How far can one keep going straight up an apparently unscaled peak without falling off a precipice? How far can the march of the human footprint on Earth continue without exceeding planetary boundaries and leading to environmental catastrophe? In an important recent paper in Nature, strangely reminiscent of the publication of The Limits to Growth by the Club of Rome in 1972, a group of scientists poses and develops tentative markers of planetary boundaries being reached or exceeded.

The paper in Nature, an accompanying editorial, the seven commentaries from leading experts, available here, are worth a read for anyone who wants an overview of what the major human impacts on the planet are and where they are headed. Specifically, the authors deal with the following nine issues:

  1. climate change
  2. ocean acidification
  3. stratospheric ozone depletion
  4. freshwater use
  5. biodiversity
  6. the global cycles of nitrogen and phosphorus
  7. land-use change
  8. atmospheric aerosol loading (to be quantified)
  9. chemical pollution (to be quantified)

The paper suggests that three boundaries related to climate change, biological diversity, nitrogen and phosphorous dumping into the biosphere, may already have been exceeded. A brief summary of the findings with relevant links is also available here at the website of the Stockholm Resilience Centre where the lead author Johan Rockström is based. The seven commentaries along with some other recent research highlights are also available here. The real meat of the paper is actually in a parallel publication in the journal Ecology and Society. Although this paper is in press, it is available here and this contains the details of the issues at stake, the underlying rationales, and references to the scientific literature based on which the conclusions are drawn.

In our context, given India’s demographic profile and dependence on agriculture, the aspects related to freshwater use and nitrogen-phosphorous cycles are really worthy of note. Water shortages in the country and the severe depletion of groundwater were recently again in the news following a paper in Nature. Anthropogenic nitrogen loading is already affecting our terrestrial ecosystems, coastal and marine areas, and rivers. Reporting high values of dissolved and sediment-bound nitrogen in Indian rivers, partly due to excessive fertiliser use and associated run-off, the authors of the last study grimly conclude: “Hence, our freshwater aquatic systems can no longer be considered natural, at least with respect to nitrogen transport.”

A quick survey of the debate emerging from the papers by Rockström and colleagues indicates two main questions are being asked (among others spurred by the publications). First, is it sensible to set a tipping-point benchmark, however scientifically tenuous it may be given the current state of knowledge? There is concern that this might cause complacence among policy makers and administrators, who may avoid responding to the situation until the benchmark is reached or exceeded. The second is the issue of benchmark itself: for instance, in the case of biodiversity loss. The authors of the study use extinction rate as a measure of biodiversity loss. In Table 1, they indicate a pre-industrial value of rate of extinction at 0.1 to 1 species per million species per year. The current rate of extinction is >100 species per million per year and the proposed boundary is 10 species per million per year. What makes this an acceptable boundary or rate of loss of species?

The overall picture that emerges is alarming, to say the least. The climate crisis is familiar; our newspapers are full of it now. Other concerns appear less commonly in the media. For instance, that our oceans, which absorb some 25% of human CO2 emissions, are undergoing acidification at a rate 100 times higher than at any time in the past 20 million years. This makes a whole range of marine organisms, such as corals and molluscs, susceptible to corrosion of their shells (made of calcium carbonate in the form of aragonite). The decline of aragonite-forming organisms and coral reefs could substantially alter marine ecosystems. Another global concern is that of human tampering of the planetary nitrogen cycles. Human activities now input more reactive nitrogen into the planet than all natural processes combined. As a large part of this enters the biosphere, it alters terrestrial ecosystems, as well as freshwater and marine ecosystems.

The paper will doubtless spur more discussions and research into the various benchmarks and their utility in tracking the human footprint. Despite the debates and shortcomings, one real value of the paper as it appears to me is that it brings into one page—onto one figure even, superimposed ominously on the globe—an assessment and visualisation of the nine-fold stranglehold that humans as a species have on Earth. Looking at it we have to keep asking: is the human journey reaching the edges of the Earth?