest thing you disrupt is going to work its way up the food chain to us in the form of food, oxygen and climate control," said Teal, who hosts Alison's Adventures, an online film and blog series. "It's pretty mind blowing what has already happened."

In contrast, Smithsonian paleontologist Doug Erwin calls the claims of a mass extinction "junk science." Erwin said that scientists have a responsibility to be accurate about such comparisons.

"Many of those making facile comparisons between the current situation and past mass extinctions don't have a clue about the difference in the nature of the data, much less how truly awful the mass extinctions recorded in the marine fossil record actually were," he told The Atlantic recently.

Eriksen emphasizes that the research speaks for itself.

"We've seen a 50-fold increase in the rate of extinction," Eriksen said. "I would call that the beginnings of a mass extinction event."

The study's authors say the window for action to reverse the current course is short, probably two to three decades at most. The changes would need to happen on a global scale, with nations making commitments to preserving land and sea as a first step.

The goal would be to create a circular economy, Erisken said. Examples include protecting waters from overfishing and fossil fuel exploration, and shifting to renewable food and water stocks.

"It's part of a much bigger global list that has to happen," he said. "It's all very doable."

However, if the mass extinction continues at the current rate, the planet could take millions of years to recover.

"We need life here," Eriksen said. "By studying other mass extinctions, we realize the vulnerable place we could be if we don't stop the one happening now."