Global tipping points in the terrestrial biosphere?
Is our planet now threatened by rapid global changes caused by human forcing of the terrestrial biosphere past a planetary tipping point? Two different articles in Nature have suggested that the answer may be yes (Rockstrom et al. 2009, Barnosky et al., 2012). Such is the question that Barry Brook, myself and colleagues evaluated recently in a peer-reviewed journal article now in press at TREE, and formed the basis for my opinion piece published today in NewScientist .
Our “eggs” are in many baskets, and our baskets are in many hands. For planetary (global scale) tipping points to exist in the terrestrial biosphere, the forces of humanity would need to act uniformly across the planet and all ecosystems would need to respond to them in the same way, and/or the response would need to be transmitted rapidly among Earth’s many ecosystems and continents to enable a synergistic global response to emerge.
It is essential to note that:
- This work is only about global tipping points within the terrestrial biosphere: it does not apply to the Earth system as a whole, or to the climate system itself. Indeed, climate science indicates that we may now in fact be heading towards a global tipping point in the climate system, as observed in past geological time periods. The potential threat of a global tipping point in the climate system demands scientific and policy attention. For a review of tipping points in the climate system, please refer to Lenton et al., 2008.
- This work does not deny that humans have caused and are causing massive global changes in the terrestrial biosphere. We have all published work supporting this (eg. Ellis 2011). Nor does our work challenge existing observations of local and regional tipping points in ecosystems. Our work challenges only the hypothesis that current and future global changes in the terrestrial biosphere will occur in the form of a global tipping point.
- Our work is based on expert review of current scientific knowledge: it is not based on controlled experiments with the terrestrial biosphere. As a result, our claims are not the strongest form of scientific knowledge or prediction. However, they do represent a peer-reviewed assessment of existing scientific knowledge in one of the most highly cited scientific journals.
As with all scientific knowledge about our planet’s future, we cannot now know with absolute certainty what the future holds. There is only one Earth, and so we must base our knowledge of how the Earth system functions based a combination of historical observations and mechanistic models. At present, global models of the Earth system have yet to reach the level of sophistication needed to answer questions about global tipping points within the terrestrial biosphere or in the Earth system as a whole. So at this point, without adequate models or prior observations of human-induced global tipping points in the terrestrial biosphere, arguments for or against tipping points must be based on informed scientific opinion: that is the only current basis for any published work on this subject.
It is my hope and belief that global change science will one day have the evidence and global modeling power to resolve these very critical questions about the nature of humanity and our planet in the Anthropocene. Today, we must merely do our best with the science that we have.
Barnosky, A. D., E. A. Hadly, J. Bascompte, E. L. Berlow, J. H. Brown, M. Fortelius, W. M. Getz, J. Harte, A. Hastings, P. A. Marquet, N. D. Martinez, A. Mooers, P. Roopnarine, G. Vermeij, J. W. Williams, R. Gillespie, J. Kitzes, C. Marshall, N. Matzke, D. P. Mindell, E. Revilla, and A. B. Smith. 2012. Approaching a state shift in Earth’s biosphere. Nature 486:52-58.
Ellis, E. C. 2011 Anthropogenic transformation of the terrestrial biosphere. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science 369(1938):1010-1035. [download].
Lenton, T. M., H. Held, E. Kriegler, J. W. Hall, W. Lucht, S. Rahmstorf, and H. J. Schellnhuber. 2008. Tipping elements in the Earth’s climate system. Proceedings of the National Academy of Sciences 105:1786-1793.
Rockstrom, J., W. Steffen, K. Noone, A. Persson, F. S. Chapin, E. F. Lambin, T. M. Lenton, M. Scheffer, C. Folke, H. J. Schellnhuber, B. Nykvist, C. A. de Wit, T. Hughes, S. van der Leeuw, H. Rodhe, S. Sorlin, P. K. Snyder, R. Costanza, U. Svedin, M. Falkenmark, L. Karlberg, R. W. Corell, V. J. Fabry, J. Hansen, B. Walker, D. Liverman, K. Richardson, P. Crutzen, and J. A. Foley. 2009. A safe operating space for humanity. Nature 461:472-475.