Crossing the boundaries

In 2009, a paper was published that measured where humanity stood in respects to the safe operating boundaries for nine environmental parameters (Rockström et al. 2009).  The nine they chose were climate change, ocean acidification, stratospheric ozone depletion, rate of biodiversity loss, biogeochemical cycles (specifically, the nitrogen and phosphorus cycles), global freshwater use, change in land use, atmospheric aerosol loading, and chemical pollution.  Using the Holocene as a baseline, they calculated threshold levels for each parameter that, when crossed, created a high risk for changes that would be damaging for human civilization.


Rockström et al. found that humanity had crossed the safe boundaries for three of the seven defined parameters in 2009 (two boundaries had yet to be defined):
  1. Climate change, with both CO2 (then 387 ppmv) and change in radiative forcing (1.5 W/m2) exceeding the respective 350 ppmv and 1 W/m2 boundary levels, 
  2. Biodiversity loss, with then extinction rate of >100 species/million species/year far exceeding the boundary of 10 species/million species/year, and 
  3. Biogeochemical cycles, with the then amount of fixed nitrogen per year (121 million metric tons) far exceeding the boundary of 35 million metric tons.  Meaning we've really screwed up the natural nitrogen cycle to go with the messed up carbon cycle.
Figure 1 from Rockström et al. (2009) showing the nine planetary boundaries as well as those which humanity had exceeded.
An update to Rockström et al. was published in Science on January 15, 2015.  Steffen et al. (2015) provided revised boundaries and rates of change.  Their results are sobering, to say the least, as they showed that we still exceed the boundaries on climate change, biodiversity loss, and biogeochemical cycles.  Indeed, recent research has shown the problems to be growing worse (e.g. McCauley et al. 2015 showed that impacts on biodiversity in the oceans is increasing).

Revised planetary boundaries from Steffen et al. (2015).
The portion of their results that gained the most attention was that land-use changes (called land system change in the new paper) now exceeded the planetary boundary of 15% of land converted to cropland.  This means that humans have now crossed four of the seven quantified boundaries.  Just as significant, in my view, is the fact that the updated numbers show that we've also crossed the boundary for the phosphorus cycle.  In 2009, the rate at which we use phosphorus hadn't exceeded the 11 million metric tons per year boundary.  Now it too joins the list of natural biogeochemical cycles that have been completely altered by human activities.

Why is the phosphorus cycle important?  Phosphorus, like nitrogen, is a key limiting nutrient to plant growth, both in terrestrial and in aquatic habitats.  Excess phosphorus leads to algal and cyanobacteria blooms in freshwater (think of what happened to Toledo, Ohio, last summer when a cyanobacteria bloom contaminated their drinking water) and dead zones in marine habitats.  Dead zones have been detected worldwide, from the Chesapeake Bay and the Gulf of Mexico to the Baltic Sea to southern Australia.  This creates numerous problems for aquatic animals (after all, every animal needs oxygen) as well as humans.

Map showing the locations of known dead zones.  From NASA GES DISC.
While the new work has gained some criticism (and the whole concept of planet-wide boundaries is controversial), I think it provides a useful framework for quantifying just how we as a civilization are progressively degrading our environment.  What should happen is that policymakers begin serious discussions and craft proposals to reverse the damage that has already been done and to keep humanity within the safe limits.  In some countries, that may happen.  In the US, unfortunately, what will happen is political gridlock amid proposals to undo much of the meager progress that has been made (see, for example, the various proposals now in Congress to make it harder for presidents to set aside public lands for monuments and parks), with nothing being done until a crisis hits.  As usual.

Comments

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