Introduction to the early-Anthropocene Special Issue

The first records of Holocene CO2 and CH4 concentrations in the atmosphere were published two decades ago (Barnola et al., 1987; Chappellaz et al., 1990), and the observation that both gas concentrations had increased during the last several thousand years has drawn considerable attention from scientists. Competing hypotheses attribute these increases either to natural forcing or anthropogenic intervention in the climate system. This ongoing debate is the focus of this volume. 

Papers in Part 1 of this issue explore the history of Holocene population growth and land clearance. Part 2 examines possible sources of the CH4 increase during the last 5000 years, and Part 3 examines possible sources of the CO2 increase over the last 7000 years. Part 4 investigates whether or not the drops in CO2 and CH4 concentrations proposed in the early anthropogenic hypothesis would have been sufficient to cause glacial inception in the Northern Hemisphere. Part 5 explores possible causes of the decreases in CO2 concentrations during the last 1000 years. A final paper provides one assessment of whether the natural or anthropogenic hypotheses for the late-Holocene greenhouse-gas increases can be rejected based on currently available evidence. 

When this issue was first planned, the editors sent out invitations to all scientists who had first-authored papers relevant to the topic or had otherwise played a prominent role in the discussion. This list was closely balanced among those favoring natural explanations, those favoring the anthropogenic explanation, and those who had not taken a strong position on either side of the debate. Roughly half of the scientists invited chose to participate, and papers that support the natural and anthropogenic explanations can be found in each of the five major parts of the issue. Some of the most prominent proponents of natural forcing did not choose to participate, but their views are frequently discussed in the 13 papers that form this issue.

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