The Tyndall History of Global Warming Lecture GC43B





The interactive transcript could not be loaded.


Rating is available when the video has been rented.
This feature is not available right now. Please try again later.
Published on Apr 14, 2014

TITLE: Early Agriculture: Land Clearance and Climate Effects

ABSTRACT: In the 2003 AGU Emiliani Lecture, I proposed the "early anthropogenic hypothesis" ---the idea that major anthropogenic effects on greenhouse gases and climate occurred thousands of years before the industrial era. In the decade since then, several dozen published papers have argued its pros and cons. In the 2013 Tyndall History of Global Change Lecture I will update where matters now stand. I will show figures from the 2003 Climate Change paper that laid out the initial hypothesis, and then update subsequent evidence from ice-core drilling, archeology, and land-use histories.

The primary claims in the 2003 hypothesis were these: (1) the CH4 rise since 5000 years ago is anthropogenic; (2) the CO2 rise since 7000 years ago is also anthropogenic; (3) the amount of carbon emitted from preindustrial deforestation was roughly twice the amount released during the industrial era; (4) global temperature would have been cooler by about 0.8oC by the start of the industrial era if agricultural CO2 and CH4 emissions had not occurred; (5) early anthropogenic warming prevented the inception of new ice sheets at high northern latitudes; and (6) pandemics and other population catastrophes during the last 2000 years caused CO2 decreases lasting decades to centuries.

The new evidence shows that these claims have held up well. The late-Holocene CO2 and CH4 rises are anomalous compared to average gas trends during previous interglaciations of the last 800,000 years. Land-use models based on historical data simulate pre-industrial CO2 carbon releases more than twice the industrial amounts. Archeological estimates of CH4 emissions from expanding rice irrigation account for much of the late Holocene CH4 rise, even without including livestock emissions or biomass burning. Model simulations show that the large pre-industrial greenhouse-gas emissions indicated by these historical and archeological estimates would have warmed global climate by more than 1oC and prevented northern glacial inception. Well-dated high-resolution CO2 (and CH4) records from ice cores show gas decreases that correlate closely with major pandemics and civil strife, but show little if any link to temperature or precipitation trends.

One significant (and intriguing) discrepancy with the original hypothesis remains. Most of the CO2 rise occurred between 6000 and 2500 years ago, well before the major increase in global population that has been hindcast from geometric models that assume a constant fractional rate of population increase. Some of this discrepancy has been reconciled by historical evidence showing much higher per-capita clearance millennia ago than later in pre-industrial time, resulting in disproportionately large early clearance and CO2 emissions. In addition, DNA studies and archeological syntheses now indicate that early farming populations initially grew at very fast rates favored by environments rich in basic resources (especially fertile soils), but then slowed in later millennia because of growing resource limitations and the effects of pandemics and civil strife in checking population growth. This emerging view of fast-rising early population trends has the potential to account for the early timing of the CO2 increase.

AUTHOR: William F Ruddiman, Environmental Sciences, University of Virginia


When autoplay is enabled, a suggested video will automatically play next.

Up next

to add this to Watch Later

Add to

Loading playlists...