Lecture 28 - The Ruddiman Hypothesis

Lecture 28 - The Ruddiman Hypothesis

1. The early anthropogenic hypothesis

   • The Neolithic transition caused CO2 concentration to increase before the Industrial Revolution.
     • This caused a deviation away from the natural feedbacks (orbital forcing-carbon cycle) within the current interglacial.
       • Causing the Earth system to move away from an ice age (via global warming).
   • Involves the decoupling of ice volume and CO2 concentration as a result of human activity.

2. Graph showing the relationship between CO2 concentration and ice volume

   • When CO2 concentration is above the glaciation threshold, Earth remains in an interglacial period.
   • Cropping and deforestation release CO2.
   • Rice paddy fields create anaerobic conditions for methane release and hydrogen sulfide production.
     • H2S is a colorless, flammable, and extremely toxic gas with a strong odor of rotten eggs.

3. The secondary Ruddiman Hypothesis

   • Reduced human population = less agriculture + more afforestation = less CO2 = global cooling = the Little Ice Age
     • This process was reversed again due to GHG emissions from the Industrial Revolution.

4. The "Hockey Stick" curve

   • Shows warming in the atmosphere and oceans over the last 1,000 years.
   • The 3 climatic phases of the last Millennium...
     • 1) The Medieval Warm Period (700-1100AD)
       • Equitable climate.
       • Golden age of cathedral construction.
       • Caused by high solar irradiance, low volcanic activity and high GHG emissions from the Neolithic.
     • 2) The Little Ice Age (1300-1850AD)
       • E.g. the London Frost Fairs (1408-1814) when the Thames froze over.
       • Shown in Pieter Brueghel's paintings of the Netherlands (1565).
     • 3) Post-industrial Revolution warming (1850-present)
       • Triggered by dramatic GHG emissions.
       • Drop in glacier height in the Mer de Glace, French Alps shown in Birman painting.
       • The Industrial Revolution took place between 1760 and 1840.
   • Sunspot cylces and solar irradiance varied over the last Millennium.
     • Solar irradiance is influenced by...
       • A) Volcanic forcing (ash blocks light).
       • B) Solar irradiation.
       • C) All other forcing (including GHG concentration).
   • Climate modelling
     • Can be used to identify major forcings in complex systems.
     • Provides strong evidence that recent warming is caused by humans.
   • GHG emissions fell at the end of the Medieval period (1300s), a period of many deadly pandemics (e.g. the black death).
     • Caused world population to fall, alongside agricultural intensity and changing land use.
     • Reversed changes caused by the Neolithic transition, moving Earth towards an ice age.