When did humans first alter the global climate?

Palaeoclimatologist Professor William Ruddiman has been working on a hypothesis that posits that pre-industrial age humans raised greenhouse gas levels in the atmosphere. Looking back seven thousand years into the Holocene - the current 11,500-year-old geological epoch - Ruddiman has proposed that early agriculture emitted enough methane and carbon dioxide to offset what would have been a global cold cycle[1].
He began his thinking about early agriculture and greenhouse gases in the late 1990s when new data from Greenland and Antarctic ice cores was made public. Examining the data, Ruddiman was puzzled by the rise of atmospheric methane around 3000 BC.

In the glacial cycles of the past four hundred thousand years, this natural methane emissions rate is linked with the earth’s approximate 22,000 years precession cycle, the orbital cycle in which the earth shifts its axis of orbit. When the northern hemisphere summer is closest to the sun via precession, the highest amount of global methane is emitted. In the current interglacial precessional cycle, the methane maximum occurred eleven thousand years ago, at which there was the expected 700 parts per billion (ppb) methane concentration in the atmosphere—expected because comparable interglacial periods have that methane level.
However, at around five thousand years ago, Ruddiman noted that instead of continuing downward to a 450 ppb level, the methane leveled off at 560 ppb, and, reversing course, rose to around 660 ppb by one thousand years ago. Ruddiman correlated the methane trajectory reversal with rice paddy agriculture in Asia, which intensified roughly around the same time, five thousand years ago. After harvesting, the rice paddy areas emit methane in a similar manner to that of natural wetlands’ organic decomposition.

Ruddiman says that in contrast to the familiar view that human-caused greenhouse gases began with the industrial revolution, “the baseline of human effects on climate started earlier and that the total effect is larger.”

He is arguing that the significant footprint of human add-on to climate began thousands of years ago and not just 150 years or so, which is still the conventional view[2]. Even today climate scientists are not in total agreement as to whether early agriculture had a significant impact on global climate. Nonetheless, support for Ruddiman’s hypothesis has broadened in recent years [3].

Ruddiman: How Did Humans First Alter Global Climate? in Scientific American – 2005
Ruddiman et al: Does Pre-industrial Warming Double the Anthropogenic Total? in Anthropocene Review -2014
Ruddiman et al: Late Holocene climate: Natural or anthropogenic? in Reviews of Geophysics – 2016

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