1

We are playing a global endgame. Humanity's grasp on the planet is not strong; it is growing weaker. Freshwater is growing short; the atmosphere and the seas are increasingly polluted as a result of what has transpired on the land. The climate is changing in ways unfavorable to life, except for microbes, jellyfish, and fungi. For many species, these changes are already fatal. 

Because the problems created by humanity are global and progressive, because the prospect of a point of no return is fast approaching, the problems can't be solved piecemeal. There is just so much water left for fracking, so much rainforest cover available for soybeans and oil palms, so much room left in the atmosphere to store excess carbon. The impact on the rest of the biosphere is everywhere negative, the environment becoming unstable and less pleasant, our long-term future less certain. 

Only by committing half of the planet's surface to nature can we hope to save the immensity of life-forms that compose it. Unless humanity learns a great deal more about global biodiversity and moves quickly to protect it, we will soon lose most of the species composing life on Earth. The Half-Earth proposal offers a first, emergency solution commensurate with the magnitude of the problem: By setting aside half the planet in reserve, we can save the living part of the environment and achieve the stabilization required for our own survival.

Why one-half? Why not one-quarter or one-third? Because large plots, whether they already stand or can be created from corridors connecting smaller plots, harbor many more ecosystems and the species composing them at a sustainable level. As reserves grow in size, the diversity of life surviving within them also grows. As reserves are reduced in area, the diversity within them declines to a mathematically predictable degree swiftly—often immediately and, for a large fraction, forever. 

A biogeographic scan of Earth's principal habitats shows that a full representation of its ecosystems and the vast majority of its species can be saved within half the planet's surface. At one-half and above, life on Earth enters the safe zone. Within that half, more than 80 percent of the species would be stabilized. 

There is a second, psychological argument for protecting half of Earth. Half-Earth is a goal—and people understand and appreciate goals. They need a victory, not just news that progress is being made. It is human nature to yearn for finality, something achieved by which their anxieties and fears are put to rest. We stay afraid if the enemy is still at the gate, if bankruptcy is still possible, if more cancer tests may yet prove positive. It is our nature to choose large goals that, while difficult, are potentially game changing and universal in benefit. To strive against odds on behalf of all of life would be humanity at its most noble. 

2

Extinction events are not especially rare in geological time. They have occurred in randomly varying magnitude throughout the history of life. Those that are truly apocalyptic, however, have occurred at only about 100-million-year intervals. There have been five such peaks of destruction of which we have record, the latest being Chicxulub, the mega-asteroid that wiped out the dinosaurs. Earth required roughly 10 million years to recover from each mass extinction. The peak of destruction that humanity has initiated is often called the Sixth Extinction. 

Many authors have suggested that Earth is already different enough to recognize the end of the Holocene and the beginning of a new geological epoch. The favored name, coined by the biologist Eugene F. Stoermer in the early 1980s and popularized by the atmospheric chemist Paul Crutzen in 2000, is the Anthropocene, the Epoch of Man. 

The logic for distinguishing the Anthropocene is sound. It can be clarified by the following thought experiment. Suppose that in the far-distant future geologists were to dig through Earth's crusted deposits to the strata spanning the past thousand years of our time. They would encounter sharply defined layers of chemically altered soil. They would recognize signatures of rapid climate changes. They would uncover abundant fossil remains of domesticated plants and animals that had replaced most of Earth's prehuman fauna and flora. They would excavate fragments of machines, and a veritable museum of deadly weapons. 

3

Biodiversity as a whole forms a shield protecting each of the species that together compose it, ourselves included. What will happen if, in addition to the species already extinguished by human activity, say, 10 percent of those remaining are taken away? Or 50 percent? Or 90 percent? As more species vanish or drop to near extinction, the rate of extinction of the survivors accelerates. In some cases the effect is felt almost immediately. When a century ago the American chestnut, once a dominant tree over much of eastern North America, was reduced to near extinction by an Asian fungal blight, seven moth species whose caterpillars depended on its vegetation vanished. As extinction mounts, biodiversity reaches a tipping point at which the ecosystem collapses. Scientists have only begun to study under what conditions this catastrophe is most likely to occur. 

Human beings are not exempt from the iron law of species interdependency. We were not inserted as ready-made invasives into an Edenic world. Nor were we intended by providence to rule that world. The biosphere does not belong to us; we belong to it. The organisms that surround us in such beautiful profusion are the product of 3.8 billion years of evolution by natural selection. We are one of its present-day products, having arrived as a fortunate species of old-world primate. And it happened only a geological eye-blink ago. Our physiology and our minds are adapted for life in the biosphere, which we have only begun to understand. We are now able to protect the rest of life, but instead we remain recklessly prone to destroy and replace a large part of it. 

4

Earth remains a little-known planet. Scientists and the public are reasonably familiar with the vertebrates (fishes, amphibians, reptiles, birds, mammals), mostly because of their large size and immediate visible impact on human life. The best known of the vertebrates are the mammals, with about 5,500 species known and, according to experts, a few dozen remaining to be discovered. Birds have 10,000 recognized species, with an average of two or three new species turning up each year. Reptiles are reasonably well known, with slightly more than 9,000 species recognized and 1,000 estimated to await discovery. Fishes have 34,000 known species and as many as 10,000 awaiting discovery. Amphibians (frogs, salamanders, wormlike caecilians), among the most vulnerable to destruction, are less well known than the other land vertebrates: a bit over 6,600 species discovered out of a surprising 16,000 believed to exist. Flowering plants come in with about 270,000 species known and as many as 94,000 awaiting discovery. 

For most of the rest of the living world, the picture is radically different. When expert estimates for invertebrates (such as the insects, crustaceans, and earthworms) are added to estimates for algae, fungi, mosses, and gymnosperms as well as for bacteria and other microorganisms, the total added up and then projected has varied wildly, from 5 million to more than 100 million species. 

If the current rate of basic descriptions and analyses continues, we will not complete the global census of biodiversity—what is left of it—until well into the 23rd century. Further, if Earth's fauna and flora is not more expertly mapped and protected, and soon, the amount of biodiversity will be vastly diminished by the end of the present century. Humanity is losing the race between the scientific study of global biodiversity and the obliteration of countless still-unknown species. 

5

From 1898 to 2006, 57 kinds of freshwater fish declined to extinction in North America. The causes included the damming of rivers and streams, the draining of ponds and lakes, the filling in of springheads, and pollution, all due to human activity. Here, to bring them at least a whisper closer to their former existence, is a partial list of their common names: Maravillas red shiner, plateau chub, thicktail chub, phantom shiner, Clear Lake splittail, deepwater cisco, Snake River sucker, least silverside, Ash Meadows poolfish, whiteline topminnow, Potosi pupfish, La Palma pupfish, graceful priapelta, Utah Lake sculpin, Maryland darter. 

There is a deeper meaning and long-term importance of extinction. When these and other species disappear at our hands, we throw away part of Earth's history. We erase twigs and eventually whole branches of life's family tree. Because each species is unique, we close the book on scientific knowledge that is important to an unknown degree but is now forever lost. 

The biology of extinction is not a pleasant subject. The vanishing remnants of Earth's biodiversity test the reach and quality of human morality. Species brought low by our hand now deserve our constant attention and care.  

6

How fast are we driving species to extinction? For years paleontologists and biodiversity experts have believed that, before the coming of humanity about 200,000 years ago, the rate of origin of new species per extinction of existing species was roughly one species per million species per year. As a consequence of human activity, it is believed that the current rate of extinction overall is between 100 and 1,000 times higher than it was originally. 

This grim assessment leads to a very important question: How well is conservation working? How much have the efforts of global conservation movements achieved in slowing and halting the devastation of Earth's biodiversity? 

Despite heroic efforts, the fact is that due to habitat loss, the rate of extinction is rising in most parts of the world. The preeminent sites of biodiversity loss are the tropical forests and coral reefs. The most vulnerable habitats of all, with the highest extinction rate per unit area, are rivers, streams, and lakes in both tropical and temperate regions. 

Biologists recognize that across the 3.8-billion-year history of life, over 99 percent of all species that lived are extinct. This being the case, what, we are often asked, is so bad about extinction? 

The answer, of course, is that many of the species over the eons didn't die at all—they turned into two or more daughter species. Species are like amoebas; they multiply by splitting, not by making embryos. The most successful are the progenitors of the most species through time, just as the most successful humans are those whose lineages expand the most and persist the longest. We, like all other species, are the product of a highly successful and potentially important line that goes back all the way to the birth of humanity and beyond that for billions of years, to the time when life began. The same is true of the creatures still around us. They are champions, each and all. Thus far. 

7

The surviving wildlands of the world are not art museums. They are not gardens to be arranged and tended for our delectation. They are not recreation centers or reservoirs of natural resources or sanatoriums or undeveloped sites of business opportunities—of any kind. The wildlands and the bulk of Earth's biodiversity protected within them are another world from the one humanity is throwing together pell-mell. What do we receive from them? The stabilization of the global environment they provide and their very existence are gifts to us. We are their stewards, not their owners. 

Each ecosystem—be it a pond, meadow, coral reef, or something else out of thousands that can be found around the world—is a web of specialized organisms braided and woven together. The species, each a freely interbreeding population of individuals, interact with a set of the other species in the ecosystem either strongly or weakly or not at all. Given that in most ecosystems even the identities of most of the species are unknown, how are biologists to define the many processes of their interactions? How can we predict changes in the ecosystem if some resident species vanish while other, previously absent species invade? At best we have partial data, working off hints, tweaking everything with guesses. 

What does knowledge of how nature works tell us about conservation and the Anthropocene? This much is clear: To save biodiversity, it is necessary to obey the precautionary principle in the treatment of Earth's natural ecosystems, and to do so strictly. Hold fast until we, scientists and the public alike, know much more about them. Proceed carefully—study, discuss, plan. Give the rest of Earth's life a chance. Avoid nostrums and careless talk about quick fixes, especially those that threaten to harm the natural world beyond return. 

8

Today every nation-state in the world has a protected-area system of some kind. All together the reserves number about 161,000 on land and 6,500 over marine waters. According to the World Database on Protected Areas—a joint project of the United Nations Environment Programme and the International Union for Conservation of Nature—they occupied by 2015 a little less than 15 percent of Earth's land area and 2.8 percent of Earth's ocean area. The coverage is increasing gradually. This trend is encouraging. To have reached the existing level is a tribute to those who have participated in the global conservation effort. But is the level enough to halt the acceleration of species extinction? It is in fact nowhere close to enough. 

The declining world of biodiversity cannot be saved by the piecemeal operations in current use. It will certainly be mostly lost if conservation continues to be treated as a luxury item in national budgets. The extinction rate our behavior is imposing, and seems destined to continue imposing, on the rest of life is more correctly viewed as the equivalent of a Chicxulub-size asteroid strike played out over several human generations. 

The only hope for the species still living is a human effort commensurate with the magnitude of the problem. The ongoing mass extinction of species, and with it the extinction of genes and ecosystems, ranks with pandemics, world war, and climate change as among the deadliest threats that humanity has imposed on itself. To those who feel content to let the Anthropocene evolve toward whatever destiny it mindlessly drifts to, I say, please take time to reconsider. To those who are steering the growth of nature reserves worldwide, let me make an earnest request: Don't stop. Just aim a lot higher. 

Populations of species that were dangerously small will have space to grow. Rare and local species previously doomed by development will escape their fate. The unknown species will no longer remain silent and thereby be put at highest risk. People will have closer access to a world that is complex and beautiful beyond our present imagining. We will have more time to put our own house in order for future generations. Living Earth, all of it, can continue to breathe.

This article appeared in the January/February 2017 edition with the headline "Fifty-Fifty."