Net energy (energy return on energy investment - EROI)
The concept of energy return on energy investment (EROI) tells us that once that ratio comes to 1:1, it is no longer profitable or cost effective to pursue. Oil for instance, once gave over 100:1 EROI with easily available oil deposits that were rich, high quality, and of vast quantities. Today that number is closer to 7:1. Due to lower quality and harder to extract oil and declining rates of EROI from exploration and energy development efforts, such as deep water or arctic drilling, which is vastly more expensive, difficult, and dangerous than conventional drilling.
Additionally, this concept applies to coal, natural gas, and other forms of energy production. As these resources become more scarce, they become more difficult and expensive to obtain. Over time, with finite fossil fuels, we will continue to approach EROI's closer to 1:1.
Regardless of the global environmental impacts that burning fossil fuels have on our planetary and human systems, we must consider the implications that EROI has on the stability of our social systems which are extremely dependent on higher EROI's. If energy return on energy investment continues to drop, there will be massive social and economic implications for the whole of humanity.
Sustainability seeks to acknowledge this fact while simultaneously addressing it within environmental, social, and economic rationality. As EROI levels approach 1:1, they will further decouple our ability to be sustainable if we pursue them. That is because the closer we approach 1:1 levels, the more dangerous and environmentally destabilizing they will need to be. All the easier, safer, and environmentally efficient resources have been mined. Only the expedient, perhaps deviant, and certainly dangerous methods of resource extraction remain available.
The absolute majority (97% according to this article) of scientists agree that climate change is anthropogenic (human-caused). By continuing down the same energy path path, one that intensifies all the negative affects (socially and ecologically) of its use, is not only dangerous to the long-term view of our economy, for the benefit of short term gains for a tiny subset of the population, it is also likely to result in highly destructive forms of violence to be perpetrated againtst nature and each other in the race towards fewer and fewer resources. Such an instance could be preempted by approaching the energy crisis, that is linked with all the other systems crises outlined by sustainability, and rationally strive for sustainable forms of energy production that can ease our economy into one of a steady state. Therefore avoiding social collapse, further biodiversity loss, and many other potential concerns.
(CC Image of DeepWater Horizon oil rig disaster courtesy of the U.S. Coast Guard via Wikimedia Commons)