Monday, June 30, 2014

The Great Warming

Recent decades have been a golden age for archaeologists.  New technology has provided tools for better understanding the past.  Researchers can now identify the climate trends of past centuries by analyzing the layers in tropical coral, tree rings, glacial ice packs, and lakeshore and seabed sediments.
Climate has played a primary role in influencing the course of human history.  It could enable the rise of mighty empires, and later reduce them to dusty ruins.  Big changes can happen suddenly, without warning, and have devastating effects.  Mighty scientists may huff and puff and stamp their feet, but climate will do whatever it wishes.
In 2000, archaeologist Brian Fagan published The Little Ice Age.  This book examined an era of cooler weather spanning from 1300 to 1850, and its effects on northern Europe.  In those days, most folks lived from harvest to harvest, with few safety nets.  In 1315, it barely stopped raining, and the heavy rains continued through 1316 and 1317, followed by horrendous weather in 1318.  At least 1.5 million folks checked out.  The famine of 1344-1345 was so extreme that even the super rich starved.
Preceding the Little Ice Age was the Medieval Warm Period, which spanned from 800 to 1300.  Fagan described this era in The Great Warming, published in 2008.  Far less was known about this time, because fewer written records have survived.  But new climate data has been filling in a number of missing pieces, revealing many forgotten events, important stuff.
When it was in the mood for mischief, the Little Ice Age was a harsh bully.  Fagan had expected the warm period to be the opposite, and in some regions, it was, sort of.  In Europe, there were fewer late frosts, and the growing season was three weeks longer.  There were vineyards in England and southern Norway.  Surplus wealth enabled the construction of grand cathedrals.
Whilst the weather was rather pleasant, the era suffered from a devastating spasm of innovation.  The diabolically powerful moldboard plow, which was able to turn heavy soils, replaced the primitive scratch plow.  A new harness allowed horses to replace pokey oxen as beasts of burden.  The new three-field fallowing system enabled two-thirds of the fields to be growing crops every year, instead of just half, with the old two-field system.
By using these new technologies, vast regions of highly fertile heavy soils could now be converted into highly productive cropland.  The only obstacle was the vast ancient forests, and their untamed wildlife.  Loggers grabbed their axes and exterminated more than half of Europe’s forests between 1100 and 1350.
Expanded cropland area, combined with a balmy climate, produced much more food, and this always resulted in a mushrooming mob.  Between 1000 and 1347, the population of Europe grew from 35 to 80 million, despite short life expectancies.  It got so crowded that folks in 1300 were worse off than their grandparents in 1200.
In other regions, the warm period brought unpleasant weather.  The Mayans of the Yucatan lowlands experienced extended droughts and abundant misery.  “Hot, humid, and generally poorly drained, the Maya lowlands were a fragile, water-stressed environment even in the best of times,” Fagan observed.  “It’s hard to imagine a less likely place for a great civilization.”
The Mayan city of Tikal may have had 300,000 residents.  It was entirely dependent on rainfall for water.  Their ecosystem did not have dependable sources of water, like rivers or underground aquifers.  They developed amazing systems for storing rainwater, and these worked really well, usually, but not during multi-year droughts.  The drought of 910 lasted six years, and generated social unrest, which led to the collapse of many Mayan cities.
At the same time, severe droughts in western North America followed similar patterns.  Irrigation systems at Chaco Canyon enabled more than 2,000 folks to survive in an arid region for several centuries.  This worked well in wetter years.  After 1100, droughts intensified, and within 50 years the city was abandoned.
California was home to hunters and foragers.  Acorns were half of the diet for many tribes.  Oaks could produce as much food per acre as medieval European farms, and foragers could acquire a year’s supply in several weeks.  Fewer acorns fell in drought years, and extended droughts killed the oak trees.
Stumps at Mono Lake indicate that a severe drought began in 1250 and lasted for over a hundred years.  Fagan noted “None of today’s droughts, which last as long as four years, approach the intensity and duration of the Medieval Warm Period droughts.”  He called them megadroughts.  They baked away the surface waters and soil moisture.
The Yellow River (Huang He) has an appropriate nickname, China’s Sorrow, because it is one of the world’s most trouble-prone rivers.  Fagan said “the Huang He basin [has] been a crucible for human misery for more than seven thousand years.”  About 45 percent of the Chinese population lives in the basin.  From year to year, precipitation can vary by 30 percent.  A dry June is a bad omen.
To reduce the risk of famines, the Chinese built complex irrigation systems, which the Yellow River enjoyed burying with silt.  The yellow loess soil of the region was highly fertile, easy to till, and 200 feet deep (61 m) on average.  It was also light and easily erodible.  Once upon a time, forests held the soil in place, but deforestation* had catastrophic consequences.  The river carried an enormous load of yellow silt downstream, and this created perfect conditions for disastrous floods, which have killed many millions over the centuries.
This region has long been a spooky place to live, but the warm period was worse, “a time of violent climatic swings nurtured thousands of miles away that brought either lengthy dry cycles or torrential rainfall that inundated thousands of acres of the Huang He basin.”  (An extreme nineteenth century drought is described in Late Victorian Holocausts.)
Today, the global climate is hotter than the Medieval Warm Period.  The warming trend has been steadily building since 1860.  Glaciers are melting and folks are getting increasingly nervous about rising sea levels.  While this is indeed a bummer, Fagan warns that extended drought is a far greater threat.  Extended drought withers agriculture, toasts pastures, and dries up lakes and rivers.  Seven-point-something billion people will be extremely vulnerable when we move beyond Peak Food, and into the climate surprises of the coming decades.
Fagan, Brian, The Great Warming — Climate Change and the Rise and Fall of Civilizations, Bloomsbury Press, New York, 2008.
* In the 1930s, W. C. Lowdermilk, of the Soil Conservation Service, visited northern China as part of a research project.  In Shensi province, he saw an ancient irrigation system destroyed by silt, which had washed down from the uplands, where erosion gullies were up to 600 feet deep (183 m). 
He published his findings in Conquest of the Land Through Seven Thousand Years.  In this document, Figure 7 is a photo of serious erosion.  The caption reads: “A severely gullied area in the loess hills of North China.  These hills were once covered with trees and grass; but cultivation started the ruinous process of erosion.  There are thousands of acres like this in China today.  It produces nothing except yellow mud to clog the Yellow River with silt.”  For scale, note the human in the foreground.


Riversong said...

Nice exposition into previous climate catastrophes, but for one statement which veers dangerously close to the language of AWG denial: "climate will do whatever it wishes".

Today, there is a 97% scientific consensus that human activity is the primary driving force for climate change. It is apparent that human civilization has reached that critical mass tipping point at which the climate no longer "does whatever it wishes".

What Is Sustainable said...

Riversong, what I meant was that the climate is out of our control. All of our science and technology is powerless to defuse the monster we have created. Even if we quit causing carbon emissions today, forever, the monster is out of the bag, and we shall reap what we have sown.

What Is Sustainable said...

Here is a 50 minute video of Fagan delivering a charming lecture on The Great Warming.