“Homo sapiens is poised to become the greatest catastrophic agent since a giant asteroid collided with the Earth 65,000,000 years ago, wiping out half the world’s species in a geological instant.” So wrote anthropologist Richard Leakey in his 1995 book, “The Sixth Extinction: Patterns of Life and the Future of Humankind.”

Because of the vital dependence we have on the “ecosystem services” provided by the rest of nature, Leakey warned, “unrestrained, Homo sapiens might not only be the agent of the sixth extinction, but also risks being one of its victims.”

Twenty years later, the great climate journalist Elizabeth Kolbert has won a very deserved Pulitzer prize for her nonfiction book “The Sixth Extinction: An Unnatural History.”

In her book, Kolbert quotes Leakey and explains that there’s no way of knowing if humanity will be wiped out in this self-inflicted disaster. For her, “what’s most worth attending to” right now, is the fact that “we are deciding, without quite meaning to, which evolutionary pathway remain open and which will forever be closed.” As she notes, “no other creature has ever managed this.”

I personally doubt homo sapiens will go fully extinct. The more important question for me is whether the planet can support upwards of 10 billion people post-2050 given that we have already overshot the Earth’s biocapacity — and the overshoot gets worse every year.

Most significantly, we are in the process of destroying a livable climate upon which so many species, including our own, rely. We are currently on a trajectory to warm the planet 4°C (7°F) or more this century and then continue warming in the next. In 2011, the UK Royal Society devoted a special issue of one of its journals to “Four degrees and beyond: the potential for a global temperature increase of four degrees and its implications.” The concluding piece warned:

“In such a 4°C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world.”

In particular, “drought and desertification would be widespread” and we’d see “large areas of cropland becoming unsuitable for cultivation, and declining agricultural yields.” At the same time, we’d “also rapidly be losing [the world’s] ecosystem services, owing to large losses in biodiversity, forests, coastal wetlands, mangroves and saltmarshes, and terrestrial carbon stores, supported by an acidified and potentially dysfunctional marine ecosystem.”

Can such a world support 10 billion people?

As for biodiversity, a 2015 study in Science said we may lose one-sixth of all species to extinction if we warm 4°C. “Other experts said the real toll may turn out to be even worse,” reported the New York Times. The paper quoted evolutionary biologist John Wiens warning the number of extinctions “may well be two to three times higher.”

As I reported a few weeks ago, another 2015 study in Science concluded that the Permo-Triassic extinction 252 million years ago (“the greatest extinction of all time”) happened when massive amounts carbon dioxide were injected into the atmosphere, first slowly and then quickly (driven by volcanic eruptions). The researchers found “During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.” This extinction killed over 90 percent of marine life and wiped out some 70 percent of land-based animal and plant life.

A 2014 review article in the journal Science led by Duke conservation ecologist Stuart Pimm, “The biodiversity of species and their rates of extinction, distribution, and protection,” concluded, “Current rates of extinction are about 1,000 times the background rate of extinction. These are higher than previously estimated and likely still underestimated.”

The current mass extinction is due to a combination of factors, many driven by humans, including habitat destruction and over-fishing and over-hunting. A number of aspects of climate change have begun contributing to species extinction, but what is of most concern to biologists today is that as the rate of global warming speeds up in the coming decades, the climate may well change too quickly for many if not most species to adapt.

Significantly, there is more to biodiversity than just the number of species, as shown in a 2011 study , “Cryptic biodiversity loss linked to global climate change.” It was the first global study “to quantify the loss of biological diversity on the basis of genetic diversity.” Cryptic biodiversity “encompasses the diversity of genetic variations and deviations within described species.” It could only be studied in detail since molecular-genetic methods were developed.

Researchers noted that “If global warming continues as expected, it is estimated that almost a third of all flora and fauna species worldwide could become extinct.” But their research “discovered that the proportion of actual biodiversity loss should quite clearly be revised upwards: by 2080, more than 80% of genetic diversity within species may disappear in certain groups of organisms.” Species may survive, but ”the majority of the genetic variations, which in each case exist only in certain places, will not survive,” as study co-author Carsten Nowak explained. A species’ genetic variation increases its adaptability to a changing climate and changing habitats. Losing genetic diversity decreases the species’ long-term chances for survival.

A similar point was made in a January 2015 Science article, “Planetary boundaries,” by 18 international experts led by Will Steffen of the Stockholm Resilience Centre.

Here is the key chart of their findings (an update of their original 2009 findings).

We are already well beyond the zone of uncertainty and into the high risk area for the “genetic diversity” component of biosphere integrity. Researchers label climate change and biosphere integrity as “core boundaries.” They could “drive the Earth System into a new state” if substantially changed. Steffen notes, “Transgressing a boundary increases the risk that human activities could inadvertently drive the Earth System into a much less hospitable state, damaging efforts to reduce poverty and leading to a deterioration of human wellbeing in many parts of the world, including wealthy countries.”

The bottom line, as the Science authors explain, is that “The relatively stable, 11,700-year-long Holocene epoch is the only state of the ES [Earth System] that we know for certain can support contemporary human societies.” As we move beyond that stable state, the risks for all species — including ours — grow and grow.