In recent decades, German scientists have sought to establish a comprehensive perspective on “sustainability”. Basic theories rooted in philosophy and ethics were linked with economic theories and knowledge from the natural sciences.
A notable example is the multilevel model developed in the 1990s. It was devised by its authors as a multi-stage process consisting of discrete mental building blocks, referred to as levels. Its aim is to derive concrete actions and measures from sustainability theory and to create a bridge between sustainability theory and real environmental policy.

• On the uppermost level, the ethical principles of the sustain­ability idea are reflected. Here it is also clarified how far people bear a responsibility towards subsequent generations and how through their behaviour, they influence the life-support base of their descendants. This discourse concludes with the demand that people living today are obliged to preserve a legacy which enables future generations to meet their own needs.
• On the second, strategic level there is discussion of what makes up such a legacy, i.e. which assets, resources and forms of capital should be preserved on what scale. At this point the authors speak out in favour of a strong sustainability model because natural capital cannot be substituted indiscriminately.
• On the third level, a framework of rules for sustainability is drafted. Top of the agenda here is the Constant Natural Capital Rule (CNCR), which imposes the obligation to conserve natural capital over time. Essentially only as much natural capital should be consumed as nature can replenish. Examples are the use of renewable energies instead of fossil fuels or the prudent management of fish stocks. For regions which were subject to large-scale destruction and consumption of natural capital in the past, an investment rule applies, its purpose being to correct as far as possible the overexploitation and mistakes of the past. The recultivation and restoration of previously degraded natural areas belong under this heading. Other management rules specify exactly whether and how much natural capital may still be used in future.
• The fourth level defines three normative guidelines for sus­tainable action. These guidelines are efficiency, sufficiency and resilience. Efficiency relates to the economy. It requires modern, more efficient technologies to be developed; for ex­ample, engines with higher energy-conversion efficiency. Sufficiency is addressed to a sustainable lifestyle. On the one hand it demands that all people worldwide should be enabled to meet their basic human needs. It sets the industrialized countries the target of striving for a lifestyle with the least possible consumption of raw materials and energy. According to this guideline, the industrialized countries are called upon to develop post-materialistic prosperity models. This is not in any way about forcing people into an ascetic way of life. Rather, it revolves around the rejection of individual utility maximization, or creating islands of deceleration and blurring the rigid boundaries between work and leisure. Resilience relates to the conservation of natural capital itself, but also to maintaining the various functions that such capital has, such as recreation. Generally resilience refers to the capacity of a habitat to withstand disturbances. Previously damaged habitats are often less resilient. One aim is therefore to protect habitats accordingly.
• On the fifth level policy-making and action areas are defined in which sustainability is to be achieved. These include areas like nature conservation, agriculture and forestry, fisheries and climate change. Such a breakdown into different areas is important in order to be able to plan and implement measures as specifically as possible.
• On the sixth level, goals are derived in the most concrete possible terms. For example, it has been resolved to reduce the discharge of nutrients into the Baltic Sea by 50 per cent in the next few years. But it is not always possible to specify a precise target value, as it can be unclear at what value sustainability is reached. For example it is not necessarily possible to determine how high the share of dead wood should be in a sus­tainably managed, near-natural forest. In such cases, a kind of target zone, a broader corridor of targets, can be defined. As a matter of principle, diverse stakeholder groups should be involved in setting target values.
• On the final level, instruments are developed to support the achievement of concrete sustainability goals, along with monitoring systems to help verify whether these have actually been attained.