The twenty-first century challenge is different from any other that humanity has faced. The planetary nature of the challenge is unique, and demands a global-scale solution that transcends national boundaries and cultural divides (Svedin 1998
). The collision of the human enterprise with the rest of nature has occurred many times in the past at sub-global scales, leading to a new paradigm of integrated social-ecological systems (Folke et al. 2011
). At the global scale, this paradigm challenges humanity to become active stewards of our own life support system (Kates et al. 2001
; Young and Steffen 2009
; Chapin et al. 2010
). We are the first generation with the knowledge of how our activities influence the Earth System, and thus the first generation with the power and the responsibility to change our relationship with the planet.
The challenge for humanity is shown by a comparison of the Human Development Index (HDI), a measure of well-being, and the Ecological Footprint (Global Footprint Network 2011
), an indicator of the human imprint on the global environment (Fig. ). The world population-weighted average HDI rose to 0.68 in 2010 from 0.57 in 1990, continuing the upward trend from 1970, when it stood at 0.48 (UNDP 2010
). The global ecological footprint has also been rising, leading to an overshoot of Earth’s annual biocapacity in the mid-1970s, corresponding to 1.5 planets in 2007. The increase is mainly due to higher demand for CO2
absorption generated primarily by fossil fuel energy usage (WWF 2010
The bottom right shaded area of Fig. represents the “sustainability quadrant”, in which the HDI reaches an acceptably high value but the ecological footprint remains with the limits of one planet Earth (Global Footprint Network 2011
). Currently, no country achieves these two levels simultaneously. However, a promising development, shown by the downwards-sloping trajectories of some countries, is that they have improved wellbeing while reducing both natural resource demand and pollution. At the aggregated global scale, however, the trends are clear. Population growth in combination with more intense resource use and growing pollution still sets the world as a whole on a pathway toward a growing total footprint (Global Footprint Network 2011
In summary, human well-being has reached high levels in many countries while our planetary life support system is simultaneously being eroded. An analysis of this ‘environmentalist’s paradox’, based on the assessment that 15 of 24 types of ecosystem services are in decline globally (MA 2005
), concluded that provisioning services are currently more important than supporting and regulating services for human well-being, as measured by the rise in HDI over the 1970–2005 period (Raudsepp-Hearne et al. 2010
). Thus, the benefits associated with food production (a provisioning service) currently outweigh the costs of declines in other services at the global scale.
Several questions have been raised about this conclusion. First, the HDI is too narrow, failing to incorporate cultural or psychological dimensions or security considerations and ignoring involuntary adaptation as a result of environmental deterioration and opportunity costs. Second, global aggregates mask the ways in which the distribution of wealth and the impacts of ecosystem service decline are skewed, between nations and within them, a factor that may have a strong bearing on well-being (Wilkinson and Pickett 2009
). Finally, an alternative explanation is the existence of time lags between the decline in ecosystem services and their effect on human well-being, particularly time lags associated with geophysical processes, such as loss of ice in the large polar ice sheets and changes in ocean circulation that operate on timescales of decades, centuries and millennia (Fig. —photo: melting ice).
The planetary boundary for climate change is designed to avoid significant loss of ice from the large polar ice sheets. Melting Greenland ice sheet (photo: Bent Christensen, Azote)
Additional insights into the twenty-first century challenge for humanity can be obtained from analyzing the interaction of human societies with their environment in the past, which highlight three types of societal responses to environmental pressures: collapse, migration, and creative invention through discovery (Costanza et al. 2007
). Collapse, which refers to the uncontrolled decline of a society or civilization via a drop in population and reductions in production and consumption, leads to a sharp decline in human well-being. Many historical (pre-Anthropocene) societal collapses have occurred, and the causal mechanisms are generally complex and difficult to untangle, usually involving environmental, social, and political interactions (Costanza et al. 2007
, and references therein).
Some hypotheses regarding the causes of collapses in the past are particularly relevant to the Anthropocene. For example, increasing societal complexity in response to problems is an adaptive strategy at first, but as complexity increases, resilience is eroded and societies become more, rather than less, vulnerable to external shocks (Tainter 1998
). Another hypothesis (Diamond 2005
) proposes that societies collapse if core values become dysfunctional as the external world changes and they are unable to recognize emerging problems. Such societies are locked into obsolete values hindering, for example, the transition to new values supporting a reconnection to the biosphere (Folke et al. 2011
). A core value of post-World War II contemporary society is ever-increasing material wealth generated by a growth-oriented economy based on neo-liberal economic principles and assumptions (McNeill 2000
; Hibbard et al. 2006
), a value that has driven the Great Acceleration but that climate change and other global changes are calling into question.
How likely are environmental pressures to trigger collapse in the contemporary world? With a more interconnected world through trade, transportation and communication and with economic structures less reliant on local agricultural production, vulnerability profiles of societies have been fundamentally altered. The nature of human-environment interactions has also changed along several dimensions—scale, speed, and complexity—which contribute to the new forms of vulnerability.
The increasingly global scale
of environmental degradation in the Anthropocene has led to some (partial) solutions (e.g., international treaties, international market mechanisms), but the distribution of environmental and economic impacts are highly uneven. Whether the local impact is sufficient to cause local collapse, and whether local collapse can propagate rapidly throughout the globalized human enterprise, as in the global financial crisis, are important questions. On the other hand, a well-connected human enterprise could lead to increased knowledge and techniques for local adaptation, averting or containing local collapse before it can spread. Another qualitatively new problem is the “democratic deficit” associated with international institutions, which are comprised of a collection of sovereign nation-states (Mason 2005
; Bäckstrand et al. 2010
). Human impacts on Earth System functioning cannot be resolved within individual jurisdictions alone; supra-national cooperation is required.
Understanding the speed
of environmental change is also important for distinguishing vulnerability to slow-onset versus quick-onset events. Many historical cases of collapse involved slow-onset or gradual change, where the rate of change was proportional to the pressure of the causal agent. Contemporary societies have a broad set of options to deal with such changes. While vulnerability to quick-onset events such as natural disasters has decreased in many respects (Parry et al. 2007
), the frequency and intensity of extreme events are expected to increase with climate change (IPCC 2007
; UNISDR 2009
). Concatenation of both slow- and quick-onset events, coupled with the increasing connectivity of the human enterprise, can lead to some unexpected global crises (Folke et al. 2011
), such as the spikes in food prices (Fig. ). The Earth System scale adds another twist to the concept of speed of change, as for the very large geophysical changes that have exceptionally long lag times but may then occur suddenly with potential devastating effects, as in the very abrupt warmings associated with the Pleistocene D/O events. Humanity, now largely in its post-agrarian phase of development, has no experience of dealing with such combinations of scale and speed of environmental change.
Finally, in addressing increasing complexity
, Walker et al. (2009
) argue that it is no longer useful to concentrate on environmental challenges and variables individually, but the challenge lies in the intertwining of multi-scale challenges across sectors (e.g., environment, demographics, pandemics, political unrest). An historical case occurred in fourteenth century Europe, when the Medieval Warm Period ended and was followed by colder and wetter growing seasons, a locust invasion, a millennial-scale flood and a pandemic (the Black Death) (Costanza et al. 2007). An oft-cited contemporary example is the food price crisis (Biggs et al. 2011
). Climate change itself is an example of such a complex challenge. Multiple crises may coincide or trigger each other, and there is a need to move beyond narrow sectoral approaches toward more coherent and effective institutions that can deal with complex systems perspectives (UNISDR 2009
; Walker et al. 2009
The scale, speed and complexity of twenty-first century challenges suggest that responses based on marginal changes to the current trajectory of the human enterprise—“fiddling at the edges”—risk the collapse of large segments of the human population or of globalised contemporary society as whole. More transformational approaches may be required. Geo-engineering and reducing the human pressure on the Earth System at its source represent the end points of the spectrum in terms of philosophies, ethics, and strategies.
Geo-engineering—the deliberate manipulation or “engineering” of an Earth System process—is sometimes argued to be an appropriate response to challenges posed by the Anthropocene, most often as a response to climate change. Manipulation of two different types of Earth System process are most often proposed: (i) those processes ultimately controlling the amount of heat entering the Earth’s lower atmosphere (solar radiation management, SRM), and (ii) those affecting the amount of heat energy retained near the Earth’s surface, that is, control of greenhouse gas concentration through manipulation of the global carbon cycle.
Both SRM and manipulation of the carbon cycle constitute a form of “symptom treatment” rather than removal or reduction of the anthropogenic pressures leading to climate change. In particular, SRM targets only the temperature change by decreasing the heat input to the lower atmosphere through, for example, production of sulfate aerosols in the stratosphere (Crutzen 2006
). This approach has no direct impact on atmospheric greenhouse gas concentrations, and other processes influenced by elevated concentrations of greenhouse gases, for example, ocean acidification (Royal Society 2005
), would continue unchecked even if SRM managed to slow global temperature increases.
Approaches that manipulate the carbon cycle, such as carbon capture and storage, could slow the rate of increase of atmospheric greenhouse gas concentrations, or perhaps ultimately reduce the atmospheric concentration of CO2. However, there are no proven mechanisms yet developed that would return the carbon removed from the atmosphere to a form as inert as the fossil fuels from which it was derived. Thus, although removed from the atmosphere, the carbon captured is stored biologically, in underground caverns or in the deep sea if the carbon capture is via chemical or mechanical means. Carbon stored in biological compartments is particularly vulnerable to return to the atmosphere with further climate change or with changes in human management.
In addition to CO2
, there are several other man-made greenhouse gases, which have contributed as much 45% to the total man-made greenhouse effect. The life times of several of these gases (methane, ozone, HFCs) are short (<15 years) compared with the century to millennium time scales of CO2
and hence actions to reduce their concentrations, possible with existing technologies, will lead to quick reduction in the total warming effect (Ramanathan and Xu 2010
Nevertheless, it may become necessary to supplement efforts to reduce human emissions of greenhouse gases with geo-engineering to prevent severe anthropogenic climate change. If this strategy is required, then SRM mechanisms would probably be the more effective as the Earth System would respond more quickly to these than to manipulation of the carbon cycle (Richardson et al. 2011
). However, in contrast to emissions reduction, the problem with geo-engineering is “not how to get countries to do it, (but) the fundamental question of who should decide whether and how geo-engineering should be attempted – a problem of governance” (Barrett 2008
). Many potential forms of geo-engineering would be relatively inexpensive, could be carried out unilaterally and could potentially alter climate and living conditions in neighboring countries. Thus, the potential geopolitical consequences of geo-engineering are enormous, and urgently require guiding principles for their application.
At the other end of the spectrum lie a number of alternative strategies to reduce or modify the human influence on the functioning of the Earth System at its source. The Planetary Boundaries (PB) approach (Rockström et al. 2009a
) is a recent example that attempts to define a “safe operating space” for humanity by analyzing the intrinsic dynamics of the Earth System and identifying points or levels relating to critical global-scale processes beyond which humanity should not go. The fundamental principle underlying the PB approach is that a Holocene-like state (Fig. , panel c; Petit et al. 1999
) of the Earth System is the only one that we can be sure provides an accommodating environment for the development of humanity.
Nine planetary boundaries have been proposed (Table ) which, if respected, would likely ensure that the Earth System remains in a Holocene-like state. Preliminary analyses (Rockström et al. 2009a
) estimated quantitative boundaries for seven of the Earth System processes or elements—climate change, stratospheric ozone, ocean acidification, the nitrogen and phosphorus cycles, biodiversity loss, land-use change and freshwater use. For some of these it is a first attempt at quantifying boundaries of any kind, that is, quantifying the supply of some of the regulating and supporting Earth System services. There is insufficient knowledge to suggest quantitative boundaries for two of the processes—aerosol loading and chemical pollution. Rockström and colleagues estimate that three of the boundaries—those for climate change, the nitrogen cycle and biodiversity loss—have already been transgressed while we are approaching several others (Fig. ).
Fig. 9 The inner green shading represents the proposed safe operating space for nine planetary systems. The red wedges represent an estimate of the current position for each variable. The boundaries in three systems (rate of biodiversity loss, climate change (more ...)
Even if a scientific consensus around boundary definitions could be achieved, much more is required to achieve successful and effective global governance and stewardship (Richardson et al. 2011
). Focusing on climate change, the outcomes of the COP15 meeting in Copenhagen in 2009 showed that (i) climate change has now been raised to an issue of high political priority internationally, and (ii) the road to achieving a legally binding international climate agreement, based on burden- or cost-sharing in the context of a global commons, is a long and complex one, with further steps beyond COP15 required to deliver such an agreement (Falkner et al. 2010
; Richardson et al. 2011
Recently, however, Ostrom (2010
) has suggested that the traditional approach of collective action to climate change based on one international treaty may be misconceived. Addressing climate change through emission reductions can, for example also bring benefits at local and regional scales, such as improved air quality in metropolitan areas. This is particularly so for the emission of the short-term climate warming gases (ozone, methane, HFCs). This approach suggests that global governance and planetary stewardship could also be built in a multi-level, cumulative way by identifying where, when and for whom there are—or could be as a result of policy—incentives to act, independently of the international level (Liljenström and Svedin 2005
). The resulting governance system would be ‘polycentric’, also allowing for more experimentation and learning.
Discussions on climate change, global change and global sustainability implicitly assume that the current global environmental changes are perturbations of the stable Holocene state of the Earth System. The assumption is that effective governance will turn the trajectory of the human enterprise toward long-term sustainability and the Earth System back toward a Holocene-like state. However, the concept of the Anthropocene, coupled with complex systems thinking, questions that assumption. The Anthropocene is a dynamic state of the Earth System, characterized by global environmental changes already significant enough to distinguish it from the Holocene, but with a momentum that continues to move it away from the Holocene at a geologically rapid rate.