Does pre-industrial warming double the anthropogenic total?

Abstract

According to the early anthropogenic hypothesis, land clearing and agriculture caused emissions of greenhouse gases to begin to alter climate as early as 7000 years ago (Ruddiman, 2003). Climate-model simulations based on the CO₂ and CH₄ concentrations proposed in the hypothesis suggest that humans caused a global mean warming of 0.9 to 1.5°C before the start of the industrial era. Additional pre-industrial effects on land surface reflectance (changes in albedo resulting from forest clearance) may have cooled climate enough to cancel 0.2 to 0.3°C of this warming effect, leaving a net early anthropogenic warming contribution of between 0.7°C and 1.2°C. This proposed early anthropogenic warming is comparable with, and likely larger than, the measured 0.85°C warming during the last 150 years. If the simulations based on the early anthropogenic hypothesis are correct, total anthropogenic warming has been twice or more the industrial amount registered to date.