Sustainability science is a young discipline that started emerging in the late 20th century, although Hans Carl von Carlowitz had already introduced ideas about sustainable management of forests in the early 18 th century. In recent times, the Club of Rome report in 1972 and the Brundtland report in 1987 developed these concepts further, and subsequently the sustainability idea became prominent in political debates as well. In both reports it was recognized that growth would have certain limits and a different style of resource utilization was therefore necessary. However, despite numerous approaches dealing with sustainability, it is still an important issue.

Nowadays humanity increasingly interferes with natural systems on a planetary scale. This holds for many subsystems of the Earth including the climate, soil and water bodies, and marine systems. During the 20th century, rapid technological development and demographic pressure advanced to a degree that we caused radical and unintended changes in the Earth's integrity. This is observable in certain subsystems, for example in the atmosphere (global warming), in marine systems (overexploitation of fish stocks), or in soils (degradation). One crucial element of sustainability is the capacity of natural resources to sustain human demands. It is foreseeable that parts of the system are overburdened beyond their capacity. This holds likewise for waste disposal, as for the atmosphere (greenhouse gases) and the utilization of resources like ores and renewables like trees and fish. To sum up, one can state that the overexploitation of natural resources and economic growth causes environmental impacts which may lead several systems to the brink of collapse. In other words, humanity causes a multitude of problems and most of them are not grounded in one sector, region, either can they be described by one scientific discipline.

Thus, sustainability science is a discipline that can be placed as the one at the meeting point of different scientific disciplines. However, during the last four decades, science made remarkable progress in regard to an assessment on how climate and global change will affect livelihood conditions, and how humanity is accelerating the above mentioned changes. The question is how we can avoid certain human activities that destroy the functionality of certain subsystems of the Earth and how we can develop potential solutions. It is a major challenge to understand the dynamics of man-made environment systems as a basis for the development of sustainable transition pathways in the sense of planetary engineering and management. In other words, sustainability science addresses the man-made environment interface.

Although all these points have bleen well-known for decades, we need to ask why it is so difficult to achieve pathbreaking scientific results, which may help us to develop clear visions of real sustainable development. It is well-known that resource consumption is an accompanying factor of economic prosperity and global resource consumption is still steeply growing. In some countries we observe-mainly the advanced ones-that resource consumption stabilizes or even decreases, while their high material intensity is still managed by exporting it to developing countries. Thus, the challenge to decouple resource consumption from economic development remains, and it is not only a question of a green economy, technological progress, or how natural resources are being utilized. It is indeed also a societal challenge. Human lifestyle changes might be a further catalyst for making headway towards sustainability. Nevertheless, current progress into this direction is slow, moreover, in large parts in the developing countries, we can see a tendency just to copy westernized lifestyles. A real innovation for the world would be a strategic approach for a sustainable economy that results in social equity and fairness, risk resilient livelihood conditions, sustainable resource use, and the avoidance of ecological scarcities-all these under consideration of planetary boundaries.

Nevertheless, sustainability is still an elusive concept. It is hard to define what sustainability really implies in terms of environmental constraints or societal development, in particular on a regional scale. Consequently, at the beginning of the 21st century, scientific bodies called for a more systematic sustainability science, e.g. International Council for Science defined sustainability as a major goal in its research strategies. Despite these efforts, concepts still lack real meaning. Thus, the aim should be to underpin activities dealing with the general aspects of sustainability with stronger and sounder scientific concepts. Questions, like: what exactly is sustainability? How can we achieve sustainability targets? And, what does 'being sustainable' mean? need to be in the foreground. Thus, sustainability science is environmental systems science.

Although all these points have been intensely discussed in recent decades, a thrilling and demanding journey still lies ahead for sustainability science. In regard to methodological terms, we need to encompass the different magnitudes of scales in terms of time, space and functions. Thus, sustainability science still invokes a lot of questions, i.e. we have to tackle, in particular, the following three challenges: 1) The provision of a methodological arsenal that allows the description and analysis of questions of sustainability in a comparable and transferable manner, i.e. we permanently have to ask ourselves what we can learn from singular cases in terms of the overarching sustainability challenge; 2) Options for solutions at different levels, e.g. regional and global, need to be assessed systematically in order to develop pathways which allow us to achieve predefined environmental targets, like the 2℃ target agreed in the Copenhagen Accord 2009; 3) As a lot of strategies are included under the term 'sustainability', there is a need to develop a concept which allows assessment and measurement of success of implemented sustainability measures.

However, sustainability itself is a challenge, because it needs ethical decisions from humankind itself whether we want to live in a safe environment or not. But how we achieve these safe limits is an issue of sustainability science, i.e. in terms of how to achieve these limits and what potential trade-offs there might be. The new journal Challenges in Sustainability provides a perfect platform for these goals.

Measuring sustainable development based on analytical models of growth and development and modern methods of growth accounting is an economic approach—often called the capital approach – to establishing sustainable development indicators (SDIs). Ecological approaches may be combined with the capital approach, but there are also other approaches to establishing sustainable development indicators—for example the so-called integrated approach. A recent survey of the various approaches is provided in UNECE, OECD and Eurostat [1]. This review note is not intended to be another survey of the various approaches. Rather the objective of this paper is twofold: to present an update on an economic approach to measuring sustainable development—the capital approach—and how this approach may be combined with the ecological approach; to show how this approach is actually used as a basis for longer-term policies to enhance sustainable development in Norway—a country that relies heavily on non-renewable natural resources. We give a brief review of recent literature and set out a model of development based on produced, human, natural and social capital, and the level of technology. Natural capital is divided into two parts—natural capital produced and sold in markets (oil and gas)—and non-market natural capital such as clean air and biodiversity. Weak sustainable development is defined as non-declining welfare per capita if the total stock of a nation's capital is maintained. Strong sustainable development is if none of the capital stocks, notably non-market natural capital, is reduced below critical or irreversible levels. Within such a framework, and based on Norwegian experience and statistical work, monetary indexes of national wealth and its individual components including real capital, human capital and market natural capital are presented. Limits to this framework and to these calculations are then discussed, and we argue that such monetary indexes should be sustainable development indicators (SDIs) of non-market natural capital, and physical SDIs, health capital and social capital. Thus we agree with the Stiglitz-Sen-Fitoussi Commission [2] that monetary indexes of capital should be combined with physical SDIs of capital that have no market prices. We then illustrate the policy relevance of this framework, and how it is actually being used in long term policy making in Norway—a country that relies heavily on non-renewable resources like oil and gas. A key sustainability rule for Norwegian policies is to maintain the total future capital stocks per capita in real terms as the country draws down its stocks of non-renewable natural capital —applying a fiscal guideline akin to the Hartwick rule.

I am honored to contribute an editorial for the inaugural issue of Challenges in Sustainability (CiS). It has provided the opportunity for me to take a step back and reflect on both the developmental progress in the field of sustainability science since its formal launch, now over twelve years ago [1], [2], and where the field might head in coming years. While it may always feel that the field is changing too slowly to keep up with the challenges it addresses, the developments have been noteworthy, especially in academia. I will discuss three areas: education, research and institutional development.

The growing offering of sustainability (science) educational programs at all levels has been an important part of the field’s evolution. Individual areas of concentration can include business and management, leadership, engineering, or policy management, to name a few. Flagship programs are now found throughout the world, including Arizona State University, Leuphana University of Lüneburg, and the University of Tokyo. In addition, programs at smaller academic institutions such as Furman and Kean Universities in the U.S. have arisen to meet the increasing demand for sustainability education. In Sweden, where I am based, there are international master’s programs in sustainability at Uppsala, Stockholm, Malmö, and Lund Universities, as well as Blekinge Institute of Technology. These programs and their different foci, seek not only to increase student knowledge to understand the complexities of sustainability challenges, but also aim to strengthen key competency development [3] in areas such as facilitation and strategic leadership.

In addition to sustainability education, the nature of research projects and programs in the field has also changed. The changes have been driven by both top-down funding priorities to finance research that is more relevant to society, and bottom-up desire from scholars to carry out more integrated work. This has led to the slow evolution from a focus on descriptive analytical research, with emphases on understanding the effects of environmental change, to transitional (or transformational) research agendas that embrace working in closer collaboration with societal stake-holders. Such research may concentrate on, for example, envisioning and scenario exercises, or problem-solving strategies beyond change strict policy change [4], [5]. Transitional sustainability science research is being carried out by individuals in innovative Ph.D. projects focused on single case studies using particular theories and approaches, and by networks of researchers in longer-term programs, such as the Earth System Governance project (www.earthsystem governance.org), united by common sustainable development themes.

To operationalize the education and research agendas in sustainability science, new organizational constellations have developed. Changes have ranged from the creation of new faculty structures at a number of universities, to the establishment of interdisciplinary research schools and programs. The Lund University Centre of Excellence for Integration of Social and Natural Dimensions of Sustainability (www.lucid.lu.se) is just one example of a longer-term program that unites senior and junior staff and Ph.D. candidates from disciplinary backgroundsincluding Economics and Economic History, Philosophy, Physical Geography, Human Geography, Political Science, and Human Ecology. The frequent interactions via discussions, debates, and joint publications have the goal of, amongst others, fostering new professionals who are capable of and accept working with the theoretical and empirical multiplicities [6] often inherent in sustainability education and research.

Despite the advancements over the past decade, there is still much to be done. Continued creativity in restructuring academic disciplines, departments, and funding and tenure incentives are necessary to pro- mote the interaction needed to achieve the interdisciplinary goals of sustainability science. Sustainability issues must also be strengthened in other areas such as the arts and humanities utilizing alternative forms of knowledge dissemination. In the area of education, additional sustainability programs are still needed,but more importantly, there must also be increased efforts in mainstreaming sustainability into all educational programs at different levels. Finally, the field must also continue to place strong emphases on reaching outside of academia in addressing pressing societal challenges.

The launch of Challenges in Sustainability represents an important step in further strengthening the field. The journal’s broad aims that focus on systemic analyses of sustainability challenges, solutions and transition processes, and associated trade-offs within socio-ecological systems, will create an important publishing outlet for scholars involved in integrative research. Furthermore, because Challenges in Sustainability is open access, it will mean that the knowledge produced in it can reach a wider range of stake-holders, adding one more attribute in a sustainability science we want to create.

A lot can be learned from the numerous pitfalls of sustainable development implementation: they outline how collective representation, short term interests and balance of power can undermine sustainability. For instance, the usefulness of global institutions in dealing with sustainable development is questionable as most are skewed toward the interests and perceptions of developed countries. The notion of sustainable development itself induces a profound cleavage between academic authors and the actors of its implementation, some of whom confuse it with sustainable growth (which favors spatial equity), whilst the others with environment management (which favors intergenerational equity). This polarization is a real problem, since originally, "Our Common Future" report promotes an inclusive approach, able to cope with both equities simultaneously. Finally, if there are obligations toward future generations, there are also obligations toward the current generation. The key issue for effective sustainability policies should be making them acceptable to everyone by including the expectations of local societies and communities. As a matter of consequence, universal solutions do not exist. They would not meet the specificities of local circumstances. The traditional prescriptive sustainable development model should give way to flexible plural sustainabilities. Singular, top-down, global-to-local approaches to sustainable development should be substituted for multiple sustainabilities.

The dominant paradigm of sustainable development (SD) where the environment is just the third pillar of SD has proven inadequate to keep humanity within the safe operational space determined by biophysical planetary boundaries. This implies the need for a revised definition compatible with a nested model of sustainable development, where humanity forms part of the overall social-ecological system, and that would allow more effective sustainable development goals and indicators. In this paper an alternative definition is proposed based on the thermodynamics of open systems applied to ecosystems and human systems. Both sub-systems of the global social-ecological system show in common an increased capability of buffering against disturbances as a consequence of an internal increase of order. Sustainable development is considered an optimization exercise at different scales in time and space based on monitoring the change in the exergy content and exergy dissipation of these two sub-systems of the social-ecological system. In common language it is the increase of human prosperity and well-being without loss of the structure and functioning of the ecosystem. This definition is functional as it allows the straightforward selection of quantitative indicators, discerning sustainable development from unsustainable development, unsustainable stagnation and sustainable retreat. The paper shows that the new definition is compatible with state of the art thinking on ecosystem services, the existence of regime shifts and societal transitions, and resilience. One of the largest challenges in applying the definition is our insufficient understanding of the change in ecosystem structure and function as an endpoint indicator of human action, and its effect on human prosperity and well-being. This implies the continued need to use midpoint indicators of human impact and related thresholds defining the safe operating space of the present generation with respect to future generations. The proposed definition can be considered a valuable complement to the recently emerged nested system discourse of sustainable development, by offering a more quantitative tool to monitor and guide the transition of human society towards a harmonious relationship with the rest of the biosphere.