GC007-0005
Did agriculture beget agriculture during the past several millennia?

Monday, 7 December 2020
Poster
Stephen J Vavrus, University of Wisconsin Madison, Nelson Institute Center for Climatic Research, Madison, WI, United States, Christopher J Kucharik, Univ Wisconsin Madison, Department of Agronomy, Madison, WI, United States, Feng He, University of Wisconsin-Madison, Madison, United States, John E Kutzbach, Univ Wisconsin-Madison, Madison, WI, United States, Navin Ramankutty, McGill University, Montreal, QC, Canada and William F Ruddiman, University of Virginia, Environmental Sciences, Charlottesville, VA, United States
Abstract:
The Early Anthropogenic Hypothesis posits that carbon emissions from deforestation and other land use changes associated with ancient farming caused global warming due to rising greenhouse gas concentrations (GHG) in the late Holocene, in contrast to declining GHG during prior interglacials. Did this hypothesized pre-industrial anthropogenic climate change also foster agriculture by creating more favorable growing conditions?

We investigate this question with transient GCM experiments and the Cultivation Suitability Index, CSI, which quantifies farming potential based on climatic and soil factors. The Community Earth System Model (CESM) simulated the climate of the last 6000 years under two forcing scenarios: (1) ACTUAL: orbital variations, estimated historical land cover change (KK10 reconstruction), and observed GHG increase; and (2) NATURAL: orbital variations, fixed land cover, and expected natural GHG decline. The CSI was computed using output from the CESM simulations and observed soil properties.

Focusing here on conditions at the start of the Industrial Era (year 1850), we find that the preceding land clearance affected climate both biogeochemically (via carbon emissions) and biogeophysically (altered surface albedo and land-atmosphere momentum and energy fluxes). The biogeochemical effects generally dominate, as evidenced by a 0.84 K warmer (and wetter) global climate in ACTUAL vs. NATURAL. But a few regions experience cooling, especially interior Eurasia during winter-spring, due to higher surface albedo from cropland.

With the expansion of agriculture, cultivation potential mostly improved in boreal mid-high latitudes and worsened in low latitudes, based on the CSI difference between ACTUAL and NATURAL. The largest and most coherent rise in CSI occurred from eastern Europe to east Asia, as opposed to a CSI decline around the Mediterranean, northern Africa, and Middle East to northern India. The higher CSI in extratropical Eurasia represents more of a poleward extension of improved growing conditions in ACTUAL than an enhancement of agricultural potential in existing favorable regions. Our results suggest that ancient farming may have promoted a “push-pull” migration influence during the late Holocene by inducing climate changes that encouraged a northward spread of agriculture.

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