Does the terrestrial biosphere have planetary tipping points?

Lots of interesting papers in the last days – one paper that caught my attention is an opinion piece by Brook et al. that just came out in TREE, titled “Does the terrestrial biosphere have planetary tipping points?”. Brooks et al. argue against the occurrence of major tipping points in the terrestrial! biosphere, essentially on the grounds of a lack of “strong” coupling between its components. They state that:

… spatial heterogeneity in drivers and responses, and lack of strong continental interconnectivity, probably induce relatively smooth changes at the global scale, without an expectation of marked tipping patterns.

Overall, I tend to think the same and I feel that this also reflects the opinion of most colleagues with which I discussed this question. Note, however, that this doesn’t mean that the biosphere is save – if there is strong change in climate due to a climate tipping point, the biosphere will of course react strongly as well, the point here is only whether the biosphere itself has major (large-scale) tipping points. Plus, the paper stresses that an absence of planetary tipping points in the biosphere does by no means say that the biosphere doesn’t react to global change – rather, we would probably expect it to react more early (but more continuously) than under a global tipping-point view. In that spirit, the authors state that

… focusing on uncertain future regime shifts at a planetary scale underemphasizes the degree to which ecosystems across the terrestrial biosphere have already been transformed by human activities over the long term (Box 1). Indeed, most ecosystems – perhaps up to four-fifths globally – have probably already undergone human-driven regime shifts of one or more kinds, at local or regional scales, over recent millennia, yielding a biosphere that today is largely characterized by post-transition, hybrid, or novel ecosystems

One has to admit, however, that there are large uncertainties in most processes that are discussed in the paper, particular if they couple with climate such as a potential Amazon die-back. The paper nicely explains different potential tipping-point mechanisms and gives verbal arguments for why we wouldn’t expect them to work, but most of that sounded pretty hand-waiving to me and I would well understand if someone who’s prior is set to tipping points is not convinced by these arguments. A more quantitative analysis would be great, but those attempts seem to drown in uncertainty about the processes, so at the moment it seems pretty much a question of gut-feeling to me. Still, I think it’s good that this opinion piece draws attention to the fact that we have to look carefully whether it makes sense to apply the tipping-point concept, which arguably is a bit of a fashion at the moment, too readily to the terrestrial biosphere.

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