The Younger Dryas Cold Phase may have been Exacerbated by Megafauna Extinctions

In my previous blog entry I explained how Pleistocene megafaunal extinction impacted ecosystems, but some scientists hypothesize the loss of megafauna influenced atmospheric conditions as well.  The existence of enormous ice caps during Ice Ages caused extremely unstable climate conditions as the below temperature graphs illustrate.  The climate alternated between warm phases known as Dansgaard-Oeschger Events and cold phases referred to as Heinrich Events.  The onset of these patterns was often sudden occurring within decades, though some cold phases occurred gradually.  The fluctuations were interrelated.  Dansgaard-Oeschger Events melted glaciers and eventually released too much cold fresh water into oceans, shutting down ocean currents that carried tropically heated water to northern latitudes.  Colder oceans caused temperatures on adjacent continental land masses to drop. The Younger Dryas, a cold phase that began 12,900 years ago, was an exaggerated Heinrich Event.  Scientists, led by F. A. Smith, a professor at New Mexico University, propose the collapse of megafauna populations in North and South America contributed to the severity of the Younger Dryas stadial.

Ice core data from Antarctica illustrates fluctuations in climate over the past 500,000 years.  The brief but severe Younger Dryas cold snap can’t be seen on this chart, but supposedly it was an anomaly compared to other fluctuations.

Ice core data from Greenland showing fluctuations in climate over the past 23,000 years.  Within decades a warming climate phase reversed, and average annual temperatures matched the coldest of the preceding Ice Age.

Large populations of megafauna produce immense quantities of manure–a source of methane (CH4), an important greenhouse gas.  Megafaunal populations collapsed shortly before the Younger Dryas began, so perhaps without the mitigating effect of this manure-sourced methane temperatures dropped further than they would have, if these animals had still been present in the environment.  Moreover, more forest replaced grasslands because there were no megaherbivores suppressing tree regeneration.  Trees help reduce CO2, another greenhouse gas.  Today, methane produced by increasing populations of livestock combined with deforestation contribute to an increase in greenhouse gas concentration and global warming.

Modern day livestock contribute to greenhouse gas emissions.  A new hypothesis suggests the extinction of Pleistocene megafauna in North and South America contributed to the severity of the Younger Dryas cold phase when average annual temperatures suddenly plummeted to levels not seen since the Last Glacial Maximum.

Reference:

Smith, F.A.; S. M. Elliott and S.K. Lyons

“Methane Emissions from Extinct Megafauna”

Nature Geoscience 3 (6) 2010

 

Tags: methane, Pleistocene climate fluctuations, Younger Dryas