Thursday, August 18, 2016

Here comes the Anthropocene

Two excellent papers examine the meaning and formalization of an Anthropocene Epoch, a geological era in which humans have a major impact on surface processes and the environment. Steffen et al. (2016) take an Earth Systems approach while Williams et al. (2016) focus on biospheric signals.  Both papers are informative and data-based, and are required reading for anyone interested in this proposed change to the Geologic Timescale, but also those of us interested in global change (that is, all of us).

The field of stratigraphy is explicitly recognized in each analysis, as it provides the foundation of Earth’s geologic timescale.  A quick primer on stratigraphy: for the past 2.5 million years, we have lived in the Cenozoic Era’s Quaternary Period, which started with the Pleistocene Epoch and, currently, the Holocene Epoch.  The addition of an Anthropocene Epoch into the geological time scale is a key motivation behind these papers, which will be decided by the International Commission on Stratigraphy, supported by an Anthropocene Working Group that includes the leads and several authors of these two papers.  Steffen and colleagues use Earth Systems science to describe our planet’s evolution from an evolving Precambrian environment into a life-dominated Phanerozoic one (since ~540Ma).   They conclude that today’s Earth system has undergone a substantial transition away from the Holocene (interglacial) state, toward a world with much less polar ice, changed atmospheric composition, and accelerated plant and animal species extinction.  Williams and colleagues’ biotic approach emphasizes that modern humans are changing the planet’s relationship through human consumption of Earth’s resources, with major consequences for the ecosphere and a change in evolutionary state.  Using different perspectives, both papers reach the same conclusion of an Anthropocene state that is unlike the Holocene, supporting the need for a new epoch.  Both also favor a chemical tracer from mid-20thC nuclear activity as its lower boundary, though that, I believe, seems less compelling from their descriptions.

The stratigraphic foundation of the Phanerozoic Eon’s geologic timescale is the preservation of hard-bodied life.  Extinctions, a relatively sudden, large decline of species, punctuate the record with five major events (excluding today) and multiple smaller events, providing global markers for stratigraphic boundaries in the geologic record.  Some extinctions were relatively fast (kyr), while others reflect longer times (myr).  The species extinction of modern time, which started with the rise of humans as the planet’s dominant consumer of resources can likewise become the base of the Anthropocene.  This latest (6th) major extinction is already underway, and continuing for decades to centuries, perhaps even culminating in human extinction.  Life, notably the radiation of species, offers another global stratigraphic marker in the tradition of the geologic timescale.  Humans exploring and conquering the world transported other life, including plants and seeds, small animals (like insects and rodents), and even large animals (like horses) that since became entrained as fossils in modern depositional strata.  This biomarker would place the start of the Anthropocene well before the 20th century, as far back as 15th century, following Medieval times.  Arguably, the current 6th extinction also started around that time.  Unlike the Holocene, which started ~12,000 years ago as a garden-variety interglacial, the Anthropocene involves vast and fast changes on a global scale, affecting life, atmosphere, land and oceans.

These patterns are not mere extensions or accelerations of the Holocene interglacial.  The Anthropocene signature is unlike that of our planet’s past icehouse-greenhouse system, leading to my earlier, alternative suggestion to adopt a Pleistocene-Anthropocene boundary that reflects this fundamental change in Earth System from an externally-driven (or Milankovitch) state to a human-driven state.  As we move toward a decision on the timescale, these papers make a compelling case for an Anthropocene Epoch, while reminding us of the large changes in environmental conditions that are already underway.

Steffen, W., Leinfelder, R., Zalasiewicz, J., Waters, C. N., Williams, M., Summerhayes, C., Barnosky, A. D., Cearreta, A., Crutzen, P., Edgeworth, M., Ellis, E. C., Fairchild, I. J., GaƂuszka, A., Grinevald, J., Haywood, A., Sul, J. I. d., Jeandel, C., McNeill, J.R., Odada, E., Oreskes, N., Revkin, A., Richter, D. d. B., Syvitski, J., Vidas, D., Wagreich, M., Wing, S. L., Wolfe, A. P. and Schellnhuber, H.J. (2016).  Stratigraphic and Earth System Approaches to Defining the Anthropocene. Earth's Future, 4: 324–345. doi:10.1002/2016EF000379.

van der Pluijm, B. (2014). Hello Anthropocene, Goodbye Holocene. Earth's Future, 2: 566–568. doi:10.1002/2014EF000268.

Williams, M., Zalasiewicz, J., Waters, C. N., Edgeworth, M., Bennett, C., Barnosky, A. D., Ellis, E. C., Ellis, M. A., Cearreta, A., Haff, P. K., Ivar do Sul, J. A., Leinfelder, R., McNeill, J. R., Odada, E., Oreskes, N., Revkin, A., Richter, D. d., Steffen, W., Summerhayes, C., Syvitski, J. P., Vidas, D., Wagreich, M., Wing, S. L., Wolfe, A. P. and Zhisheng, A. (2016).  The Anthropocene: a conspicuous stratigraphical signal of anthropogenic changes in production and consumption across the biosphere. Earth's Future, 4: 34–53. doi:10.1002/2015EF000339.

Modified from AGU Editors’ Vox;