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Abrupt
Shifts in Weather and Climate Regimes Have Occurred in Human History
 We
are accustomed to thinking of climate change as a gradual, millenia-long
process of slow shifts in temperature and rainfall. Nevertheless,
abrupt climate change has been a fact of history. Individual countries
or entire regions have experienced rapid climate changes that have
perturbed agricultural production, disrupted the continuity of fresh
water supplies, and forced large populations to migrate. While it
is rarely a sole cause of political collapse the wide-scale dislocations
that climate changes can engender have often destabilized political
regimes in the course of human history.
In
recent American history, the conditions of the "dust
bowl" appear now to have been engendered by a climate anomaly
that NASA scientists are beginning to identify as a pattern related
to changes
in tropical sea surface temperatures. Local farming practices
-- including the overplowing of land, the destruction of wind-breaking
tree cover and the overtapping of groundwater -- may well have contributed
to the severity of the dust bowl conditions, but the driving force
behind this abrupt shift in weather may have been a climate phenomena
whose origins were thousands of miles away.
During
the 1930s the drought and dust bowl conditions contributed to the
mass migration of American farmers to California, and the impact
of the dust bowl was considerable in both economic and political
terms in American history. Since the 1930s many more people in the
world have come to depend upon the seemingly stable and continuous
flow of grain surpluses generated in the American mid west. If prolongued
drought were to return to this mid-continental region -- for whatever
reason -- the social impact would be enormous in scope, contributing
to major strains to an international food and grain distribution
system upon which China and many other countries in the Third World
have come to depend. |
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Further readings:
| Spencer
Weart |
2003 |
"The
Discovery of Rapid Climate Change," Physics Today,
Vo. 56, 8, (2003),p. 30 ff.
Only within the past decade have researchers warmed to the possibility
of abrupt shifts in Earth's climate. Sometimes, it takes a while
to see what one is not prepared to look for. |
Changes
to the Thermohaline Circulation (THC) System in the Atlantic and
the Mediterranean Weather Delivery System
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There
have been several partially documented abrupt climate shifts
in the Holocene at 8.2 kyrbp (8.2 thousand years before the
present), 5.2 kyrbp and 4.2 kyrbp. The latest of these --
the 4.2 kyrbp climate "event" -- seems now to have
been implicated in the dramatic, swift and devastating collapse
of the northern Sumerian empire and the subsequent southern
migration of large numbers of people into Babylonia.
This
phenomena has been studied in depth by Professor Harvary Weiss
of Yale University. See particularly:
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and the lecture
by Professor Weiss to
the 2005 Class of ENVR E130, the
research for which is scheduled to be published
in December 2006 as a book entitled:Collapse.
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[Click Book Cover to Access Lecture]
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It is
not clear what the causal connections are between the thermohaline
circulation (THC) system and the Medterranian system that
controlled the climate of ancient Mesopotamia. Nevertheless,
NASA evidence (see picture from space, above) suggest that
there is a systemic link between the Mediterranean weather
delivery system and the climate system in the northern Atlantic.
In the light of that interdependence it is important to keep
the dynamic of this west-to-east moving system clearly in
mind when you "click" on the following two simulations.
Initially, some observers thought the 4.2 kyrbp event might
have been caused by volcanic activity in the Anatolian peninsula
(Turkey), but Harvey Weiss pointed out that tephra events
are not normally associated with century-scale cooling or
century-scale aridification. It now appears from evidence
that Professor Weiss has amassed that this event is reflected
in ice cores from glaciers and ice caps around the world.
This suggests that this "local collapse" in Tell
Leilan may have been a manifestation of a global climate anomaly,
perhaps linked to something like a temporary shift of the
THC. None of this is proven, but it will remain a very important
element of paleoclimate investigation for years to come.
Animation:
A simplified model of global ocean circulation] Abrupt Climate
Change from: "Abrupt Climate Change: Should We Be
Worried? - Prepared for a panel on abrupt climate change at
the World Economic Forum Davos, Switzerland, January 27, 2003,"
Ocean and Climate Change Institution - Woods Hole Oceanographic
Institution, (27 January 2003).
and
Animation:
"If too much fresh water enters the North Atlantic..."
Abrupt
Climate Change: Should We Be Worried? - Prepared for a panel
on abrupt climate change at the World Economic Forum Davos,
Switzerland, January 27, 2003," Ocean and Climate Change
Institution - Woods Hole Oceanographic Institution., (27 January
2003).
Click
on and observe these animated sumulations and think through
the logic of what might "flip" in the Atlantic thermohaline
system if certain thresholds in temperature and Greenland
ice sheet melting (as yet not fully understood) were to be
exceeded. This raises the question:
What relevance does holocene climate history have
to our contemporary circumstance?
See:
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Videos
of Class12 Lectures
