What Causes the Jet Stream to Change its Course?

Usually changes in “normal” weather patterns reflected in the movement of the jet stream are short lived. However, sometimes the jet stream moves north or south for more extended periods of time. So what can make the path of the jet streams change?

Remember that it is the sun's heating of the earth, of its land, and its water that drives the weather. To change the weather then, it is necessary to change the absorption and distribution of the sun's energy.

There are two basic ways to do this:

1) Change the amount of solar radiation that reaches the earth or

2) Change the amount of sun's energy that radiates back into space.

Clouds, for example, block the sun’s rays from reaching the ground by reflecting the sunlight back into space. We all have experienced the cooling effects of clouds on a warm summer day. Clouds also prevent the heat radiating back from the earth from reaching outer space. As any farmer knows, cloud cover can prevent a very cold winters night from reaching freezing temperatures. Because cloud cover has both a cooling and a warming effect the net effect on the overall climate is nil.

Dust in the upper atmosphere also reflects sunlight back into space but unlike clouds its net effect is anything but nil. Dust tends to cool the high latitudes more that those at the equator. Remember that the path of the sunlight that reaches the poles is longer than that at the equator. If the atmosphere is dusty, the sunlight has a longer path through the dust and more of the sunlight is reflected back. This of course increases the temperature differentials at the boundary between Arctic and equatorial air and therefore increases the force of the winds and the strength of the storms. One of the most important sources of atmospheric dust is violent volcanic explosions. These explosions can throw tremendous amounts of debris high into the atmosphere which then circulate around the globe for years and are believed to affect the climate all over the globe.

The correlation of volcanic eruptions and climate has been made relatively recently although the consequences of such eruptions have been recorded, albeit unwittingly, throughout history. For example, Roman poets tell us of the eruption of Mt. Etna in Sicily in 42 BC and at the same time an historian in China writes of a "veiled and indistinct" sun and crop failures. More recently, Benjamin Franklin wrote in May 1784 of a constant fog over all Europe during the summer months of 1783 and the most severe winter in Europe for many years. He knew about volcanic eruptions in Iceland about that time and speculated that there might be a connection but he did not know of the eruption in Japan in the 1783. There were crop failures and poor harvests in France during this time that are believed to have contributed to the social unrest preceding the French Revolution in 1789. In June 1991 the dust from the eruption of Mt. Pinatubo in the Philippines more than doubled the normal amount of dust in the air. The dust cooled the climate, brought spectacular sunsets to Hawaii, caused ash induced damage to jet plane engines, and some believe, disturbed weather patterns in southern Africa. In 1992 and 1993 Mozambique, Zimbabwe, and South Africa suffered a drought which withered crops and caused a food shortage throughout the region.

In his book Catastrophe, D. Keys believes there is strong evidence that in 535 AD there was a major volcanic eruption that devastated the climate all over the world. Its location was mostly likely in Indonesia between the islands of Sumatra and Java where the notorious volcano of Krakatoa is located. The climate disruption from this massive eruption lasted for 20-30 years and affected every know civilization of the time. If this is true, and it probably is, this was the final blow to the Roman Empire.

For many places in the world, recovery did not come until 800 AD or later. But this recovery was short lived as the global temperature fell again with devastating effects throughout the world.

Ice cap studies together with historical records and tree ring growth data showed that there was a long time of increased volcanic activity from 1250-1500. This was exactly at the time of the Bubonic Plaque in Europe, the end of the Mississippi culture (the Mound Builders) in North America, and demise of the Viking colonies in Greenland and Iceland. It was the beginning of another period of global cooling that became what scientists call the Little Ice Age.

Next Post: After the Fall of Rome

Different Weather for Different Places

Let’s summarize what we know about the connection between the world’s climate and the rise and fall of human civilizations so far.

When the Last Ice Age of the Pleistocene ended around 10,000 BC, the world’s average temperature rose slowly, then between 6000 and 3000 BC average temperature increased at a slightly faster rate. By 3000 BC, global temperature was about 2 degrees C above the temperature of 6000 BC. Then from 3000 BC to around 1000 BC, the global climate began to cool. By 1000 BC, the global average temperature was about the same as it had been in 6000 BC. After 1000 BC, the average global temperature began to become warmer again.

It appears that the warming climate after the last ice age and an increasing human population precipitated a change from food gathering to food production in many regions of the world. By 3000 BC, people all over the world had domesticated plants and many began to settle in permanent farming villages. The cooling temperatures after 3000 BC, however, had different effects depending on where these farmers were located.

The cooling weather weakened the jet stream and in some places that meant the rainfall became more inconsistent. In the Near East, state governments formed to conserve water and grow more food to feed their growing population. During this time, the changing climate had little effect on the social organization in China. Society remained one of small villages and chiefdoms. By 2200 BC however, the wanderings of the weakened jet stream brought increased droughts, fewer monsoons, and the “east wind” to the Near East. It also brought increasing rains to Northern China.

Around 2200 BC a state society evolved in China to control the flooding caused by the increased rain. As we learned earlier, while agriculture failed in the Near East, it grew in China. As the influence of the Zhou increased southward, rice became important to the Chinese economy along with millet and their carrying capacity rose. In the Americas, the changing weather must have allowed for the growing of surplus food and farming villages of the Olmec and the Chavín grew into state societies. While in the Mediterranean, civilizations fell apart during the time period between 1200 and 700 BCE.

Then around 750 BCE, the global temperature began to rise again. As the global climate warmed, the path of the jet stream and its patterns of rainfall would have changed again. During this same time period China, the Chavín and the Olmec fell into crisis and by 200 BC the civilizations of the Chavín and the Olmec were a memory. Between 700 and 200 BC the state government of China could no longer maintain control and the country became segmented and ruled by warlords.

Civilization along the Mediterranean however, began to recover. Around 200 BC Rome emerged as the “greatest power the West had ever known” but by 450AD, the glory of Rome was over. Between 200 and 600 AD the global climate was cooling. This means that the paths of the Jet Stream were unstable yet again.

Next Post: What Causes the Jet Steam to Change Its Course?

Rust Again

In 700 BCE, when Rome was only a few farming villages along the Tiber River, the Roman farmers knew well of this rusting sickness that made their wheat useless for food or for seed. To stop this plague the priests of primitive Rome began to placate a new god. His name was Robigus, the god of rust. The ceremony that evolved to calm the anger of Robigus was held during the Spring Festival in the third week in April and included the sacrificing of a red dog, the pouring of red wine, and the following prayers:

Spare our crops, O Robigus.
Is it not enough that thou couldst harm them?
Accept, O mighty god our sacrifice.
Take it, O Robigus, and not our wheat!
Drink the red wine O god!
As we stand humble before thee

Spare us from the baleful glare of Sirius the dog star.
Still the blasting winds from the eastern sky.
Grant us our crops, the labors of our hands,
And we in holy gratitude
Will honor thee forever more.

Quoted from Famine in the Wind by G. L. Carefoot and E. R.Sprott

Did you catch it??? Still the blasting winds from the eastern sky.

In addition it must be remembered that the territory of the Roman Empire by 117 CE included all the land surrounding the Mediterranean Sea, north and south, including Egypt (see map below). And by 200 CE most of Rome’s food was being shipped from North Africa.

The soil of Africa has been given over by nature entirely to Ceres [goddess of agriculture and especially grains]; the oil and the vine have almost been refused; all the glory of the country is in its [wheat] harvest.

Pliny the Elder, a Roman scholar.

During the time of the Roman Empire, as in the time of the New Kingdom in Egypt, grain harvests in the Mediterranean were vulnerable to the whims of the region’s temperamental weather. These whims include winter floods that inundated newly-sown fields, scorching dust-laden winds that blew from the South (siroccos) that could cause the wheat kernels to shrivel before attaining maturity, clear skies too early in the spring that portended severe frost damage, and the east wind. As before, the east wind meant moisture. An east wind during the growing season meant wheat plants soaked with water, which enabled the spores of the wheat rust fungus to germinate and infect their hosts. Although many farmers knew there was some connection between wet weather and bad rust years, there was not much they could do about it except pray to Robigus.

Historical records and studies of growth rings of the old trees on both the African and European shores of the Mediterranean indicate that from 100-300 CE the climate was unusually wet. We can only conclude that the crops of the Roman Empire must have been ravaged by rust during the later part of the Empire. The three components for an epidemic were present, the plant (wheat), the pathogen (the rust fungus), and wet weather. The destruction of the food supply caused by rust probably contributed to the “Crisis of the Third Century”.

Like all other civilizations before and after, the economy of the Roman Empire, the defense of the Roman Empire, and therefore the survival of the Roman Empire were tied to agriculture. And if you are dependent on agriculture you are dependent on the weather - too dry and you lose your food source to drought - too wet and you lose it to flooding or disease.

Next Post: Different Weather for Different Places

Rome and More

In the Mediterranean, the time period between 1200 and 700 BCE is known as the Greek Dark Ages. Basically civilization there fell apart. Then around 750 BCE, civilization along the Mediterranean began to recover. Notice on the climate graph that the global temperature was rising.

First Mesopotamia evolved into the Persian Empire (612-330 BCE), then Greece recovered (750-146 BCE) and final the Roman Empire took over the entire Mediterranean (264 BCE-235 AD). By 450AD however, the glory of Rome was over. What happened?

Today, most historians would concede that no single factor can be blamed for the decline of this great civilization. As with all “extinctions” many things contributed to its fall. But perhaps one of the most interesting things about this period is that Rome was not the only empire to decline during this time.

Rome was not alone in its plight. In the years between 200 and 600 AD, empires were under siege all across the broad civilized belt that stretched from Rome in the west to China in the east.

Empires Besieged by the Editors of Time-Life Books

The world has grown old and lost its former vigor ... the mountains are gutted and give less marble, the mines are exhausted and give less silver and gold ... the fields lack farmers, the sea sailors, the encampments soldiers ... there is no longer any justice in judgments, competence in trades, discipline in daily life ... the Day of Judgment is at hand.

Cyprian, Bishop of Carthage, c. 250 AD

In China, the Han Dynasty, which had taken control of China sometime after the Warring States period and who had ruled successfully for nearly 400 years, fell apart and Chinese society was again plunged into a series of vicious civil wars. The political instability and conflict lasted from 189-589 AD. Nomads from the north (the Mongols and the Huns) took advantage of this turmoil, moved in, and took over.

By 500 AD the Gupta Empire in India (which included Indus) and New Persian Empire (which included Mesopotamia) were also toppled by the pressure of barbarian invasions. Like the time between 1200-1000 BC, there seemed to be a “general crisis of the civilized world”. This time the crisis also included China and again the “uncivilized” people from the Eurasian Steppes and the deserts of Arabia and North Africa were on the move. The previous crisis of 1200 BC was correlated with a changing global climate. Was there a climate change this time as well?

If you look back at our climate graph, you will see that indeed there was a climate change from 200-600 AD. For the first 200 years of Imperial Rome the climate was stable, then around 200 AD, the temperature began to drop. You will notice that the magnitude of the drop is not nearly as dramatic as that when the Near East Civilizations declined. Yet the average global temperature around 400 AD is about the same as it was around 2200 BC, the beginning of the 300 year drought in Mesopotamia. This probably means that the northern jet stream had moved enough to cause the same general effects that it had before.

The first people that would have felt the effects of these climate changes would have been the “uncivilized” nomads of the North and the South, Rome’s, India’s, Persia’s, and China’s “barbarians”. These people lived in very fragile ecosystems with minimal carrying capacity. In addition, there were a lot more people in both the civilized and uncivilized worlds in 200 AD than there were in 2000 BC. In 2000 BC the world population was only about 25 million people. In 200 AD there were over 200 million people. In both the civilized and uncivilized worlds, the populations were at or were approaching the carrying capacity of their respective environments. Even a relatively small decrease in the global climate would have lowered that carrying capacity enough to send the nomads in search of greener pastures, literally. The nomads that took over China, for example, turned farmland into pastures for their livestock. For the “civilized” farmers, the decreasing temperature would have meant decreasing yields and for the farmers of the Roman Empire, it must have meant rust epidemics.

Next Post: Rust Again