The project has two major components. The first looks at changes to the biosphere in an attempt to identify key planetary boundaries, that is "human-determined values (of key ecological variables) set at a “safe” distance from a dangerous level" such that major tipping points will be avoided and the biosphere will continue to function more or less as it currently does.
The image at the left summarizes the findings from this portion of the study: 1) the group identifies 10 key ecological variables (climate change, ocean acidification, biodiversity loss, etc. represented by the slices of the pie), 2) defines a 'safe' level for each variable (represented by the green region) and 3) compares the current measures for each variable (red regions) to the save level. The study concludes that for three variables (climate change, biodiversity loss and nitrogen cycling) we have already crossed the planetary boundary.
Second, Rockstrom argues that humanity is putting the planet into a “quadruple squeeze” through pressures of human growth and inequality, climate change, ecosystem loss, and the problem of surprise – rapid tipping points.
It is interesting to compare this analysis with that provided by Tad Homer-Dixon in The Upside of Down. Homer-Dixon contends that five "tectonic stresses" are accumulating deep underneath the surface of today's global order:
- energy stress, especially from increasing scarcity of conventional oil;
- economic stress from greater global economic instability and widening income gaps between rich and poor;
- demographic stress from differentials in population growth rates between rich and poor societies and from expansion of megacities in poor societies;
- environmental stress from worsening damage to land, water forests, and fisheries; and,
- climate stress from changes in the composition of Earth's atmosphere.
The effect of the five stresses is multiplied by the rising connectivity and speed of our societies and by the escalating power of small groups to destroy things and people, including, potentially, whole cities. Interaction among the tectonic stresses and multipliers, according to Homer-Dixon, increases the possibility of unexpected and potentially catastrophic 'synchronous failure', a concept very similar to Perrow's characterization of a system accident.
Comparison shows a substantial amount of similarity in the two. With the exception of Homer-Dixon's emphasis on energy, they focus on the same factors: demographic, economic, environmental/ecological and climatic stresses and the importance of thresholds and surprise. Each, however, extends the analysis of the other in new and important directions. Thus, the Planetary Boundaries provides details on the entire range of key ecological operations necessary for viable operation of the biosphere; a topic not covered in as much detail by Homer-Dixon. Similarly, Homer-Dixon provides substantial additional insight on trends within human social systems that will affect our ability to implement the changes necessary to live within the planetary boundaries. Specifically, he adds the multipliers of globalized transportation and communication networks and the redistribution of power resulting from the proliferation of cheap weapons to the shared concern for economic inequality.
In short, the two present complementary rather than competing accounts.