Climate Feedback

Planetary boundaries

1.bmpDespite the apparent stress that humanity is causing to the Earth system, defining sustainable limits for our own existence has proved to be something of an intractable problem. But what if we could define global sustainability numerically?

In this issue of Nature, a group of renowned earth system and environmental scientists led by Johan Rockström of the Stockholm Resilience Centre make a first attempt at estimating boundaries for the biophysical processes that determine the Earth’s capacity for self-regulation.

Using existing data, Rockström and colleagues put ‘acceptable’ upper limits on seven environmental parameters: climate change, ocean acidification, stratospheric ozone depletion, freshwater use, biodiversity loss, the global cycles of nitrogen and phosphorus, and land-use change. Crossing even one of these boundaries, they say, would risk triggering abrupt or irreversible environmental changes. And if one boundary is transgressed, then others are at serious risk of being breached.

For some parameters, such as nitrogen loading and atmospheric CO2 concentrations, we may have already stepped out of our safety zone and need to back-pedal quickly. For others, such as ocean acidification, we may still have enough time to avoid catastrophic change if we act wisely.

But do we understand the Earth system well enough to know the real limits to environmental degradation? And if we can define them, even roughly, would doing so would ultimately help or hinder efforts to protect the planet? We posed these questions to seven leading experts, who were invited to respond to the ‘planetary boundaries’ proposal. Each author brings specific expertise to evaluating one aspect of the proposed framework. Their responses can be freely accessed at Nature Reports Climate Change. We’ve weighed in with our own thoughts in an editorial in Nature, and with a podcast. All of Nature’s coverage, plus a full length version of the paper by Rockström and colleagues, can be accessed here.

The commentaries are available individually at the following links:

William H. Schlesinger, President of the Cary Institute of Ecosystem Studies in Millbrook, New York comments on the boundary for global nitrogen and phosphorus cycles (html|pdf).

Steve Bass, senior fellow at the International Institute for Environment and Development, UK comments on the boundary for land-use change (html|pdf).

Myles Allen, physicist and climatologist at the University of Oxford, UK comments on the boundary for climate change (html|pdf).

Mario J. Molina, director of the Mario Molina Center for Strategic Studies in Energy and the Environment in Mexico City comments on the boundary for stratospheric ozone depletion (html|pdf).

David Molden, deputy director general for research at the International Water Management Institute in Sri Lanka comments on the boundary for freshwater availability (html|pdf).

Peter Brewer, ocean chemist and Senior Scientist at the Monterey Bay Aquarium Research Institute in Moss Landing, California comments on the boundary for ocean acidification (html|pdf).

Cristián Samper, Director of the Smithsonian National Museum of Natural History in Washington DC comments on the boundary for biodiversity loss (html|pdf).

For the most part, our respondents agree that the ‘planetary boundaries’ framework is a useful and worthwhile endeavour. But they also issue words of caution in choosing upper limits on environmental degradation. Some such as ocean chemist Peter Brewer question whether we know enough to choose the right parameters; on ocean acidification, for example, does an upper bound on aragonite saturation fully represent the potential detriment of loading the ocean with CO2?


Similarly, climatologist Myles Allen warns that setting a limit on long-term atmospheric carbon dioxide concentrations may distract from the much more immediate challenge of limiting warming to 2°C. Whatever the long-term target, keeping temperatures to no more than 2°C above pre-industrial values will require substantial emissions reductions over the coming decades, says Allen. Without knowing what future generations might do, and without a good understanding of the long-term behaviour of the carbon cycle, perhaps we should focus on what’s achievable in the near-term?

Others, such as Schlesinger say there are dangers associated with setting boundaries. While thresholds are comforting for policymakers, they can also be risky, says Schlesinger. After all, waiting for thresholds to be crossed can merely allow the continuation of misbehaviour that might be better nipped in the bud. And, as Molden points out, defining boundaries on a global scale, by necessity, overlooks the role of local circumstances in exacerbating or ameliorating the problem of managing scarce resources.

But the planetary boundaries concept and its first estimate of numeric values also gives us an important warning call of how close we are to overstressing the Earth.

Do you agree? Is the planetary boundaries framework a useful way of measuring sustainability? Join the discussion here on Climate Feedback.

Image: Shutterstock

Olive Heffernan

Comments

  1. Report this comment

    Kevin Matthews, ArchitectureWeek said:

    Wonderful, profound, vitally important, beautifully presented stuff. Thank you, Nature!

    And facing this science, let’s not mince words. It’s not just “… an important warning call of how close we are to overstressing the Earth.”

    It’s a blueprint of how we are now, seriously, already, overstressing the Earth. Our one and only Earth.

    This science is a wayfinding guide for us to back off dramatically, especially and most urgently in the most over-stressed dimensions.

  2. Report this comment

    Jesus Rosino said:

    How interesting! Great stuff, Nature, thanks a lot! (I’m lucky, because I’ve just began to dig into this stuff with “The Limits to Growth”.

    At first sight, they seem a bit strict with CO2 boundary. They set 350 ppm, but the usual value for 2ºC is 400 ppm (eg. Steering Committee). AFIK, Hansen is the only scientist that has suggested such a lower limit.

    With that (more common) boundary, at the current rate of 2 ppm/year, we would overstep the limit in 7 years.

    On the other hand, that 350 ppm boundary would give us a radiative forcing boundary of 1.2 W/m2 (instead of 1). With the usual 400 ppm it would be 1.9 W/m2) [F = 5.35ln(C/C0)].

  3. Report this comment

    Michael Lardelli said:

    I found this feature by Nature, the associated commentaries, and the original paper by the Stockholm Resilience Centre to be incredibly depressing reading. Not because of the information they contain but for what they do not say. The entire motivation for examining these limits is because the size of the human population and its associated resource consumption and production of waste products have now become so huge that they threaten our own survival. Indeed, with our total ecological footprint now >1.25 planet Earths we are already in a state of population overshoot. However, the topic of population growth is such a vexed and contentious issue that not even a supposed centre of objectivity such as Nature appears able to handle it openly (though full credit to commentators David Molden and Peter Brewer for at least mentioning it). The scientific community seems to tie itself in knots trying to avoid talking about population issues. The language at the Stockholm Resilience Centre website is typical. Phrases such as “rapid expansion of human activities” and “the expanding human enterprise” do, of course, encapsulate the idea that environmental impact is a product of both population size and consumption. However, with the world population expanding linearly by 79 million people per year (the growth rate is no longer decreasing, http://www.worldwatch.org/node/6257) and each person needing about 2000 kcal from food per day to survive there truly are limits to growth that we appear to be rapidly approaching (especially in light of concerns that we have probably passed the peak of world oil production last year, see http://www.energybulletin.net/50077). In language better suited to a report by a management consultant the paper by Rockström et al. states, “The proposed concept of ‘planetary boundaries’ lays the groundwork for shifting our approach to governance and management, away from the essentially sectoral analyses of limits to growth aimed at minimizing negative externalities, towards the estimation of the safe space for human development”. This perpetuates the illusion that there is still uncertainty about the suicidal trajectory on which the human species is set and how close the edge of the cliff is. If we cannot talk openly and clearly about population growth (i.e. if we cannot, as a scientific community, recognise the scale and urgency of the problem) then there is no hope that the world can take the necessary action to avoid – or even ameliorate – the approaching ecological disaster (population crash).

  4. Report this comment

    Marc Hendrickx said:

    This model makes a lot of major unfounded assumptions, perhaps the biggest being that humans are incapable of thriving under climate scenarios that differ greatly from the Holocene.

    Under current population growth scenarios its quite clear humans will have an increasing impact on the planet but this is not necessarily a negative.

  5. Report this comment

    Jarl Ahlbeck said:

    Playing with phantasy numbers is just bad science

  6. Report this comment

    Atte Korhola said:

    The concept of planetary boundaries is interesting and stimulating, but the management of the idea is poorly handled in the paper. First of all, the idea lacks a falsification criterium, which is essential – according to science philosopher Popper – for any scientific theory. The authors claim that climate change has already crossed the critical boundary (350 ppm), and they try to show some evidence for this. However, much more interesting question would be what is the evidence that would falsify this theory? For instance, should the Arctic sea ice develop only uni-directionally after crossing the boundary? Or is its present behaviour, where a slight ‘recovery’ is observable since 2007 in the summer sea ice extent, actually falsifying the overall concept of ecosystem thresholds, tipping points and regime shifts? The biggest problem of the whole climate science at the moment is that it lacks falsification criteria. If reversals, improvements or recoveries happen in any atmospheric or ecosystem characteristics, they are explained only afterwards, posterior, as “fitting to the data or predictions”. It would fine to have a priori stated falsification criteria for any scientific theory. What data or phenomenon would dispute the theory of planetary boundaries?

  7. Report this comment

    Michael Jensen said:

    We’ve been closely following what we call the “converging emergencies” for a few years now, and this marvelous essay (and thank you, Nature, for making it fully free online) brings the science to the fore.

    At ApocaDocs.com, we’re trying to “humor the horror of environmental collapse,” because so few people are willing to listen — and fewer still willing to understand — without a little humor thrown in. It’s also become a resource for background on particular crises — stories going back a couple of years on the Plastic Gyres, on White Nose Syndrome, ocean acidification, and on and on.

    It has been quite depressing to see the non-“global warming” emergencies getting such short shrift in the media; perhaps this majestic work can wake up even politicians to the fact that nature and humanity are all interrelated.

    Spraying sparkly things into the skies to ward off warming won’t fix ocean acidification, or phosphorus/nitrogen cycles, or species collapse… geoengineering is a fool’s game, given our track record so far.

    Anyway, thanks — on to read the editorials!

  8. Report this comment

    Oleksandr Ignatov said:

    It is rather strange that the authors do not mention the most evident and primary cause for all-mentioned environmental changes, i.e. overpopulation. The quantitative threshold for human population on Earth is mistakenly not defined or omitted in this article.

  9. Report this comment

    Ray Culver said:

    This article is and will have profound ripples through scientific and regulatory arenas. The idea of segregation of the science into earth boundaries and the quantification of limits does spark a new opportunity; if quantified boundaries are accepted by all stakeholders then another feedback loop of statistical control needs to be recognized and implemented for this proposed planetary boundary model to become a working model to the benefit of all.

    I agree that setting upper control limits on some of the proposed boundaries is or can be dangerous. However if we understand that boundaries are not static but variable pending further research and measurement methodology then our opportunity is to start narrowing in on key environmental impact metrics which would not only provide upper control limits (UCL) but lower control limits (LCL). Perhaps this research will eventually answer the key question “what is a normal earth biodiversity system”. Today we don’t know since what one thinks is normal the other deems unacceptable.

    Kudos to the authoring team and I look forward to much anticipated debate and infusion of research coming out of this piece.

  10. Report this comment

    Oleksandr Ignatov said:

    I completely agree with Michael Lardelli, omitting and silencing overpopulation issue will cause (and already does) disastrous consequences for Earth’s ecosystems and for our civilization

  11. Report this comment

    Steve Soltesz said:

    Perhaps I am not interpreting the planetary boundary graphic correctly, but the size of the red wedge extending beyond the threshold does not seem to correlate with the degree of overshoot. For example, the threshold for climate change is 350 ppm CO2, so the graphic would indicate the planet is currently at 525 ppm (50% higher than the threshold) if one uses the concentric circles as a guide, or more than 100% beyond the threshold if one compares the areas of the wedges.

  12. Report this comment

    Mikep said:

    I hope this work is better than the Limits to Growth. This was comprehensively refuted on its first publication in the 1970s (when we were supposed to be running out of key materials, causing death by pollution etc by 2000). See in particular papers by William Nordhuas in Brookings Papers on economic Activity, or the excellent book by Christopher Freeman and colleagues from the Science Policy Research Unit at Sussex. this has not stopped the authors from essentially repeating the same flawed approach in several new editions. But there was no reason to believe them in the early 1970s and there is no reason to believe them now.

  13. Report this comment

    Alice Friedemann said:

    Why aren’t scientists shouting from the rooftop that humanity may go extinct if we don’t change our ways? Schlesinger’s “if we cross a threshold … that leads to deep oceanic anoxia, we risk a truly dire situation” is way too subtle.

    Scientists need to explain to the public clearly how and why we could go extinct if we go over certain thresholds, as Peter Ward does so well in “Under a Green Sky” (1).

    I didn’t see any mention of preventing coal from being burned (to replace declining oil and natural gas) as a key part of limiting climate change – are the maximum amounts of coal that can be burned per year known? Clean coal is not going to happen (2).

    Nor will alternative energy ever replace oil and natural gas, the EROEI is too low, there’s no funding, and the scale required is too large (3) – so what exactly is the plan to keep coal underground?

    (1) Peter Ward. “Under a Green Sky. Global Warming, the Mass Extinctions of the Past, and What they can tell us about Our Future”. http://www.amazon.com/Under-Green-Sky-Warming-Extinctions/dp/006113791X

    (2) Liz Jackson. “The Coal Nightmare” ”http://www.abc.net.au/4corners/content/2009/s2678936.htm." rel="nofollow">http://www.abc.net.au/4corners/content/2009/s2678936.htm.

    (3) H Goldstein. “Joules, BTUs, Quads-Let’s Call the Whole Thing Off”

    http://www.spectrum.ieee.org/energy/fossil-fuels/joules-btus-quadslets-call-the-whole-thing-off

  14. Report this comment

    (Dr) Andrew Glikson said:

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

    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

    27-9-09

    andrew.glikson@anu.edu.au

  15. Report this comment

    Jonathan Loh said:

    I largely agree with Christian Samper’s comments. I would add that, for maintaining ecosystem functions (and services), the important thresholds for biodiversity loss should be measured in terms of the local abundance of particular species, not the overall global extinction rate.

  16. Report this comment

    Johan Rockström said:

    Thanks for the important comments posted so far, which will guide the continued research on the challenges of global sustainability.

    I would like to share the reasoning among the authors of the Planetary Boundaries paper regarding one of the fundamental comments so far, namely why we never included population growth as a planetary boundary.

    This was a deliberate choice, justified by the core rationale behind the planetary boundaries concept. What we propose with the concept is that we need to define the Earth system processes that determine the ability of the Planet to remian in a desired “Holocene” state (the conducive state we assume is a pre-condition to suppport a growing population). This definition can thus be done irrespective of our human impacts on the planet, i.e., they – the planetary boundary processes – define the regulatory capacity of the Planet to remain in a stable state, irrespective of how many we are. Once this is done (and we tentatively propose a set of quantifications for 7 of the 9 boundaries we have been able to identify), humanity should be able (is our assumption) to thrive within the safe operating space that the boundary levels provide. To succeed, population growth (and the level of social and economic development associated with it)must then occur within the safe space to be sustainable. Population growth – as ample evidence clearly shows – within the current (largely) unsustainable growth paradigm, generates major negative environmental impacts, and is thus a key driver of change challenging the planetary boundaries – but it is not a boundary in itself. The planetary boundaries concept thus provides us only with a definition of global sustainability based on growing insights of the risks of non-linear change at the Earth system or major sub-system scale. It does not provide a paradigm of how to achieve sustainable development, which includes population growth and human welfare as key ingredients.

  17. Report this comment

    Jesus Rosino said:

    Atte Korhola:

    a slight ‘recovery’ is observable since 2007 in the summer sea ice extent

    There isn’t any recovery since 2007. This year’s minimum extent (5.2) is even below the linear melting trend (5.6) and below the IPCC projections.

    The long-term melting trend is a nosy one and what we are seeing these last two years is exactly what is expected and what we’ve seen before: interannual zigzag variation along the melting trend. There’s nothing unsual in these last two years of data.

  18. Report this comment

    Daniel M. Gilsdorf said:

    I had trouble downloading and reading the file from Stockholm in its full length, with the details on all of the proposed boundaries.

    The boundary that concerns me is ocean acidification, so my questions are for Peter Brewer.

    First, how well do we understand the factors influencing local saturation limits for carbonic acid in ocean water? In addition to the well-understood effects of temperature, the movement of the water (waves, tides, hurricanes) could affect it like the shaking of a carbonated drink. Also, what else is dissolved in the same water will influence the limits of carbonic acid uptake. I suspect that these factors are poorly quantified by reseach to date.

    Second, do we know the limits of tolerance for acidified environs on algae? Oceanic algae are known to be the main photosynthetic source of carbon dioxide uptake on Earth, with marine diatoms in polar waters and dinoflagellates in tropical waters being the largest contributors.

    Third, if human-generated carbon dioxide has contributed most of the current level of carbonic acid in the oceans, that may indicate that a large portion of carbon dioxide generated since the Industrial Revolution (at least – perhaps even earlier) has not been taken up through photosynthesis as previously supposed. If this is taken into account, and the carbonic acid saturation levels for the world ocean are exceeded, how fast will atmospheric levels of carbon dioxide rise?

  19. Report this comment

    Keith Alverson said:

    In setting the stage for the interesting feature “A safe operating space for humanity” (Nature, 461, 472-5; 2009) Rockström et al repeatedly state that the climate of the Holocene has been stable. They further imply, by contrasting this purported past stability with the many large anthropogenic changes impacting the Earth system today that climatic stability has been of key importance to allowing human civilizations to develop and flourish. This paradigm can be criticized in two important ways. First, the Holocene was not climatically stable, and second there are good reasons to believe that it is precisely the dramatic and dynamic history of Holocene climatic variability that ‘saw human civilizations arise develop and thrive’ and ‘enabled human development’.

    Although it is true that global atmospheric temperatures did not change much during the Holocene, we know, from well-dated quantitative paleorecords, that regional hydrological change during this period has been extremely large. The extent of glaciation in the European Alps, for example, has been highly variable during this time, including periods when the glaciers were substantially more restricted than today. Lake levels in much of semiarid Africa have risen and fallen by order 100 meters on similar timescales. Large, and often rapid, regional climatic changes, often involving hydrological balance, have occurred throughout the Holocene, and have had substantial impacts on human societies, including playing a role in the demise of several civilizations (eg. Mayan, Akkadian, Greenland Norse). Furthermore, it seems quite plausible that human societies, developed, adapted and grew strong precisely in response to such variability, not because they were ensconced in a cocoon of stability. Although there is no doubt that we live in both unusual and challenging times, human induced changes to the Earth system are not happening against a backdrop of 10,000 years of Holocene climatic stability, and it is precisely our ability to make societal changes and adapt to variability that is now required in the face of the dire global anthropogenic changes that Rockström et al. highlight.

  20. Report this comment

    Jesus Rosino said:

    Keith Alverson,

    It seems to me that migration is the best sometimes only adaptation strategy when the climate becomes hostile. But this strategy is quite an ordeal when much of the society’s wealth is invested in real state and with the modern political divisions and restrictions on migration.

    Although, IMHO, catastrophes have probably played a role in decay more frequently than in development, it’s certainly true that, when overcomed, difficulties such as hunger, wars or climate changes, and their challenges are a boost to development. However, the society thrives by facing (i.e. avoiding) those hardships, and the sooner the problem is anticipated and faced, the more the chances of success.

  21. Report this comment

    Mark Thompson said:

    I thoroughly enjoyed reading this article. Figure 1 captures attention and highlights the gravity of the situation immediately.

    My primary concern is in reference to the section on biodiversity loss. The article refers to species extinctions and biodiversity loss. It is important to distinguish the two. Species extinction is an important factor, but as Paul Ehlrich et al. have noted: population level extinction is occurring at ~20X the rate of species extinction. The sum is greater than the parts. Population levels of biodiversity are the more sensitive indicator of natural capital. Conservation biologists have been reporting on this for a number of years and are asking other scientists to stop pointing at species loss alone. We need to understand and measure the net loss to biodiversity as a process and across all levels of measurement. Species loss is bean counting, whereas populations are the more important indicator in this context. Measurements based on species extinctions alone grossly underestimate the extent of damage that has been done – if it isn’t already depressing enough.

    Great article otherwise.

    Thanks!!

  22. Report this comment

    Patrik said:

    How can science even speak of such a thing as “sustainable” in this context – if there is no way of quantifying it?

    Are the authors here seriously saying that all this talk about sustainability is non scientific at this stage?

  23. Report this comment

    Patrik said:

    Atte Korhola>> When it comes to the dangers of anthropogenic global warming, I believe a falsification would be:

    To find a historic period, when CO2 levels have risen with at least ~35% during a fairly short period, and no discernable reaction in other forcings have occured, such as increased H2O gas, increased CH4, decreasing ice masses and rapidly rising temperatures (more than 0.4-0.7 C/50-100 years).

    If one or more such instances could be identified in the present geological age, then I think at least AGW would be falsified.

  24. Report this comment

    Atte Korhola said:

    Jesus Rosino:

    “There isn’t any recovery since 2007. This year’s minimum extent (5.2) is even below the linear melting trend (5.6) and below the IPCC projections.

    The long-term melting trend is a nosy one and what we are seeing these last two years is exactly what is expected and what we’ve seen before: interannual zigzag variation along the melting trend. There’s nothing unsual in these last two years of data."

    You miss my point. My question was: what would be an unusual development regarding Arctic sea ice extent that would falsify your theory? Can you state it a priori? Many researchers cannot find any evidence for a critical threshold (or “tipping point”) beyond which the

    ice cover melts away in an irreversible process (see e.g. Eisenman & Wettleaufer, PNAS 106:28-32, 2009). What sort of sea behaviour would be inconsistent with your theory of planetary boundaries? This very same question could be asked also regarding other examples that you present for boundary crossings.

  25. Report this comment

    Atte Korhola said:

    Thanks Patrik. However, I am not questioning AGW at all. I’m only interested as how to develop falsification criteria for certain predicted consequences (e.g. extreme weather events) or in this case for boundary crossings.

  26. Report this comment

    Jesus Rosino said:

    Atte Korhola,

    Thanks for your clarification. I don’t know what a falsification evidence would be like, I just meant to answer your question “is its present behaviour, where a slight ‘recovery’ is observable since 2007 in the summer sea ice extent, actually falsifying the overall concept of ecosystem thresholds, tipping points and regime shifts?”, in the sense of “I don’t think so, because this behaviour falls within the expected behaviour as described by the theory”.

    In my view, the concepts of threshold and tipping points arise from the fact that constant exponential consumption growth cannot hold indefinitely within a finite system such as Earth’s natural resources. There might be disagreement on where those threshold are, but IMHO not on the existence of such limits.

  27. Report this comment

    Paul Ramos said:

    Michael Lardelli posted a crucial subject which should be adreessed by Nature as a subject in the boundaries here discussed. The boundaries discussion should also bring to the scenary the technosphere boundaries, mainly because of the imprudent desregulations through abusive liberations of the new tecnologies like transgenes OGMs, nanotecnology, geoengenhary and so on. These fields of sciences do not answer for the prudence appeal once they are dominated by the greatest and richest companies in the world.

    And “scientists” are there to sell theirs souls giving background to keep the insanity going on at costs of human healthy insecurity, food insecurity, contamination of earth sistems and extintions of species both natural and cultural ones.

  28. Report this comment

    Drew Barton said:

    This is a very important contribution to the goal of connecting basic climate change science with climate change policy. The authors have been very careful to explain that the numbers proposed here are a rough draft. But it’s a start that should lead to focused research in addressing the question: what are the boundaries within which humanity can proceed in a way that is acceptable and tolerable. No thoughtful person could think that the future of humanity is in danger. Doubtless humans will be able to survive under any climate change scenario. The key question is, however, what future do we find acceptable, and what physical boundaries would allow that? This is a challenging set of questions but one that can be addressed by science. This contribution provides a start.

  29. Report this comment

    James Peter (Hamish) Kimmins said:

    As a long-time student of Limits to Growth and human population growth, I enjoyed reading this arlicle which represents to me a first small step to try to integrate our increasing but still rudimentary understanding of the earth as a system. Such endeavors are replete with shortcomings, but one has to start somewhere.

    My major concerns about such useful beginnings are that concepts developed from understanding of processes and characteristics at lower levels of biological organization sometimes get applied uncritically at higher levels of organization and integration. In particular, I am troubled when I read about uncritical applications of concepts from the genetics, individual organism or population levels applied to ecosystems – especially forest ecosystems – that have been the focus of my studies. I believe that an organismal interpretation of ecosystems is not supported by our current understanding of the ecosystems with which I am familiar. Similarly, I am unable to accept an organismal interpretation of the earth, such as in the Gaia theory. While there is little doubt and much empirical evidence that the terrestrial biota plays an important role in hydrology, climate, biogeochemistry and soil development, amongst other things, I cannot accept that this is purposive, but rather the result of genetically-controlled tolerances and adaptations to the physical environment, that becomes altered to a degree by their functional processes.

    A particular concern is the frequently vague treatment of ecosystem “resilience”. Users of this term should define their use of it at the outset, and address the issues of the ecologcal role of natural disturbance at the species, community and ecosystem levels, and then contrast this with the ecological consequences of human effects on both global and local ecosystems. I find that much of the discussion of biodiversity and its relationship to various interpretations of “resilience” lacking in detail – what measures of biodiversity over what spatial scales, and over what time scales? And there is a persistent confusion between biodiversity and ecological diversity. In the metaphor of “ecological theatre”, ecological diversity (climate, topography, soils, hydrology, and physical disturbances such as fie and wind) consitutues the “ecological stage” which determines which of several alternative “ecological plays” can be acted out on that “stage”. The “ecological play” is the sequence of biotic communities that successively occupy and are replace by other communities over time as a result of proceses of ecological succession, new “plays” being initiated by disturbance that causes successional retrogresion (or sometimes accelleration). The “actors” (species)involved in the “play” are one measure of biodiversity.

    As we think about planetary boundaries, we are thinking about altering parameters of the “ecological stage” and the “ecological play”, according to how the genetic diversity within species and communities react to changes in the “stage”.

    Consideration of boundaries at the global level is very necessary, but just as the Limits to Growth analysis was weak (despite which I support the overall conclusion of limits and the problem of linkages between issues)because it lacked adequate bottom-up linkages,the present useful exercise must ensure that our knowlegde of genetic diversity, species and community diversity, the diversity of “ecological stages” (from the ecological theatre metaphor), as well as our current understanding of ecological processes are fed upwards to the global assessment. The current popularity amongst some of simple “bioclimatic” models to predict plant species shifts in resonse to climate change is an example of an unfortunate level of simplification.

    I look forward to reading updates of this necessary and useful analysis.

  30. Report this comment

    Will Steffen and Johan Rockström said:

    We appreciate the several comments that we have received on the climate change boundary. These have helped us to sharpen the line of argument for the boundary as well as reiterate some important features of the planetary boundaries concept.

    1. Regarding the proposed boundary itself (350 ppm atmospheric CO2 concentration and 1 W m-w radiative forcing relative to pre-industrial), we emphasise the uncertainly zone of 350 to 550 ppm, and our use of the precautionary principle to place the proposed boundary at the lower end of the uncertainty zone. The current global discussion of CO2 targets, much of it centred around 450 – 500 ppm CO2, as a target concentration is thus entirely consistent with our planetary boundary approach. The key issue is risk: how much risk is society willing to take with the potential impacts of climate change. Thus, setting a boundary for climate change is a highly normative process, informed, we hope, by the type of arguments we have made in suggesting a climate change boundary.

    Our approach is entirely consistent with the aggregate emissions approach, which has been recently advocated as a policy option for managing emissions trajectories. We should add, however, that the translation of aggregate emissions into an atmospheric CO2 concentration depends also on the behaviour of carbon sinks on the land and in the ocean, and the relationship between the rate of emissions and the behaviour of the sinks.

    2. Several comments have questioned the rationale for our proposed boundary.of 350 ppm CO2 and 1 W m-w radiative forcing relative to pre-industrial, particularly since humanity has already transgressed this boundary. We emphasise that our proposed boundary is based on a synthesis of three lines of evidence:

    (i) Climate sensitivity, which in our view should also include the “slow feedbacks” in the climate system – primarily albedo changes associated with changes in the large polar ice sheets and in the distribution of vegetation – which are not currently included in GCM simulations. Inclusion of these feedbacks roughly doubles the climate sensitivity, from about 3 deg C to about 6 deg C, for a doubling of atmospheric CO2 concentration above pre-industrial. The timescale associated with these slow feedbacks – several centuries to a millennium – is within the ethical timeframe we consider in our planetary boundary analysis.

    (ii) Destabilisation of the large polar ice sheets, which would lead to significant increases in sea-level in addition to contributing to the slow feedbacks described above. At our current atmospheric CO2 concentration of about 387 ppm, we are committed to an equilibrium global temperature rise of about 1.3 deg C above pre-industrial, which, according to palaeo-climate evidence, will lead to a sea-level rise of several metres over the next centuries. Both the Greenland and West Antarctic ice sheets have shown net mass loss at an increasing rate over the past decade.

    (iii) At the current CO2 concentration of about 387 ppm, serious impacts that can reasonably be linked to anthropogenic climate change, are already occurring in many parts of the world. These include the rapid melting of glaciers in the Hindu Kush-Himalaya region with implications for water resources over much of Asia; the 2003 heatwave in central Europe with thousands of excess deaths, increased bleaching of coral reefs around the world; crippling drought and violent bushfires in southeast Australia; and increased inundation events on many low lying islands in the Indian and Pacific oceans.

    3. The fact that our two boundaries – one based on atmospheric CO2 concentration (350 ppm) and the other based on radiative forcing (+1 W m-2 compared to pre-industrial) – are not precisely equivalent has triggered several comments. The slight mismatch in quantitative values is deliberate; otherwise we would have had a single boundary. The radiative forcing boundary, which is the slightly more stringent boundary, is also the more fundamental from a scientific perspective and includes all of the ways in which human activities modify the climate. We included the CO2-based boundary because it is the most important of the anthropogenic greenhouse gases and is central to the current policy debates. Planetary boundaries need to be both relevant to policy and grounded in the best science. Given the complexity and urgency of the climate change debate, we believe that a double-barrelled boundary is the most effective.

    4. The debate about our proposed climate change boundary – and indeed about the appropriateness of the control variables we have selected – is just what we hoped would happen. The planetary boundaries article is basically a proof-of-concept paper that represents the beginning of both a scientific and social discussion, and not the final word, on our relationship with the Earth. We are pleased that the article has added momentum to this important discussion, and we appreciate the challenging but constructive nature of the comments.

    Will Steffen and Johan Rockström

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    Stefan Hellstrand said:

    The ambition of the paper is good. Having a background in agriculture, I have looked closer on the planetary boundaries related to use of phosphorus and nitrogen. The results are disturbing. Keeping in mind the trends of (i) population growth, (ii) increased welfare per capita causing a shift in the food intake towards more animal products, (iii) urbanisation, (iv) globalisation with more and more agricultural products traded longer and longer distances, (v) the increasing demand on bioenergy; and (vi) the limitedness of the area of land available, we have a demand on increased production per ha land over time. Since the Second World War, mineral fertilisers such as nitrogen and phosphorus have been major means increasing yields per ha globally. That was a major element in the so-called Green Revolution.

    Sweden during the 18th and 19th century suffered from starvation. The introduction of crop rotation systems in which the nitrogen fixation by leguminouses was utilised for nitrogen supply contributed to its elimination.

    The food supply perspective related to the biophysical conditions of the planet is missing in the article. Regarding plant nutrients it focuses on the wellbeing of water systems, while allocating a lower priority to the biophysical preconditions for the wellbeing of people, including food supply. Regarding phosphorus, it is the wellbeing of oceans the coming 1 000 years that defines the set boundary, while possible restrictions in agricultural production affecting global food security due to lack of phosphorus are ignored. The given estimate of global available reserves has no reference. It is up to 9.6 times higher than the reserves estimated in U.S. Geological Surveys, and 3.1 times higher than the reserve base estimated by the same source. Estimates suggesting that the phosphorus reserves will be used up within 100 years are quite common within agricultural sciences.

    Regarding nitrogen, the concern is once again the quality of water systems. The planetary boundary suggested implies that the amount of mineral nitrogen fertiliser globally used should be reduced from an application rate of on average 57 kg per ha arable land to 13 kg. That reduction cannot be compensated by increased use of leguminouses. On the contrary, their contribution should be reduced from 29 to 7 kg nitrogen per ha arable land and year. Thus, in total an average supply of 86 kg nitrogen should be reduced to 20 kg per ha and year. This will reduce global agricultural production while demands are increasing.

    My conclusion is that if these planetary boundaries regarding plant nutrients in reality were introduced, then the number of starving people would within one year have increased substantially. In ten years, the number of people had decreased significantly. Lack of energy and food had increased social disorder among people, regions, and nations. Due to the interpretation of the biophysical constrains for humanity expressed in the article, the socioeconomic foundation for a safe operating space had deteriorated.

    To the editor: The article is not peer-reviewed research. What does that imply with regard to the quality of the probing of its quality before publishing? How to value its scientific merits without the peer-review process?

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    Mark Stewart said:

    Iagree with Schlesinger that there are some dangers to setting ‘boundaries’. Natural system scientists understand that these are estimates with considerable uncertainty on either side. I fear that policy makers do not share that understanding. A boundary becomes a line, ok on one side, bad on the other. Also, all of these natural systems contain substantial lag times between system stresses and system responses. By the time you reach a stated boundary condition, it is very likely that even greatly reducing the stress at that time will not prevent going well past the boundary. If the estimate of the boundary includes the lag time, it wiould be less likely to be misunderstood. For example, there is a proposed boundary of 2 degrees of warming. As we are at about 0.8 degrees, it would seem that we have ‘time’ to take action. However the reality is that if we cease carbon emissions now, temperatures will continue to rise for decades, and there would be a significant chance of hitting the 2 degree boundary.

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    Scott Jenkins said:

    Ignoring the elephant in the room-namely that human population growth is at the heart of every issue of a threatened planet along with an unadmittedly weak knowlege base as to how these various systems interact makes this very poor science with an agenda to boot. Shame on you for publishing it.

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    Jesus Rosino said:

    Thanks for your comments, Will Steffen and Johan Rockström. Your answer regarding climate change boundary is fair enough to me (in fact, my comment was preliminar, just at first glance; later on, I read the whole article with more detail and I saw that the risk-aversion argument was explicitly stated in your paper). Within the current scientific range you’ve taken a lower limit. That’s fine. I think that the slow feedback estimation is a bit speculative, but, as you said, that goes into the politics of risk aversion, and I wouldn’t like to distract the attention from the real issue: that we are already moving just around the dangerous boundary.

    I look forward to more work on this line. I find Chameides’s last statement particularly compelling: “at some point the human boundary will squeeze into the planetary boundary — leaving no space for sustainable society. Now there’s a predicament we would do well to avoid.”

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    Itsuki C. Handoh said:

    Very stimulating work! Indeed, we need to have some feasible measures for full Earth-system sustainability, not just anthropospheric dimensions.

    ‘Planetary boundaries’ will be employed by various scientific communities and policy decision -makers.

    By the way, we, MATRIX (Mathematical Arts and Transdisciplinary Research Initiatives for Climate Systems, Ehime University, Japan), have developed a novel Earth-System model for persistent organic pollutants (POPs). The model could well be applicable to any toxic pollutants which include those in the Stockholm Convention on POPs. We would like to contribute to better defining the planetary boundary for Chemical Pollution.

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    Rob Jones said:

    Amidst talk of environmental/ecological/social/economic chaos related to climate change, it is shameful and puzzling to not see emphasis on how overpopulation cancels all efforts at reducing the spiraling effects of global climate change. As noted by Alan Weisman (The World Without Us, 2007), Bill McKibben (Maybe One: A case for smaller families, 1999; The End of Nature, 2006), Storms of My Grandchildren, 2009 by James Hansen, and other publications, all efforts to reduce our carbon footprints (i.e., stomping the life from our only Earth) will have no real lasting effect unless population growth is reversed.

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