The Case for a deep time Anthropocene

In a recent publication in the journal Science, Motti et al. (2021) use nearly 1200 pollen records, with some sequences dated to 18 thousand years ago, collected by researchers from all over the planet, to show that the highest rates of plant species turnover are associated with human occupation; specifically, with phases of agricultural expansion and intensification. Why does this matter?

In short, the period of their analysis (the late Pleistocene and early Holocene) shares similarities to our own, the so-called Anthropocene: that of a planet gradually filling up with groups of socio-economically bound humans in ever greater densities, effecting ever greater impacts on the landscape. We expect this to continue, even in the best case scenario of a radical transformation of prevailing systems of human-environmental interactions. Next, environmental changes during the Holocene have been modest compared with those over the last hundred thousand years. For example, the end of the Pleistocene (about 12.5 thousand years ago) was marked by a dramatic cooling event called the Younger Dryas over which the Northern Hemisphere swung wildly from a stable climate similar to that of today and ice-age conditions in the space of a few centuries, and possibly as swiftly as a few decades. From this perspective, the idea that anthropogenic environmental disruptions could be even more dramatic is remarkable.

The paper underscores how critical hydrologic changes are to ecology writ large (e.g., big impacts from inferred changes in monsoon conditions). In some sense, this should be a no-brainer, but temperature change dominates the popular discourse. I suspect this is because temperature change is easier to measure and model. But planners are far more interested in drought and flood risk than heat-waves. (It seems non-intuitive, but cold snaps are far more damaging to crops than heat-waves, for instance, provided sufficient water is available.) Of course, both temperature and precipitation are essential, but it’s worth reminding ourselves from time to time that Earth is ultimately a water-world.

What’s especially interesting, from my point of view, is the authors’ point that “large vegetation changes challenge the common myth of the stable tropics and suggest a strong sensitivity of the Neotropics to temperature, hydroclimate variability, and orbital precession during the Early Holocene”. Arguably the most valuable contribution that a study like this can provide is understanding the scale and duration of major disruptions to stable environmental systems, of which the Neotropics is just one example. (The only more valuable contribution that comes to mind is determining causal teleconnections between events, like ocean circulation changes and drought; but these are less valuable in isolation, without comparisons to sophisticated Earth system-general circulation models.) Environmental stability and social cohesiveness in the Neotropics (i.e., Latin America, politically speaking) will be enormously important in any future Earth scenario that I’m interested in living through.

Before I get to the final point, a note on perspective: it is extremely difficult to distinguish the effects of natural climate change from human influences in the paleoenvironmental record. Most researchers’ claims to do so rely on inferences, between this record and other well-dated records showing periods of human occupation and climatic change. The “well-dated” requirement means that your records are dated with comparable precision and relative sampling frequency; i.e., you are comparing “apples” to “apples”.

The authors end their paper with a kick-to-the-pillow for the global community, and it bears repeating here: “[T]he magnitude and extent of Late Holocene rates of vegetation change suggests that the global transformation of the terrestrial biosphere by humans now resembles or exceeds in rate and scope even the profound ecosystem transitions associated with the end of the last glacial period. Moreover, the global ecosystem changes for this century may be greater yet given current climate commitments and given that the climate changes expected for higher-end emission scenarios are similar in magnitude to those of the last deglaciation.”

Image: chironomid subfossil recovered from lake sediment. (I didn’t have an image of a pollen grain to hand.)

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