The early anthropogenic hypothesis: Challenges and responses

Ruddiman (2003) proposed that late Holocene anthropogenic intervention caused CH 4 and CO 2 increases that kept climate from cooling and that preindustrial pandemics caused CO 2 decreases and a small cooling. Every aspect of this early anthropogenic hypothesis has been challenged: the timescale, the issue of stage 11 as a better analog, the ability of human activities to account for the gas anomalies, and the impact of the pandemics. This review finds that the late Holocene gas trends are anomalous in all ice timescales; greenhouse gases decreased during the closest stage 11 insolation analog; disproportionate biomass burning and rice irrigation can explain the methane anomaly; and pandemics explain half of the CO 2 decrease since 1000 years ago. Only ∼25% of the CO 2 anomaly can, however, be explained by carbon from early deforestation. The remainder must have come from climate system feedbacks, including a Holocene ocean that remained anomalously warm because of anthropogenic intervention.