Posts Tagged ‘Younger Dryas’

Surprise: Increased Hurricane Activity During the Younger Dryas

November 28, 2017

 

The vast ice sheet that covered Canada during the last Ice Age began to melt rapidly about 15,000 years ago, creating enormous glacial lakes. The largest glacial lake, known as Lake Aggasiz, was bigger than all of the present day Great Lakes combined.  The ice dam impounding this incredible volume of water collapsed 12,900 years ago, and a massive flood of cold freshwater, icebergs, and debris gushed into the North Atlantic via the St. Lawrence River.  This event caused a sudden drop in global temperatures and a reversal back to Ice Age conditions at northern latitudes because the influx of cold fresh water shut down ocean currents that brought tropically-heated salt water north.  The cold climate phase lasted for about 1500 years, and climate scientists refer to it as the Younger Dryas.

The colder ocean of the Younger Dryas should have spawned fewer hurricanes than the warmer oceans of today.  Hurricanes are a product of energy released from warm ocean water.  However, scientists discovered evidence hurricane activity increased off the coast of Florida during the Younger Dryas.  They discovered deposits of turbidite near the Dry Tortugas Islands, dating to the Younger Dryas.  Turbidite is sediment and rock resulting from underwater perturbations.  Earthquakes can cause turbidite formation, but this region is not prone to seismic activity.  Instead, hurricanes produced underwater currents that formed turbidite here.

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Map of the Dry Tortugas–site of the study referenced in this blog entry.

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Image showing how turbidite deposits are formed.

Scientists aren’t sure why hurricane activity increased during the Younger Dryas at this locality.  Some of their climate models suggest the oceans were much colder to the north and west of the Florida coast but only slightly colder than present day ocean temperatures off the modern Florida coast.  Perhaps the tropically-heated water that pooled near the equator spawned hurricanes that reached the Florida and south Atlantic coasts.

Increased hurricane activity contributed to the expansion of longleaf pine savannahs.  The wind felled forests, and the accompanying lightning-sparked fires maintained longleaf pine savannah ecosystems while repressing closed canopy hardwood forests.  Pleistocene megafauna became extinct during the Younger Dryas, even though longleaf pine savannahs are ideal habitat for grazers such as mammoths, bison, horses, giant tortoises, and many other species.

Reference:

Toomey, M. ; et. al.

“Increased Hurricane Frequency Near Florida during Younger Dryas Atlantic Meridional Overturning Circulation Slow Down”

Geology 45 October 2017

 

 

 

 

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

July 26, 2017

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.

Image result for Antarctic ice core data average annual temperatures for last 500,000 years

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.

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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.

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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