Carbon cycle and climate commitments from early human interference

Abstract:

According to the early anthropogenic hypothesis proposed by Ruddiman (2003), human influence on Earth’s climate began several thousand years before the beginning of the industrial era. Agriculture and deforestation starting around 8000 years before present (BP) and slowly increasing over the Holocene, would have led to an increase in atmospheric methane (CH₄) and carbon dioxide (CO₂) concentration, preventing a natural cooling of Earth’s climate.

Here, the emphasis is not on testing Ruddiman’s hypothesis, but rather on exploring the carbon cycle and climate commitment from potential early CH₄ and CO₂ emissions. In contrast to modern greenhouse gas emissions, early emissions occurred over millennia, allowing the climate system to come to near-equilibrium with the applied forcing.

We perform two transient Holocene simulations with an Earth system model of intermediate complexity – the University of Victoria Earth System Climate Model (UVic ESCM). The first simulation is a standard transient Holocene simulation, forced with reconstructed changes in CO₂ and CH₄ concentrations and orbital and volcanic forcing. The second simulation is forced with CO₂ and CH₄ concentrations corrected for the net anthropogenic contribution postulated by Ruddiman (2007), with other forcings evolving as in the standard simulation. The difference in diagnosed emissions between the two simulations allows us to determine the anthropogenic emissions. After year 1850, anthropogenic CO₂ and CH₄ emissions are set to zero and the simulations continued for several hundred years. In this paper, we analyze the carbon cycle and climate response to the applied forcings, and quantify the resulting (post 1850) commitment from early anthropogenic interference.