• Increase in seasonality in the Northern Hemisphere

• Stronger Monsoons.
  

When the suns shines during the day, the land warms up more than the ocean does.

• The air over the land rises

• The rising air is replaced at the surface by inflowing air from the ocean.
  • The Monsoons are a large scale version of the same phenomena.
    • Replace "sea breeze" with "Monsoon" and "warm day" with "summer".
      

  • Monsoons affect whole continents, while sea breeze only affects coastal areas.

  • It's important to know that in the Monsoon case the winds from the ocean bring rain
    

  ---

  Why Stronger Monsoons II

  
  
  
• During winters the land cools down fast, and the oceans slowly.

• During summers the land warms up fast, but the ocean slowly.

• In early summer there is a large land- ocean temperature contrast. (the land warmer than the ocean surface)
  • This temperature contrast is the engine that drives the monsoons.

• If the winters are colder and summers warmer this temperature contrast will increase.

• If the temperature contrast increases the strength of the Monsoons increases.

  
  

  • This is why during the first half of the Holocene the Monsoons were stronger than they are now.
    

  ---

  
  

  What were the consequences of the stronger Monsoons?

  
  
  
• Much more rainfall in areas that now are deserts.

• The Sahara was mostly a savanna, and "a land of lakes"

• Similarly the Northwestern Indian deserts had greater summer rainfalls.

  Areas out of reach of the Monsoon moisture source, were dryer than presently (Asia Minor, Iran)

• Similarly in N-America the area between the Rockies and the Mississippi was dryer between 7Kyrs and 4Kyrs ago than it currently is.

  ---

  The Climatic optimum

  
  
• The climatic conditions from c.a. 9 Kyrs to c.a. 4 Kyrs ago are known as the Climatic optimum, also known as the Hypsithermal.
  • Many parts of the world were warmer than they are now.

  • Usually 1 - 2 degrees warmer than they are now.

  • Places that now are deserts were much less arid then.

  
  
• Much of this warming can be explained in terms of warmer summers and a corresponding atmospheric circulation that differed from the present one. (See the COHMAP article in your Course Package)
  

  ---

  The late Holocene Climatic decline
  

  
  
• In the latter half of the Holocene the climatic conditions have not been as warm as in the first half.

• Increasing aridity led to changes in agriculture.
  • The Ancient hydraulic civilizations in Egypt, Mesopotamia, India and China all developed in the same physical context:
    • In major river valleys in climates of low or unreliable rainfall.

    • Large scale control of the hydraulic resources.

    • Centralized organization developed in all of these places.

    
    
  • It is interesting to speculate on how much this development was preconditioned by the climatic changes.
    

  ---

  A Case study: Mesopotamia

  
  
  
• In Mesopotamia the control of the water resources was necessary.
  • Early rules ensuring water rights.

  • Rules had to be enforced by centralized power.

• The water that evaporated on the irrigated land left minerals behind.

  
  

  • Increasing salinization became a problem, and eventually the soil became infertile.

• The biggest increase in population in the area, occurred after the invention of improved water distribution techniques.
  
  

  ---

  The neoglaciation and minor climatic variations

  
  
  
• In some places the climatic decline led to neoglaciation:
  The reappearance of mountain glaciers, where they had vanished during the hypsithermal.

  
  

• Climate improved in several places (e.g. the Mediterranean) from about 2.5 Kyrs ago to 1.5 Kyrs ago (500 BC to 500 AD).
  
  (For historical context: This about the time of the Roman Empire.)

• The next thousand years were rather cold, but ended with a patchy warm climatic period, called the Medieval warm period.
  

  ---

  The medieval warm period (or the little climatic optimum)

  
  
• Patchy in space and time.
  • Some places in the world may have had warm conditions through most of the interval 400 AD to 1200AD.

  • In Northern Europe it was cold from about 650 AD to 850 AD, with the medieval warm period peaking from 1000 AD to 1200 AD.

• The transition from the cold conditions to warm conditions in the North Atlantic coincided with Norse expansion westward (to Iceland &Greenland)
  • First attempt to settle Iceland 865 AD, failed due to a cold winter (and earned the country a cold name).

  • The climate was warming so the next attempt in 874 AD fared better and led to the establishment of a permanent settlement.

  • About 100 years later when Greenland was settled it had already been warm for a century, so it was covered with vegetation (the name is not just another unfair public relation stunt).
    
    

  ---

  
  

  The little ice age
  

  
  
• After around 1200 AD the climatic conditions got colder around the northern Atlantic.

• The Norse settlement in Greenland, ended in the 15th century, when conditions were colder than they had ever been since settlement.

  
  

• Most likely a sequence of bad summer (10 - 15 in a row) were sufficient to make the settlement uninhabitable. (See Science Feb.1997)

  
  

• The period from about 1600 to 1850 has been termed the Little Ice Age.
  
  • There are record of the Thames river freezing over (England) with ice that was thick enough to allow for a fair to be held on the ice.
    

    Just as medieval warm period the little ice age was also patchy both in time and space. In some places there are no signs of the little ice age.

    • In the next lecture we will discuss the medieval warm period and the little ice age in more detail.