2016-01-19. Early Agriculture Has Kept Earth Warm for Millennia.

posted Jan 22, 2016, 9:39 AM by Alan Gould   [ updated Jan 22, 2016, 9:44 AM ]
By Sarah Stanley, Earth & Space News (EoS; AGU). For GSS Life and Climate chapter 12. Excerpt: Ice core data, archeological evidence, and other studies suggest humans had a significant influence on Earth's preindustrial climate. Modern human activity is known to drive climate change, but global temperatures were already affected by farmers millennia before the Industrial Revolution. For years, scientists have been debating about the size of preindustrial warming effects caused by human activities. Now, according to Ruddiman et al., new evidence confirms that early agricultural greenhouse gas emissions had a large warming effect that slowed a natural cooling trend. Earth’s climate has cycled between warmer interglacial and cooler glacial periods for 2.75 million years as a result of cyclic variations in the Earth’s orbit. The current Holocene epoch, which began about 11,700 years ago, is an interglacial period. In an earlier study, Ruddiman compared Holocene trends with data from previous interglacial periods over the past 350,000 years. Instead of slowly decreasing—as observed early in previous interglacial periods—carbon dioxide levels began to rise 8000 years ago, and methane levels started increasing 5000 years ago. These increases correspond with the onset of early agriculture, which, Ruddiman hypothesized, may have produced enough greenhouse gases to slow the normal cooling trend. Now Ruddiman and 11 colleagues have more thoroughly compared the Holocene with past interglacial periods.  ...The team also reviewed archaeological and paleoecological evidence. Studies show that the spread of rice irrigation is likely responsible for much of the increase in atmospheric methane between 5000 and 1000 years ago. The spread of livestock across Asia, Africa, and Europe—as well as other agricultural activities like burning weeds and crop residues—contributed as well. Deforestation that accompanied early agriculture could be responsible for the carbon dioxide increase that began nearly 7000 years ago....  https://eos.org/research-spotlights/early-agriculture-has-kept-earth-warm-for-millennia