|Fig.1. AWI Core Repository (Credit: Hannes Grobe/AWI, CC-BY-SA-2.5)|
In 2003, paleoclimatologist William Ruddiman at the University of Virginia hypothesized that early humans significantly altered the climate by burning large areas of forests to clear land for farming and grazing. The greenhouse gases released into the atmosphere - mainly carbon dioxide and methane - halted a natural cooling cycle and possibly prevented another ice age.
This argument is based on ice core record data that show a spike of atmospheric carbon dioxide and methane levels in early interglacial periods and then a gradual decline into another ice age. Unlike previous interglacials, Ruddiman found that the current Holocene actually saw a sustained increase in atmospheric carbon dioxide and methane levels coinciding with early large-scale agriculture. This idea, called the early anthropogenic hypothesis, is controversial, and critics cite issues with ancient methane emission records and very small population sizes in prehistoric times.
|Fig.2. Slash and burn agriculture in modern day Amazon |
(Credit: Matt Zimmerman via Flickr, 2007)
Ice Cores and IrrigationIn a recently published article in the journal Reviews of Geophysics, Ruddiman and colleagues from around the world dug deeper into this issue with Antarctic ice-core records dating back to 800,000 years ago. They compared these records to archaeological evidence and ancient pollen samples to understand climate shifts during the time of early humans.
The researchers found that slash and burn techniques widely used by early farmers catalyzed an increase in atmospheric carbon dioxide levels 7,000 years ago and a similar rise in methane levels around 5,000 years ago. This method of farming is unsustainable, requiring farmers to move on to new forest and repeat the process as soil quality becomes too poor to support crops. Such techniques harm the soil quality, cause large-scale erosion, and contribute to global carbon emissions.
Despite small population sizes at the time, Ruddiman et al. argue that the area of land necessary to sustain slash and burn practices would have been large enough to significantly affect atmospheric levels of carbon dioxide and methane. They also cite recent research that suggests early rice irrigation and livestock farming were the culprits of the abnormally sharp rise of atmospheric methane levels around 5,000 years ago.
|Fig.3. 19 cm long section of GISP 2 ice core from a depth of 1855m, showing eleven annual layers with the lighter summer layers designated by arrows (Credit: NOAA/U.S. Army Corps of Engineers, 2007)|
Evidence in IceAlthough controversy remains about the full effect of early human agriculture, there is little argument among scientists that humans have an outsized impact on their environment. Ruddiman and his team are part of a larger group of scientists who examine ice core records and other deep-time environmental samples to study the world before and during human activity.
Snowfall that collects on glaciers - and ultimately becomes packed into ice - traps pollutants, gas bubbles, pollen, and other organic traces. In 2013, researchers found surprisingly high methane emissions and pollutants recorded in Greenland ice-cores starting around 100 B.C. They hypothesized that particles from metallurgy and greenhouse gases from large-scale agriculture around this time were caused by ancient Roman and Chinese civilizations. Stable isotope ratios of water reveal nuances in temperature through the years and sharp peaks in sulfate have even been used to accurately date volcanic eruptions. Ice core records are essential for our understanding of the ancient world and contribute to ongoing research on how humans affect their environment.
Ruddiman, W. F., et al. (2015, December 29). Late Holocene Climate: Natural or Anthropogenic? Reviews of Geophysics, 53. doi: 10.1002/2015RG000503
Tollerfson, J. (2011, March 25). The 8,000 year-old climate puzzle. Nature. doi: 10.1038/news.2011.184.
Wade, L. (2015, February 9). Ice core suggests humans damaged atmosphere long before the industrial revolution. Science. doi. 10.1126/science.aaa7829