One or the Other:Sustainable or Unsustainable   The amount of throughput in the global economy is either sustainable or unsustainable. All economic throughputs degrade ecosystems to some extent; the key issue is whether the rate or total amount of throughput is greater than the natural regeneration of affected ecosystem functions. If the impact of economic throughput degrades ecosystems faster than they can regenerate, there is an inevitable loss of ecosystem functions; this is unsustainable.  If the impact allows the ecosystems to totally replace the resource or sink functions it interferes with, then there is no loss of function and they can endure; this is a sustainable scale.  Economic throughput either degrades ecosystems faster or slower than they can regenerate; it draws down either more or less natural capital than ecosystems reproduce.  This is a basic feature of ecological sustainability.  Whether or not economic activities are sustainable (in terms of the ecosystems that contain and sustain it) has only become a relevant policy issue as evidence accumulates that current levels of economic throughput have crossed the sustainability threshold, and are continuing to increase (see Brief Overview, and Areas of Concern).

Levels of Sustainable Throughput   There are many levels of sustainable throughput. Available evidence indicates that up until at least the Industrial Revolution the level of throughput in the global economy was sustainable.  Certainly the level of hunter-gathers’ throughput was sustainable, and considerably lower than the level of throughput in the 16^th century, which was also sustainable. No global ecosystems were challenged by the level of economic throughput until the 19^th or 20^th century. Some levels of throughput provide very little in terms of material goods and services, and some provide considerably more.  Two conceptual levels of sustainable throughput are of particular interest:

• Maximum Sustainable Scale is the highest level of material throughput that remains sustainable; that is, where the rate of throughput is theoretically identical to the rate of regeneration.  Any increase in throughput becomes unsustainable. Maximum sustainable scale is determinded by the biophysical limits of the ecosystems affected by the throughput in question. This conceptual notion of maximum sustainable scale is difficult to quantify precisely, but is nonetheless an important conceptual tool in understanding the dynamics of sustainable scale.
• Optimal Scale is a level of material throughput within the sustainable range ( i.e. where throughput is less than regeneration), which provides the most benefits relative to costs, where the notions of benefits and costs includes ethical and social as well as economic values. Optimal scale is therefore determined by socio-political limits, in addition to biophysical limits of ecosystems.

Levels of Unsustainable Scale   There are also many levels of unsustainable scale, ranging from a level where throughput is only slightly higher than regeneration, to a level where throughput exhausts or destroys the ecosystem’s capacity to regenerate, where resilience is exhausted.  Obviously, these different levels of unsustainable scale represent different levels of threats to the well being of humans and other species. Unsustainable levels of throughput that are only slightly higher than maximum sustainable scale (see above) are less dangerous than levels of throughput which greatly exceed the rate of regeneration. If the level of throughput is so much higher than the rate of regeneration that it overwhelms the ecosystem’s capacity to continue functioning, the basic ecosystem dynamic is changed and a new equilibrium established, then Maximum Scale is exceeded. The concept of maximum scale represents the point of no return, the level of throughput where the ecosystem functions upon which we depend, are no longer available.  Maximum scale is defined by the biophysical limits of affected ecosystems. Such a level of throughput may be difficult to identify empirically with precision; the concept nonetheless serves as a useful reminder of the inevitable consequences of too much throughput relative to what ecosystems can process.

Policy Implications of Scale Dynamics   Many policy implications flow from these dynamics:

• Its is more desirable for economically driven levels of throughput to be sustainable than unsustainable (as sustainable levels maintain ecosystem functions that economic and other vital life support activities depend on)
• If levels of economically driven throughput are unsustainable, efforts should be made to reduce throughput levels, and return to sustainable levels as soon as possible (as the longer we operate at unsustainable levels the more we degrade ecosystems and their ability to regenerate, and the more difficult and costly will be the process of restoring sustainable scale)
• Maximum scale is to be avoided.  Exceeding maximum scale represents an irrevocable loss of critical ecosystem functions that threaten human well being and survival.
• Optimal scale is the policy priority. If a sustainable level of throughput is close to maximum sustainable scale there is always the possibility that some unforeseen natural or anthropogenic events could push the level of throughput to an unsustainable level.  It is desirable to manage our level of economic throughput so that such an inadvertent transition to unsustainable scale does not occur.  This involves assessing risks, as well as other ethical issues, for current and future generations of humans and other species. Also to be considered is the level of material goods and services desirable for human well being.  Broad participation is needed to determine the socio-political priorities of optimal scale.

Optimal Scale: the Policy Priority    The concept of optimal scale incorporates ecological sustainability as well as social and ethical concerns.  It identifies the level of economically driven material throughput that is most desirable from these multiple perspectives, each so fundamental to human well being – the level of throughput which provides the greatest economic, environmental and social benefits.  One of the social and ethical considerations involves the responsibilities of the present generation to future generations of humans and other species. One aspect of such considerations is the safety margin that is acceptable in terms of inadvertently exceeding sustainable scale, and thereby slipping into unsustainable practices. 

Creating a Safety Margin: Accepting Limits    Unsustainable levels of throughput are dangerous because of the degradations they impose on critical life support systems; they are to be avoided.  But the biophysical limits which determine ecological sustainability are complex, dynamic, and non-linear, making it difficult to describe the boundary between sustainable and unsustainable throughput with great precision. This means limiting the level of material throughput below the level that is maximally sustainable, to create a safety margin.  But the less material throughput we have, the fewer material goods and services we have to enjoy.  The current economic paradigm assumes that more material goods and services mean greater human happiness and well being.  Optimal scale recognizes that material goods are only one of the determinants of human happiness and well being (see Real Determinants of Human Happiness and Well Being).

Managing Global Material Throughput   The concept of sustainable scale provides a conceptual framework for managing the level of economically driven material throughput so that it remains within the safe, sustainable range. This framework assists us to get the most benefits from economic activities, as well as from the non-market benefits of life support ecosystems.  As a relational concept (throughput relative to optimal regeneration) it requires a new way of thinking about how to integrate economic, ecological and social/justice priorities and activities.  What we have learned to do effectively (if not necessarily efficiently) in the last 150 years is how to drive throughput in economic activities.  We have yet to learn how to mange throughput levels to remain within the biophysical limits of ecosystems, and the ethical limits of our highest common aspirations.