23 Sep 2009: Analysis

Provocative New Study Warns
of Crossing Planetary Boundaries

The Earth has nine biophysical thresholds beyond which it cannot be pushed without disastrous consequences, the authors of a new paper in the journal Nature report. Ominously, these scientists say, we have already moved past three of these tipping points.

by carl zimmer

Human civilization has had a stable childhood. Over the past 10,000 years, as our ancestors invented agriculture and built cities, the Earth remained relatively stable. The average global temperature fluttered slightly, never lurching towards a greenhouse climate or chilling enough to enter a new Ice Age. The pH of the oceans remained steady, providing the right chemical conditions for coral reefs to grow and invertebrates to build shells. Those species, in turn, helped support a stable food web that provided plenty of fish for us humans to catch. The overall stability of the past 10,000 years may have played a big part in humanity’s explosion.

Now, ironically, civilization has become so powerful that it can reshape the planet itself. “We have become a force to contend with at the global level,” as Johan Rockstrom of the Stockholm Resilience Center in Sweden, puts it. Humans have changed the chemistry of Earth’s oceans, lowering their pH and causing ocean acidification. We are shifting the composition of the atmosphere, raising levels of carbon dioxide higher than they’ve been in at least the past 800,000 years.

A number of scientists have warned in recent years that if we keep pushing the planet this way, we will cause sudden, irreversible damage to the

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systems that made human civilization possible in the first place. Typically, they’ve just focused on one of these tipping points at a time. But in today’s issue of the journal Nature, Rockstrom and 27 of his fellow environmental scientists argue that we have to conceive of many tipping points at once. They propose that humans must keep the planet in what they call a “safe operating space,” inside of which we can thrive. If we push past the boundaries of that space — by wiping out biodiversity, for example, or diverting too much of the world’s freshwater — we risk catastrophe.

Unfortunately, the authors of the Nature paper maintain, we’ve already started pushing out beyond these boundaries without knowing where they actually are. “We’re sitting on top of a mesa right now, and we’re driving around, but we don’t have our lights on and we don’t even have a map,” says Jonathan Foley, a co-author of the new study and the director of the University of Minnesota’s Institute on the Environment. “That’s a dangerous way to move around.”

In their new study, Foley and his colleagues put down stakes to mark where they believe seven of these boundaries lie. By their estimate, we have already pushed beyond three of these boundaries, and are moving quickly toward the other four. “We’re running out of time,” says Rockstrom.

The new paper has already drawn strong reactions from other scientists, some glowing, some harsh. “This kind of work is critically important,” says Christopher Field, the director of the Department of Global Ecology at the Carnegie Institution at Stanford University. “Overall, this is an impressive attempt to define a safety zone.”

But other scientists wonder whether a planetary safety zone is a concept worth bothering with. “I don’t think this is in any way a useful way of thinking about things,” says Stuart Pimm, a conservation biologist at Duke University.

Rockstrom and his colleagues developed the concept of planetary boundaries from earlier work on how natural systems change. Those changes are sometimes gradual, but they can also come in jolts. A lake, for example, can absorb a fair amount of phosphorus from fertilizer runoff without any sign of change. “You add a little, not much happens,” says Shahid Naeem of Columbia University, who was not involved in the Nature paper. “Add a little more, not much happens. Add a little... then, all of sudden, you add a little more and — boom! — phytoplankton bloom, oxygen depletion, fish die-off, smelliness. Remove the little phosphorus that caused the tipping of the system, and it does not reverse. In fact, you have to go back to much cleaner water than you would have imagined.”

In recent years, some scientists have argued that the entire planet behaves in a similar way. Adding extra greenhouse gases can raise the planet’s temperature in a steady, proportional rate. But there may come a point when the climate will get pushed into a drastically new state. Some climate scientists have argued, for example, that global warming may trigger the runaway collapse of ice sheets in Greenland and Antarctica. Even if we then immediately stopped emitting greenhouse gases, the ice sheets would keep collapsing into the sea. And then we couldn’t do anything to reverse the change. “We don’t know how to refreeze the Greenland ice sheet,” says Rockstrom.

Rockstrom helped organize a workshop in Stockholm in April 2008 where environmental scientists talked about the other possible thresholds that might exist on a global scale. They concluded that there was good evidence for nine kinds of thresholds: climate change, ocean acidity, the ozone layer, freshwater use, the movement of nitrogen and phosphorus, the amount of land used for crops, aerosols (haze and other particles), biodiversity, and chemical pollution.

The scientists then reviewed each of those factors to mark boundaries that the world should not push beyond. “The idea is to say, ‘Let’s put up some guard rails,’” says Robert Costanza of the University of Vermont. “Maybe the guard rails are for a slope we could have taken and survived, but maybe not. We owe it to human civilization to be more careful.”

Ultimately, the scientists felt confident in estimating seven boundaries, three of which we have already pushed past. For one thing, they argue, we’ve already put too much carbon dioxide in the atmosphere. James Hansen, a NASA climate scientist and co-author of the Nature paper, has argued that to avoid catastrophic melting of ice sheets, we should keep carbon dioxide levels no higher than 350 parts per million. Before the Industrial Revolution, the concentration was at about 280 parts per million, but today we’re up to 387. In other words, we’ve moved out of the safe operating space — and into risky territory.

The scientists also argue that as we spread fertilizer on farmland and burn coal, we are pumping far too much nitrogen into the environment. Human activity releases 121 million tons of nitrogen, much of which ends up polluting rivers, lakes and oceans and potentially pushing their ecosystems into irreversible changes. At most, the scientists argue, less than 35 million tons of nitrogen would be a safe boundary.

The rate at which species are becoming extinct is also far beyond a safe boundary, according to the scientists. During most of the history of life, species have become extinct at a slow, fairly regular pace. And as old
The concept of boundaries is provocative because it highlights how much scientists don’t yet understand.
species have become extinct, new ones have been evolving. There have been times when many species have become extinct at a much faster rate, and these pulses have sometimes ushered in a global collapse of ecosystems. The authors of the new Nature paper propose that to avoid collapse, the extinction rate cannot rise above 10 times the long-term background rate. Today, however, scientists estimate that the extinction rate is 100 to 1,000 times higher.

In five other areas, the scientists found, we have not yet crossed the boundary into the danger zone. As we release carbon dioxide, for example, some of it goes into the oceans and makes it more acidic. In acidic seawater, coral reefs have a harder time building skeletons, because the minerals they produce for their skeletons quickly dissolve. Invertebrates have the same trouble making shells. According to recent surveys, the ocean is now acidifying 100 times faster than at any time during the past 20 million years. Yet the Nature co-authors estimate that we have not yet reached the point where acidity may cause ecological collapse. But we are close.

While the paper makes for a sobering read, its authors think we should also find some cause for optimism in it. Humanity nearly crossed another threshold by destroying the ozone layer with chlorofluorocarbons. But we recognized the crisis in time and banned chlorofluorocarbons, allowing the ozone layer to slowly recover. If we had waited much longer we might have been too late to do anything. “We were able to avoid a global disaster,” says Rockstrom. He hopes we can do the same again, and keep human civilization from falling off the environmental mesa.

“The authors make a strong case for their selection of key boundaries,” says the Carnegie Institution’s Christopher Field, “and the proposed locations for the boundaries are conceptually reasonable.” Field said he would not be surprised if other researchers argue for shifting the boundaries based on further research. “But most would agree with the general theme that we are pushing very hard on the Earth system, so hard that we should not be surprised if key parts begin to break.”

Other researchers agree with the basic concept of the new paper, but question whether we should be trying to pin down planetary boundaries. Michael Mann, a climate scientist at Penn State University, agrees that

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there probably is a dangerous threshold for climate change, but he thinks that 350 parts per million might be too strict a limit. And on a practical level, Mann points out that the policies being considered by the U.S. Congress probably won’t even be able to keep carbon dioxide levels down to 550 parts per million in 2100. “I sometimes worry that there is the danger that if we dramatically move the goalposts and argue that 350 ppm must be the stabilization target, policymakers will just throw their hands up in futility,” says Mann, or reach instead for a quick-fix geoengineering scheme, “which frankly terrifies me.”

But some critics question the basic concept itself. “The notion of a single boundary is just devoid of serious content,” says Stuart Pimm. “In what way is an extinction rate 10 times the background rate acceptable?”

One reason that the concept of planetary boundaries is so provocative is that it highlights how much scientists don’t yet understand about the thresholds built into our planet. “I think this is interesting and I’m glad the paper is coming out,” says Naeem, “but it could lead to the false sense that we understand the biosphere better than we do.”

POSTED ON 23 Sep 2009 IN Climate Climate Policy & Politics Asia Europe North America 


Dear Mr. Zimmer,

Your article speaks of tragedy averted in the ozone layer. Not too fast: NewScientist "Laughing gas set to deplete the ozone layer" 02 September 2009
Magazine issue 2724.

NITROUS oxide - also known as laughing gas - is the new villain in the fight to save the ozone

I don't think your scientists are keeping up with each other.

Posted by carol isaac on 24 Sep 2009

As observations mount and information piles up, we will understand the limits to growth and the tipping points ever more clearly, as well as the repair required when we mess up our Earthly nest. Precaution is better than repair and an insurance policy is better than running with risk without cover. Unfortunately, the need for applying precautionary principles appears to have become an historical curiosity as human numbers grow and the demands for technological advancement increase. If we would like to consider a Churchill-like emergency plan to save the Earth, ensure our future survival and keep the wheels of our civilization turning, we may need two pages to work with that are mutually supportive. On the left page will be the sustainability plan for human habitation of Earth, describing the shape of the society that would allow a healthy environment indefinitely into the future.

On the right hand page would be the natural extension of our civilization and technology into space. Securing a sustainable presence in space may prove to be the insurance policy that we cannot afford to live on Earth without, as from a confident survival presence in space, we would have the choice to work for a healthier Earthly environment. Should human civilization be dragged back in time to a new Stone Age, or worse, by slow or fast global heating induced catastrophe, or simply our indecision and lack of vision, what choice would we have but to join the Age of Stupid paid-up members club. Links across the page that take us to a survivor role on Earth and in space could be an adjustable sunshade in space at Lagrange 1 and the construction of solar power stations in space that could provide all Earth' energy needs and leave fossil fuels as fossils.

I hope the people of Earth vote for a two page solution to our present planetary emergency, as a focus on the Earthly nest alone could prove to be the royal road to the extinction of the human species. Unfortunately many seem to have given up hope and are waiting for the end in some black fantasy of collective suicide. We don't need to go there. We live in an age where we need a vision of hope for our survival.

Kim Peart
Posted by Kim Peart on 24 Sep 2009

A few points. Corals have survived vast changes of ocean alkalinity over millions of years (not just 10,000 years). The changing conditions is one of the drivers of the evolution of species. Secondly, the oceans are not acidic, they are strongly alkaline; all talk of acidifying the oceans is ridiculous. The earth's atmosphere is currently much less than ideal for plant life; something nearer 1000ppm CO2 is much better. There is much uncertainty about what the atmospheric concentration of CO2 has been in the past. "Greenhouse gases can raise the planet’s temperature in a steady, proportional rate" is untrue - there is no such linear proportionality. "Global warming may trigger the runaway collapse of ice sheets in Greenland and Antarctica" is nonsense (note the word 'may'; the ice sheets are held within mountains and the latent heat needed to melt them is astronomical - what is the average temperature of the Antarctic?
Posted by Phillip Bratby on 24 Sep 2009

When are we going to turn our attention to what I regard as a 10th threshold: too many people? I know this is a difficult subject that many have addressed in the past, but we have to rekindle that education effort.

The size of our populations needs to be recognized as a significant part of the problem. Too large numbers of people put too much pressure on natural systems and resources, hmm?

It occurs to me that all these threshold measures are primarily driven by our population needs, in fact.

Posted by Kirk Leonard on 26 Sep 2009

I read the article thoroughly. Thanks for adding one brick to build a carbon free new world or saving one brick from the falling old world before "a little... then, all of a sudden, ... a little more and — boom!".

As we recognized the crisis in time and banned chlorofluorocarbons, allowing the ozone layer to slowly recover, shouldn't we realize the in coming crisis before it's too late to do anything?

We hope the present policymakers will rethink before they raise their hands up because they also share the same world.
Posted by Padam Pande on 27 Sep 2009

I find the attempt to compartmentalize the planetary environment in terms of a division into 9 separate systems arbitrary, since it neglects the close interaction between several of these systems.

Several of the "planetary systems" defined by the authors are intertwined.

For example:

1. Ocean acidification is related to climate change (i.e. CO2 and SO3 contents of the atmosphere).
2. Chemical pollution and aerosol loading overlap.
3. Change in land use and climate are related in terms of the carbon cycle.
4. Biodiversity loss is related to land use and climate.
5. Availability of freshwater is related to climate and land use.

The synergy of changes including all the 9 components results in global environmental change.

Andrew Glikson
Earth and paleoclimate scientist
Australian National University

Posted by Andrew Glikson on 27 Sep 2009

Re: Phillip Bratby

On Ocean acidity..you're confused with the terminology here. No-one's saying the oceans have become battery acid - they are however becoming more acidic (or less alkaline) which if taken too far will stop coral and other marine life from forming a skeleton. And yes, ocean pH has changed up and down before, but now "100 times faster than at any time during the past 20 million years" which means nothing will have time to evolve to survive the change.

On 1000ppm CO2 being great for plants - incorrect. The limiting factor in growth in most places on the planet is water or sunlight not CO2, so adding more if it won't make plants "happier". It will however make the planet warmer and alter rainfall pattern in lots of places so that many plants (including the ones we rely on for food) will find themselves suddenly in a sub-optimal location.

On "the ice sheets are held within mountains"...unless both Greenland and Antarctica are perfect bowl-shapes that doesn't make much sense...glaciers empty into the sea.. And both of them are already melting, particularly the Arctic which is heating faster than global average due to albedo feedback.

So no, everything isn't fine. And we probably will have to stop burning these polluting finite resources (which we'd need doing eventually anyway as they are finite) and produce electricity in a way that doesn't level mountains or give our children asthma.

What a pity.
Posted by DT on 27 Sep 2009

Mr. Zimmer,

I find it fascinating that you began your article with the advent of agriculture. The type of farming which was practiced in our culture from 10,000 years ago until today is a particular type of agriculture -- one that advocates continual increases at the expense of biodiversity. It is the basis of civilization as well as the environmental and societal problems we have caused. I invite you and your readers to see the narrated slide show at www.panearth.org entitled "World Food & Human Population Growth." You can also find other resources at this site.

Thanks for your good work.

Posted by Russ Hopfenberg on 27 Sep 2009

Basically a Malthusian argument mixed with the precautionary principle. Their limits are almost purely conjecture. Here's a simple fact -- with 9-10 billion people wanting to live healthy, prosperous lives, we will change the planet. Getting 27 environmental scientists to state that no change is safer than change is pretty easy.

What we really need is to answer how best to adapt the world to best deal with those 10 billion people. Controlling CO2 emissions may not be as important as adequate education for girls, adequate urban sewage treatment and fresh water, and adequate pollution control of the more traditional, non-greenhouse gas variety.

I worry that our crusade to limit greenhouse gas emissions is blinding us to the many other imperatives that will decide whether we end up with 10 billion educated, middle class people capable of rational choices, or a seething mass of 10 billion poor people led by warlords desperate to claim limited resources.
Posted by Tom on 30 Sep 2009

Mr. Zimmer,

Thank you for this enlightening analysis. There is so much that we can do to help the environment and yet, things are moving so slowly.

I found it interesting that you started your article with a history of the earth and how, as humans, we were able to sustain a stable environment which in turn provided us with a stable food web.

Humans have been able to do extraordinary things, but unfortunately probably did not think about the long term effects of their actions. The earth did not come with a training manual and we are now witnessing the effects of not learning and listening at what it had to teach us.
Posted by Gina Dupont on 02 Oct 2009

Securing a sustainable presence in space may prove to be the insurance policy that we cannot afford to live on Earth without, as from a confident survival presence in space, we would have the choice to work for a healthier Earthly environment. Should human civilization be dragged back in time to a new Stone Age, or worse, by slow or fast global heating induced catastrophe, or simply our indecision and lack of vision, what choice would we have but to join the Age of Stupid paid-up members club.
Posted by Padam Pande on 04 Nov 2009

The planetary boundaries concept is already gaining popular interest outside the scientific community, and will be analysed from different points of views. What we take from Nature and what we deposit on the planet environment has been called our footprint.

And we know that our footprint has ceased to be sustainable a long time ago. We have created several imbalances, and they have become increasingly more acute, through an intricate feedback system.

Posted by alex on 11 Jan 2010

We're about to cross carbon dioxide omission limit in 8 to 10 years. Once it is crossed, no one will be able to save the earth ... No matter how many trees we plant, no matter we stop factories .... the CO2 omission will be automated once that tipping point is crossed.

Posted by Jilesh Patadiya on 28 Jan 2010

Yes, I think it is definetely true, that we’re running out of time. Something needs to be done, and done fast.
Posted by HP Jeschke on 17 Feb 2010

Well, I have started to dedicated my weekends for growing more trees ... Everyone should dedicated at least a day in a week to fight against global warming ...

Posted by Jilesh Patadiya on 25 Feb 2010

Everyone knows that we are on a collision course with disaster and I don't think we are a bigger force in the universe than mother nature. Can anyone guess who's going to win this battle? The whole thing makes me sick. We're selling out future generations with our irresponsibility.
Posted by Lou Alessi on 26 May 2010

The good news is they used so much water when they watered down the US climate bill that sea level stopped rising.

Posted by David Lewis on 08 Aug 2010

"The Earth has nine biophysical thresholds beyond which it cannot be pushed without disastrous consequences, the authors of a new paper in the journal Nature report. Ominously, these scientists say, we have already moved past three of these tipping points."
by carl zimmer
To set up "nine biophysical boundaries" is in itself a stupendous feat of pseudoscience, and the fact that Nature published it is a black mark against Nature.. But Carl Zimmer buys it and then tells us how we ought tp behave to avoid those boundaries. On top of that he has a box extolling the "Limits to Growth" fantasy of the Club of Rome in 1972. Can I remind you that "Limits to Growth" told us that the world would run out of oil in the nineties? And that raw materials would be exhausted in the early twenty-first century, we would starve, and civilization would collapse? This drivel is then touted as "ground-breaking" by no less a personage than Bill McKibben, the author of "Nine Degrees" which is another disaster fantasy.

How do you get this much unreality into a little newsletter that is supposed to belong to a school of forestry? It takes true cultists to accept this. Someone had to know how the moonies did it because it is spreading. Scientific societies as well as journals are in the grip of global warming madness and cannot be reached by reason. It is a huge bubble and when it bursts things will not be nice. And it will burst because there is no warming now and sooner or later someone will look at a thermometer and wonder where it went.

Posted by Arno Arrak on 12 Nov 2010

Comments have been closed on this feature.
Carl Zimmer writes about science for The New York Times and a number of magazines. A 2007 winner of the National Academies of Science Communication Award, Zimmer is the author of six books, including Microcosm: E. coli and the New Science of Life. In previous articles for Yale Environment 360, he has written about the high-tech search for a cleaner biofuel alternative and about using assisted migration to save species threatened by climate change.



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