How Long Have We Been in the Anthropocene Era?

With great interest we have read Ruddiman’s intriguing article which is in favor of placing the start of the Anthropocene at 5–8 millennia BP instead of the late quarter of the 18th century. He shows how land exploitation for agriculture and animal husbandry may have led to enhanced emissions of CO2 and CH4 to the atmosphere, thereby modifying the expected changes in the concentrations of these gases beyond those expected from variations in the Milankovich orbital parameters. Much of his argument depends on the correctness of their projected CH4 concentration curve from 7,000 years BP to pre-industrial times showing a decline to about 425 ppb, according to Milankovich, instead of the measured 700 ppb. It appears, however, strange that in Ruddiman’s analysis the proposed increase of CH4 due to anthropogenic activities stopped at about 1000 years BP, because ice core data showed almost constant mixing ratios of CH4 between 1000 years BP and about 200 years ago before the rapid rise of CH4 in the industrial period (IPCC, 2001). A major feature of Ruddiman’s argument is that natural atmospheric CH4 concentrations depend strongly on geological varying summer time insolations in the tropical northern hemisphere, controlling tropical wetlands and methane release from decaying organic matter under anaerobic conditions. The choice of the start of the anthropocene remains rather arbitrary. The records of atmospheric CO2, CH4, and N2O show a clear acceleration in trends since the end of the 18th century. For that reason, the start of the anthropocene was assigned to about that time, immediately following the invention of the steam engine in 1784 (Crutzen and Stoermer, 2000; Crutzen, 2002). The consequences of this innovation have been astounding, for instance, there has been a tenfold rise in human population to 6000 million, during the past three centuries, and a fourfold increase in the 20th century (Turner et al., 1990; McNeill, 2000). This expansion was made possible by medical advances and a major growth in agriculture and animal husbandry leading for instance to a current cattle population of 1400 million (globally averaged about one cow per average size family). Let us give a few more examples. In a few generations mankind is exhausting the fossil fuels that were generated over several hundred million years, resulting in large emissions of air pollutants. The release of SO2, globally about 160 Tg/year to the atmosphere by coal and oil burning, is at least two times larger than the sum of all natural emissions, occurring mainly as marine dimethyl-sulfide from the oceans (IPCC, 2001). The oxidation of SO2 to sulphuric and NOx to nitric acid has led to acidification of precipitation, causing forest damage and fish death in biologically sensitive lakes in regions, such