A better way to measure Planetary boundaries: global plant production

Running, Steven W. 21 Sep 2012. A Measurable Planetary Boundary for the Biosphere. Science.

Forty years ago Meadows et al, in “Limits to Growth 1972” concluded that by the first part of the 21st century, limits to essential global resources would emerge.  Graham Turner’s reanalysis in 2008 found that these projections were remarkably accurate.

Another groundbreaking analysis by Rockström et al. proposed 9 areas we could measure to see how soon one or all of them would render the earth uninhabitable from climate change, ocean acidification, land-use change, biodiversity loss, and five other boundaries.

These 9 boundaries are hard to measure.  Running has proposed a new, more measurable boundary: world-wide Net Plant production (NPP), for which we have a lot of data. The NPP measurement encompasses 5 of Rockstrom’s 9 boundaries:  land-use change, freshwater use, biodiversity loss, global nitrogen and phosphorus cycles, and NPP is affected by 2 of the others, climate change and chemical pollution.

We’re already using 40% of NPP, and some day may consume all of it (Vitousek et al.). The latest satellite data indicate this number is at least 30%.

This data is very robust — Global NPP comes from satellite measures of vegetated cover and density, combined with daily weather observations that analyze light, temperature, and water constraints to plant growth.

Consideration of current land use patterns and the projected rise in the human population suggest that human consumption may reach the global NPP boundary within the next few decades.  Therefore, it is reasonable to ask whether the earth can support a 40% increase in global population projected for 2050 and beyond.

What about the roughly 60% of NPP we’re not using yet?  Most of this can’t be harvested, i.e. the roots, wilderness areas with no transportation, and so on.  At best, another 10% is theoretically possible for future use by humans.

Some of this would be from undeveloped land in South America and Africa.

“Cropland under irrigation has roughly doubled in the last 50 years, and fertilizer use has increased by 500% (Foley).  Many analyses now conclude that freshwater use for irrigation has already reached a planetary boundary. As some rivers are completely drained for agriculture and groundwater withdrawal limits are reached in some regions, irrigated crop area could decrease in coming decades (Vorosmarty). Likewise, Rockström et al.concluded that the nitrogen and phosphorus cycles may have already exceeded planetary boundaries, as evidenced by massive river pollution and ocean anaerobic dead zones. If anything, future increases in NPP must be achieved with less, not more, irrigation and fertilizer use.”

We certainly don’t have enough land, water, and so on to grow additional plants to for bioenergy (Smith).

Foley, J. A. et al,. 20 Oct 2011. Solutions for a cultivated planet. Nature, vol 478 : 337-342.

Smith, W. K. et al. 2012. Global Bioenergy Capacity as Constrained by Observed Biospheric Productivity Rates. BioScience 62(10):911-922.

Vitousek, P. M. et. al. 1986, Human Appropriation of the Products of Photosynthesis. nearly 40% of potential terrestrial net primary productivity is used directly, co-opted, or foregone because of human activities. Bioscience 36, 368 (1986).

Vitousek, P. M. et al. 25 July 1997. Human Domination of Earth’s Ecosystems. Human alteration of Earth is substantial and growing. Between one-third and one-half of the land surface has been transformed by human action; the carbon dioxide concentration in the atmosphere has increased by nearly 30 percent since the beginning of the Industrial Revolution; more atmospheric nitrogen is fixed by humanity than by all natural terrestrial sources combined; more than half of all accessible surface fresh water is put to use by humanity; and about one-quarter of the bird species on Earth have been driven to extinction. By these and other standards, it is clear that we live on a human-dominated planet. Vol. 277 no. 5325 pp. 494-499

Vorosmarty, C. J. et al. Sep 2010. Global threats to human water security and river biodiversity. Nature Vol 467 555-561

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