When we think about the concept of mass extinctions, we tend to think of something pretty dramatic. For instance, we now know that the dinosaurs were wiped out by a six-mile-wide asteroid that hit the Earth at thousands of miles per hour. Its impact, according to the new Smithsonian Channel documentary Mass Extinction: Life at the Brink (airing Sunday night)had the force of “a hundred million nuclear bombs,” unleashing tsunamis hundreds of feet in height that hurtled across the ocean "at the speed of a jet."

So yeah, that's pretty dramatic. And yet many scientists think that today we may be on the verge of another creeping mass extinction -- the sixth the planet has seen -- even as most people barely notice it happening.

Consider just one species highlighted by Mass Extinction: the African lion, or Panthera leo. There are some 32,000 to 35,000 lions left, according to a recent scientific estimate. But as of 1950, their numbers were vastly higher; one group of experts puts them at 500,000, and Mass Extinction uses the number 400,000. Either way, that's a 90 percent or more decline.

The lion numbers, stark as they are, are pretty solid, says Anthony Barnosky, a biologist at the University of California-Berkeley who is featured in the film, and who authored the book Dodging Extinction: Power, Food, Money and the Future of Life on Earth.

"We know that from historic records of where lions used to be, and where they clearly are not any more. So it’s a combination of using the historical data about what we know distributions were over the past couple of centuries, combined with some very detailed studies, censuses of how many lions are out there in known populations over the past half century."

The chief cause of lion declines? "Indiscriminate killing in defense of life and livestock, coupled with prey base depletion," says the International Union for Conservation of Nature, which has the African lion on its "red list" of threatened species.

African lions aren't unique. A similar story, according to Barnosky, could be told about tigers, rhinos, and any number of other species. "We have killed about 50 percent of the world’s vertebrate wildlife in just the past 40 years," he says. "We've killed half the numbers of individuals. We've fished 90 percent of the fish out of the seas. So these are big things we’re doing to the world."

Mass Extinction examines both at the asteroid strike that killed off the dinosaurs 66 million years ago, and also the "Great Dying" at the end of the Permian period 252 million years ago, when some 90 percent of Earth's species vanished in the wake of massive volcanic eruptions in Siberia.

In each of these cases, the global extinction was caused not only by an immediate dramatic event, but by its subsequent effect on the planet's oceans and atmospheres. The asteroid impact led to so much smoke in the atmosphere that the sun's radiation was cut down dramatically, leading to great climatic changes. And the dramatic vulcanism that ended the Permian triggered global warming and ocean acidification by putting lots of carbon dioxide in the atmosphere.

In other words, these past extinctions have been tied to dramatic changes in the global climate. That is not the principal cause of the current extinction -- yet. So far, we've been threatening species by taking their habitat away for farming and for our growing populations. But global warming may now act on top of that.

"It's like adding a match to gasoline," says Barnosky.

You can watch a preview of Mass Extinction below: