“Come gather ’round people
Wherever you roam
And admit that the waters
Around you have grown
And accept it that soon
You’ll be drenched to the bone
If your time to you
Is worth savin’
Then you better start swimmin’
Or you’ll sink like a stone
For the times they are a-changin’. “
– Bob Dylan, “The Times They Are A-changin’ “ (1964)
Oh Bob, if we only knew back then what we know now, maybe more people would have learned to swim. Thankfully, we have seen a lot of progress since 1964, in social justice (the ostensible subject of his song), and in environmental regulation and scientific understanding of global systems (the subject of this blog, believe it or not). It is clear that “the line it is drawn, the curse it is cast,” and major anthropogenic global change is inevitable, in the form of climate change, habitat loss, and biological invasions. Thankfully, we do not have to rely on Bob Dylan for clarity of mind or inspiration anymore. Scientific advances in measurement and modeling are better poised to advise us on how best to respond.
How will Earth’s systems respond to the pressures of anthropogenic global change? Well, according to the authors of a recent TREE paper by Brooks et al. (part of a special issue on planetary tipping points), the responses can be broken down into three categories, each more catastrophic than the last. Change will be a) monotonic, b) non-linear, or c) non-linear with hysteresis (irreversible change). Importantly, the latter two involve thresholds, or ‘tipping points’, which they define as “the critical point at which strong nonlinearities appear in the relationship between ecosystem attributes and drivers.”(pg. 396) These can be viewed as ‘points of no return’ beyond which an ecosystem is irreparably degraded. There are many historical examples on multiple scales. Examples include reversal of the Atlantic thermohaline circulation due to fluxes of surface freshwater, and the destruction of coral reefs due to ocean acidification. In both cases, a gradual increase of an important driver eventually led to a tipping point.
These examples seem feasible, but Brooks et al. ask whether such tipping points exist in the entire terrestrial biosphere. Do we stand at the threshold of dramatic shifts in Earth’s biota, and can a dramatic change in one part of the planet cause a ripple effect across the entire planet?
The authors think the answer is no. Obviously this is not a testable hypothesis, but thankfully they have articulated an argument for that conclusion. They examined four drivers of biotic change, and concluded that their impacts on global ecosystem attributes (community composition and diversity, ecosystem function) should not have a tipping point, based on three a priori criteria.
1. The four drivers they examined were: changes in climate and atmospheric CO2, land use change, habitat fragmentation, and global species richness.
2. The a priori criteria include a) ecosystem attributes should respond similarly to the same drivers across continents, b) continents should have strong and redundant biotic and abiotic connections, c) drivers should be consistent across continents. In their words, “if drivers and responses are spatially homogeneous or inter-regional or intercontinental connectivity is strong, ecological change might display a tipping-point pattern at a global scale.” (pg 397)
In other words, the global “line that is drawn” is a relatively straight one, a gradual decline toward a fully degraded biosphere. But still, the “waters around you have grown.” Without a tipping point, the reflex to protect the terrestrial biosphere may be less strong, but the curse is still cast.
Why do I like this paper?
1. It attempts to address a question that is certainly on everyone’s mind, scientists and non-scientists alike. They lay out a clear argument. I am not totally convinced that I buy their argument, but it is great starting point and it defines the terms well.
2. It harkens back to early ecology, when the theory was built on expositions of logical arguments backed up by observations and ideas, such as in the classic HSS paper. Many such theoretical arguments have been overturned with math and/or data, but many have simply been refined and remain solid at their core.