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. They assessed ice core records from Antarctica, which provide a record of greenhouse gas levels and temperature-sensitive geochemical indices going back 800,000 years. If preindustrial warming were due to natural causes, the Holocene trends should fit the patterns of past interglacial periods.
Instead, the team found that Holocene patterns deviate from the norm—suggesting human influence. The comparisons confirmed that gas trends during the last few millennia have been anomalous and thus anthropogenic. An interglacial period near 800,000 years ago is the best analog to the Holocene in terms of natural orbital variations. Toward the end of this analogous period, carbon dioxide (CO2) levels had decreased by 17 parts per million (ppm), but by the same point in the preindustrial Holocene, the CO2 levels had risen by 20 ppm. The anthropogenic greenhouse gas emissions necessary to explain this 37-ppm difference is very close to the 40-ppm amount originally hypothesized by Ruddiman in 2003.
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. New pollen data from Europe reveal mainly preindustrial deforestation, and archaeological data from north central China suggest major forest loss as well.
More research is needed to reveal exactly how much carbon dioxide and methane was produced by these early agricultural practices, the scientists say. It seems, however, that the argument of whether early farming emitted enough preindustrial gas to keep Earth warm has been largely put to rest. (Reviews of Geophysics, doi:10.1002/2015RG000503, 2015)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2016), Early agriculture has kept Earth warm for millennia, Eos, 97, doi:10.1029/2016EO043793. Published on 19 January 2016.