What does Ruddiman’s hypothesis state?

Introduction

This post summarises the evidence and the hypothesis set out in Ruddiman's 2003 paper which considers two atmospheric gasses, methane (CH₄) and carbon dioxide (CO₂), and compares the currently observed concentrations with the concentrations that would be expected under natural climate change driven by orbital forces.

The Methane anomaly

For most of the last 350,000 years, levels of atmospheric methane have tracked the axial precession of the orbit of the earth through the rising and falling of summer insolation. An increase in the quantity of solar energy falling on land masses causes air to rise which sucks in air from over the oceans. As this moist air rises over land it heats and deposits the moisture as monsoon rains, causing methane to be released from soils. The Greenland Ice Core Project (Blunier et al, 1994) evidence showed an anomalous increase in methane levels since 5000 BP when a decrease was predicted. The actual increase in methane was ~100 parts per billion. Given the predicted reduction, the likely excess methane is ~250 pbb.

The anomalous increase in atmospheric methane since 5000 BP (modified from Ruddiman, 2003)

The carbon dioxide anomaly

Atmospheric carbon dioxide concentrations also track orbital forces with varying degrees of latency. The forcing is more complex as it is affected by the axial precession cycle of 23,000 years, the axial tilt variation cycle of 41,000 years and the orbital eccentricity cycle of 100,000 years. Hays et al (1976) showed that these cycles are responsible for a substantial amount of climate variation including glaciation events and atmospheric carbon dioxide concentrations. The carbon dioxide levels predicted by those cycles and matched to the three of the most recent interglacials are not matched by those observed for the current interglacial. The predicted contemporary concentration is 240-245 parts per million but the observed concentration is 280-285 ppm, an anomalous increase of 40 ppm; this divergence commenced ~8000 BP.

Explaining the anomalies

Loss of terrestrial vegetation would reduce the uptake of carbon dioxide by plants. Various scenarios were modelled (Foley, 1995) to assess whether natural forces, such as desert expansion caused by reduced monsoon precipitation, could explain the anomaly. An alternative theory (Broeker et al, 1999) proposed that the expansion of forests after the Last Glacial Maximum had reduced atmospheric carbon dioxide; when forest expansion ceased the ocean's acidity increased dissolving calcium carbonate and releasing carbon dioxide into the atmosphere; Ruddiman chooses to dismiss these explanations because the underlying theories should have manifested the same outcomes in previous interglacials, those outcomes were not observed.

The alternative explanation offered is that biomass was reduced by anthropogenic action, the clearance of Eurasian forests for agriculture. There is considerable and persuasive archaeological evidence that such deforestation has taken place since 8000 BP, complemented by historical evidence from Greek and Roman writers. Carbon dioxide concentrations also track human population variations caused by plagues.

Commentary

This is a long post but I wanted to establish the arguments for the hypothesis and the complexity of the natural forces that affect climate. What I learned by writing it is that:

• the effects of astronomical forcing on climate vary as the different cycles harmonise and compete with one another
• those astronomical forces continue to affect climate as anthropogenic effects also manifest themselves
• both the methane and carbon dioxide anomalies must be explained in a complementary manner

Next, I will explore the archaeological evidence.