Latest attempt to visualize the mess we're in: A wheel of misfortune

The journal Nature has just published a massive feature series on, to use a well-worn phrase, "the limits to growth." The centerpiece is a graphic created by Johan Rockström of the Stockholm Resilience Centre and his colleagues as part of "a new approach to defining biophysical preconditions for human development. For the first time, we are trying to quantify the safe limits outside of which the Earth system cannot continue to function in a stable, Holocene-like state." Here's their wheel of misfortune:

The green is the safe limit, the red represents more or less where we are. For those having trouble reading the left and right red wedges: the left-hand wedge is Biodiversity loss, the right the nitrogen cycle. Nature's editors put a number of caveats on the science that informs the image:

First, the wheel and the rest of the feature package are not peer-reviewed. Second, Rockström et al concede that the numbers they chose are "merely our first best guesses." The Nature editors write:

For the most part, the exact values chosen as boundaries by Rockström and his colleagues are arbitrary. So too, in some cases, are the indicators of change. There is, as yet, little scientific evidence to suggest that stabilizing long-term concentrations of carbon dioxide at 350 parts per million is the right target for avoiding dangerous interference with the climate system. Focusing on long-term atmospheric concentrations of the greenhouse gas is perhaps an unnecessary distraction from the much more immediate target of keeping warming to within 2 °C above pre-industrial levels. Nor is there a consensus on the need to cap species extinctions at ten times the background rate, as is being advised.

Furthermore, boundaries don't always apply globally, even for processes that regulate the entire planet. Local circumstances can ultimately determine how soon water shortages or biodiversity loss reach a critical threshold.

Assigning 'acceptable' limits to processes that ultimately determine our own survival is risky in other ways, too. After all, some of the suggested limits may be easier to balance with ethical and economic issues than others. Human interference in the nitrogen cycle may well have damaging long-term consequences, but the production of nitrogen for agriculture has also fed large parts of humanity.

Still, it's a "creditable attempt to quantify the limitations of our existence on Earth." Will this visualization project gain any traction where it's needed most, in the halls of power? I am doubtful, but I'm doing my part to spread the meme.

The most interesting, and likely the most contentious, limit assigned by Rockström and his group is a safe atmospheric CO₂ concentration of 350 parts per million, which is some 37 ppm below what it is today. They do this because they agree that "long-term reinforcing feedback processes" mean that doubling the CO₂ will lead to a 6 °C (~10 °F) increase in global average temperatures, not just the 3 °C that has formed the basis of most of the climate policy discussions so far, and because glacier and ice cap melt and sea level rise have all proceeded faster than previously anticipated. In other words, they buy co-author James Hansen's arguments, which until now have received more support from activists and journalists like Bill McKibben (and me) rather than other climatologists. This could be a sign that that's about to change, lack of peer-review notwithstanding.

However, among the commentaries published alongside this report is one from Oxford's Myles Allen, who argues that setting CO₂ concentrations misses the point. Instead, he writes, we should be thinking about our cumulative emissions of carbon, a notion that Allen has been pushing hard since his Nature paper on the issue was published earlier this year, and it deserves our attention. He even takes issue with the entire planetary boundaries idea in the first place:

There is no need to speculate about the behaviour of the climate system into the next millennium to make the case that emission reductions are urgently needed to avoid dangerous climate change.

Rockström et al believe their holistic analysis can be useful, though, and it's hard to argue that a sincere effort to look at the big picture won't be useful, regardless of the merits of the individual thresholds they've assigned. As they write:

Although the planetary boundaries are described in terms of individual quantities and separate processes, the boundaries are tightly coupled. We do not have the luxury of concentrating our efforts on any one of them in isolation from the others. If one boundary is transgressed, then other boundaries are also under serious risk. For instance, significant land-use changes in the Amazon could influence water resources as far away as Tibet. The climate-change boundary depends on staying on the safe side of the freshwater, land, aerosol, nitrogen-phosphorus, ocean and stratospheric boundaries. Transgressing the nitrogen-phosphorus boundary can erode the resilience of some marine ecosystems, potentially reducing their capacity to absorb CO₂ and thus affecting the climate boundary.