The Anthropogenic Greenhouse Era Began Thousands of Years Ago by William F. Ruddiman Faisal Abd Razak ESS 433

Hypothesis Anthropocene actually began thousands of years ago as a result of the discovery of agriculture and subsequent technological innovations in the practice of farming

Arguments Cyclic variations in greenhouse gases Explanations on natural forcing for mid – to late- Holocene can be rejected based on paleoclimatic evidence Historical evidence point out to viable explanations tied to anthropogenic changes.

Early Anthropogenic Greenhouse Emission Cyclic Variations in CO2 and CH4 driven by Earth-orbital changes during the last 350,000 years predict decreases throughout the Holocene However, the CO2 trend began an anomalous increase 8000 years ago, and the CH4 trend also increases 5000 years ago.

Early Anthropogenic Methane Emission The anomalous late Holocene CH4 increase cannot be explained by natural forcing, when natural processes lost control of methane trends For hundreds of thousands years, the CH4 in Vostok ice followed the 23,000 years orbital insolation cycle It is coincides closely with the innovations in agriculture that produce methane in abundance Measured CH4 increase of 100 ppb can be explained by a simple linear scaling of 1990 population and anthropogenic CH4 emissions to 1750 population levels But the full 250-ppb anomaly was most likely due to the inefficiency of early rice irrigation, where extensively flooded wetlands harboring numerous weeds

Early Anthropogenic CO2 Emission Much more abundant gas than methane, variations have had a larger climatic impact over all time scales

Pre-Industrial Land Clearance Can explain the Holocene CO2 rise

Pre-industrial forest clearance in Eurasia explains he CO2 rise between 8000 yrs BP and 1800 AD Validated by 3 tests 1. Clearance must begin near 8000 yrs BP (when the CO2 rise began) on a small, yet ‘non-negligible’ scale 2. Clearance must grow large enough by ~2000yrs BP to explain ~80% of the pre-industrial CO2 anomaly 3. the negative CO2 oscillations of 4 to 10ppm after 2000 yrs BP also need an explanation

Test 1: Significant land clearance occur near 8000 yrs BP Agriculture in Europe consisted of slash-and-burn clearance of small patches of forest, planting of crops, and movement from site to site Alteration becomes far noticeable after the arrival of bronze-age plows near 6000 to 5000 years BP A pervasive decrease in forest pollen in China by 6000 yrs BP due to increase disturbance of natural vegetation by human

Test 2: Extensive deforestation occur by 2000yrs BP Almost every major food crop grown today was cultivated by 2000 yrs BP From calculation of many factors, about ~85-95% target validate the hypothesis that humans caused the rise in CO2 after 8000yrs BP Forest clearance appears to have been by far the major early-anthropogenic source of carbon

Test 3: Why did several CO2 minima interrupt the rise after 2000 yrs BP Local and regional epidemics, pandemics The deaths of tens of millions of people because of plague would slow the long term rate rise of carbon emission Caused the abandoned of farms and rural villages, where extracted carbon from the atmosphere Black Death (1347-1352 AD) correlates to CO2 anomaly of -5 to – 10 ppm in the ice core records

Effect of Early Anthropogenic Greenhouse Gasses on Climate Large areas of high terrain would be at glaciated threshold today due to the absence of the industrial-era warming Altitude of the glaciation limit during the Little Ice Age would have been lower about ~2 degrees C cooler Significant part of Northeast Canada should have been glaciated in last millennial Ice sheet should have begun to grow in the last 3000-6000 yrs based on the numerical model

Reference Ruddiman, F. William: 2003, ‘The Anthropogenic Greenhouse Era Began Thousands of Years Ago’.