From the foothills of Tucson, Arizona, a 4200 foot climb will bring you to the summit of Mt. Kimball. From the peak there are good views to the north and east of Cathedral Rock, looming over the Wilderness of Rocks, and further still, the steep south face of Mt. Lemmon. But look to the west, and the mountains drop away to reveal wide bajadas, the low desert alluvial fans formed over millions of years by the erosion of these mountains. This landscape has been abruptly disturbed by the sudden appearance of this.

You are seeing Sun City, a retirement community located in the town of Oro Valley. Their marketing materials are please to note that “Common areas and individual homes have a pleasingly homogenous appearance”. They feature their 18-hole golf course, which “was created in harmony with the natural desert surroundings”. I wanted to show you this travesty of land development and irrigation not to directly criticize this property or its owners, but rather to make a broader point.

Recently in ecology the concept of a ‘tipping point’ has been becoming very popular. The idea is that a system (say, the earth’s climate, or the biodiversity of a desert landscape) may appear to be stable – but with a small push past some threshold, the whole system may collapse. The math is complex, but one way to conceptualize this is a ball balanced on the top of a hill – with any random gust of wind, it will begin rolling, though left or right remain unpredictable. There is evidence that this kind of transition does happen – for example, at the Permian-Triassic boundary (approximately 252 million years ago) we have evidence that eighty to ninety percent of all marine species went suddenly extinct. It seems reasonable to worry that current planetary changes may have the same effect.

A recent publication (Rockström et al.) have called for the identification of ‘a safe operating space for humanity‘. The argument is that we should maintain our various impacts on the planet (e.g. atmospheric carbon dioxide levels, deforestation, phosphorus mining) below some critical values, because exceeding certain levels could lead to a runaway catastrophe for biodiversity and functions of the earth that are necessary to sustain human civilization.

The impacts of humans on the natural world are large. A recent study by Vitousek et al. has indicated that humans dominate the earth, using (for example) more than half of all available freshwater and half the available land. As a result, last year Barnosky et al. wrote a paper arguing that we are ‘approaching a state shift in Earth’s biosphere’. This perspective is reasonable, given the large and obvious impacts of humans on the landscape. Take that Oro Valley development, or alternatively the abandoned Bisbee copper mine seen below, filling with wine-dark waters leached from the tailings.

But there is a controversy over this perspective from within the scientific community – just last week, Brook et al. published a paper with the title ‘Does the terrestrial biosphere have planetary tipping points?’. Here, this group of researchers argues that evidence and concepts for tipping points may be weak. The point out that a planetary state shift would require that ‘the terrestrial biosphere is likely to respond heterogeneously in time and space to drivers, which implies that the pattern of change in any global aggregate ecosystem attribute will be relatively rate constant and cumulative, rather than exhibiting any iden- tifiable tipping points’. That is, they don’t think the earth system, as a whole, will respond in a coordinated fashion to the large and real stresses we are placing upon it.

I think that this is a good and important point, that will perhaps be the first challenge to a perspective that has been attractive mostly for its apocalyptic implications and not considered as critically as it need be. Moreover, it makes the good point that indices of human impacts (e.g. loss of species diversity) are unlikely to provide robust indicators of earth’s functioning or imminent loss-of-functioning. However I do think that this publication overlooks two important issues. First, I do not think we have a sufficient amount of evidence for the responses of modern ecological systems to large perturbations. This study highlighted many empirical scenarios where perturbations have had heterogeneous and regionally limited effects – yet we are now introducing perturbations that are fundamentally larger and less well understood than previous ones. It is difficult to detect a nonlinear response or bifurcation in a system if it has never yet been perturbed far enough to see one.

Second, I think the authors lean too heavily on the presumed independence of different components of the earth system. The logical argument is good – if different regions respond independently to a perturbation, then no coordinated planetary state shift will occur. But the evidence is weak. We in many cases simply do not know how different parts of the earth are coupled together. For example, it was recently shown that wildfires in one region create dust that accelerates snowmelt and changes water availability in far away river systems.

I think we are particularly vulnerable to these unexpected linkages when we think about the causes and effects of land use change. Here, as an example, are two images from a recent trip to Puerto Rico, showing the impacts of urbanization and deforestation in very recent times.

Puerto Rico actually used to be mostly forested, but for a time had less than five percent total cover, a number that has now increased rapidly. These rapid swings have much to do with changing geopolitics and local economic policies. And economic systems show strong couplings across the planet, so that an event in one region instantly and dramatically affects other regions. I think we underestimate the importance of these processes for driving and synchronizing ecosystem responses. Yet the data to address this question adequately remain limited. Predicting the ecological consequences of economic dynamics is hard, but I think very worthwhile.

What does the Sun City development in the Arizona desert presage? We don’t know, but probably should.