HOW much can our poor Earth take? We’ve already transformed most of the biosphere beyond anything our early ancestors could have imagined, clearing, ploughing, burning, building, damming, domesticating, driving to extinction, dousing with chemicals and even changing the climate. Surely at some point, the biosphere will simply collapse in the face of such a massive and unrelenting onslaught.

Or will it? This is a question that inspires intense debate among ecologists and global change scientists. Some say that we are heading rapidly for a global tipping point – a threshold beyond which the entire biosphere will shift into a new and mostly undesired state. Others, like me, are convinced that no theoretical or empirical evidence exists for such a claim, and that a widespread belief in the existence of such a point of no return threatens to push ecological science and its application in the wrong direction.

Let us examine the evidence. Ecologists have long been aware that tipping points exist in local and regional ecosystems. For example, when nutrients are added to a lake, its ecological properties tend to continue as before until the lake suddenly shifts to a new state. The water changes from clear to turbid; communities of plants, fish and other species change almost completely. Shifting the lake back into its previous state is possible, but requires massive efforts.

Among other examples of local and regional tipping points are the rapid collapse of coral reefs in the face of rising ocean acidity and the transformation of ecosystems by the extinction of a dominant species, or the introduction of a new one.

With such strong evidence of tipping points in regional ecosystems, why wouldn’t we expect such tipping points to exist in the biosphere as a whole? Examine the mechanisms that produce tipping points, and the answer becomes clear.

Tipping points happen when the components of a system respond gradually to an external force until a level of change is reached at which the response becomes non-linear and synergistic. This amplifies the effect of the force and rapidly drives the system into a new state.

To respond in this way, systems must meet certain requirements. Either external forces are applied uniformly and each part of the system responds in the same way, or the system must be highly interconnected to allow synergistic responses to emerge. Or both.

Do these criteria apply to the biosphere as a whole? I think not. For planetary tipping points to exist, the forces of humanity would need to act uniformly across the planet, all ecosystems would need to respond to them in the same way, and the response would need to be transmitted rapidly across Earth’s many ecosystems and continents.

Even the force of human-induced climate change, so evident across the planet, does not meet these requirements. For example, it warms and dries some regions while cooling and moistening others. Even if it did uniformly heat Earth’s ecosystems, this would not produce a coherent global shift in ecology because local ecosystems respond so differently, often in opposing ways.

Finally, organisms and ecosystems in different biomes and on different continents are not strongly connected. Animals, plants and microorganisms are limited in their interactions by distance and barriers such as oceans and mountain ranges. Even with human-induced species invasions, there is no species capable of colonising all of Earth’s biomes – not even the mighty cockroach.

So there is little chance of anthropogenic climate change leading to a global tipping point in the biosphere. When it comes to other changes, including land use, habitat fragmentation and extinction, the case for a global tipping point is even weaker.

How, then, does the biosphere as a whole respond to human pressures? To put it simply: every ecosystem changes in its own way. We are driving massive long-term changes in the ecology of our planet, one ecosystem, one community, one species at a time. The biosphere’s response to human pressures is merely the sum of all of the changes.

Viewing things this way puts the emphasis back where it belongs: on understanding and managing ecosystems at the local and regional level. While we must continue to think and act globally, it is the local and regional levels that are the key for conservation and management.

To deny the likelihood of an impending global tipping point is not to deny that we are transforming the biosphere profoundly and permanently in ways that are likely to disgrace us in the eyes of future generations. Much of our planet’s ecology can and will be lost unless we focus much greater effort on conserving and restoring it.

“To deny a global tipping point is not to deny that we are profoundly changing the biosphere”

With this in mind, the concept of a global tipping point has major policy implications. It suggests that below some threshold nothing serious will happen, but after that all will be lost. Holding such a view risks breeding complacency on one side and hopelessness on the other. Both are misplaced: to lose even one species is more than we should accept lightly. The same holds for our local ecosystems. To conserve them is to conserve the biosphere.

The claim that the biosphere is approaching a global tipping point remains no more than a contested and untested hypothesis. As we strive towards more sustainable stewardship of our planet, we must think globally – but let us not lose track of problems on smaller scales. The fate of the entire biosphere depends on it.

This article appeared in print under the headline “Back from the brink”

Humans have transformed Earth beyond recovery – but rather than look back in despair we should look ahead to what we can achieve

THE Holocene, with its mild climate so remarkably stable and good for us, is over. We humans have transformed Earth’s climate, geology, biology and hydrology so extensively, profoundly and permanently that geologists are proposing the formal designation of a new geological epoch: the .

International scientific panels will ultimately decide whether to recognise the new epoch, and it could be a decade or longer before we get a final ruling. Nevertheless, it’s high time that we – and I do mean all of us – take stock of the new Earth we have created. One reason to do this is to help answer a basic geological question: will the Anthropocene last long enough to justify its designation as a new epoch, or will it remain a mere geological event akin to the impact of an asteroid? It will also help us answer a more profound question: what do we do now?

The first lesson of history is simple: the Anthropocene was a long time in the making. Significant human alteration of the biosphere began more than 15,000 years ago as Palaeolithic tribes evolved social learning, advanced hunting and foraging technologies, and the use of fire, and used them to open up forested landscapes and kill off megafauna.

These Palaeolithic human impacts were significant and extensive, but they were minor compared with the impact of the rise of agriculture more than 8000 years ago. By domesticating plant and animal species and engineering ecosystems to support them, humans introduced a wide range of unambiguously anthropogenic processes into the biosphere.

Human alteration of Earth systems tends to be far more extensive and complex than one would expect based on numbers alone. Even 8000 years ago, with a population of just 10 million or so, humans had already altered as much as a fifth of Earth’s ice-free land, primarily by using fire to clear forest. The reason small populations had such extensive impacts is that early agriculture emphasised labour efficiency. Early farmers did not use the plough, and that meant constantly shifting cultivation to the most fertile areas. As a result, most of the landscape was in some stage of recovery, giving rise to “semi-natural” woodlands. These were among the first anthropogenic biomes, or ““.

In this way, human populations were able to increase and expand for millennia, converting vast tracts of pristine forest into semi-natural woodlands and less productive land into rangeland. As populations grew larger and more dense they created ever more intensively transformed anthromes by tillage, irrigation, manuring and cropping. By 1750, more than half of the terrestrial biosphere had been converted into anthromes, leaving an ever greater permanent record in soils, sediments and the atmosphere. This process ultimately gave rise to the densely populated village and urban anthromes most of us live in today.

The rise of industrial systems in the past century has transformed the majority of the terrestrial biosphere into intensively used anthromes dominated by novel ecological processes. Now more than 7 billion strong and growing, we continue to transform the last wild biomes into anthromes – a process that must end soon as we reach the limits of the usable biosphere. Already, more than 12 per cent of Earth’s ice-free land is used continuously for crops and 16 per cent for livestock.

Thus we find ourselves in the Anthropocene. Today, even if the population were to decline substantially or land use to become far more efficient, the extent, duration and intensity of human activity has altered the terrestrial biosphere sufficiently to leave an unambiguous geological record differing substantially from that of any prior epoch. Earth’s biodiversity, biogeochemistry and evolution are now profoundly reshaped by us – and are therefore in our hands.

There will be no returning to our comfortable cradle. The global patterns of the Holocene have receded and their return is no longer possible, sustainable or even desirable. It is no longer Mother Nature who will care for us, but us who must care for her.

“The global patterns of the Holocene have receded and their return is not possible or even desirable”

This raises an important but often neglected question: can we create a good Anthropocene? In the distant future will we be able to look back with pride?

We have seen what we can do, and it is awesome. In just a few millennia, humanity has emerged as a global force of nature – a networked system of billions of individuals creating and sustaining an entirely new global ecology. We live longer than ever, and our average standard of living has never been higher. These unprecedented achievements clearly demonstrate the remarkable ability of our social systems and technologies to evolve and adapt, often to changes we ourselves have induced.

Yet it is also easy to see what we have lost and are even now destroying. Wild fish and forests are nearly gone. We are warming the atmosphere, melting the ice caps, acidifying the ocean, polluting land and sea, driving species to extinction and inducing invasions by species from around the world – and in some areas leaving only a wasteland of monocultures and weeds. Clearly it is possible to look at all we have created and see only what we have destroyed.

But that, in my view, would be our mistake. We most certainly can create a better Anthropocene. We have really only just begun, and our knowledge and power have never been greater. We will need to work together with each other and the planet in novel ways. The first step will be in our own minds. The Holocene is gone. In the Anthropocene we are the creators, engineers and permanent global stewards of a sustainable human nature.