Transcript U3A-ClimChange06 10384KB Oct 27 2012

CLIMATE CHANGE
THE GREAT DEBATE
Session 6
HOLOCENE CLIMATE CHANGE
• The Holocene is generally taken to
begin at about 12,000 BP, following the
end of the Younger Dryas cold period
we discussed last week
• The Holocene epoch has been a period
of climatic stability compared with the
Last Glacial that preceded it
• Nevertheless, even minor changes
have had major impacts on human
activities, both in prehistoric and
historic times
The Younger Dryas was followed by rapid climate
warming with relatively stable temperatures during
the last 10,000 years
HOLOCENE CHANGES
• The warmer temperatures caused rapid
melting of the continental ice sheets in the
Northern Hemisphere and rapid retreat of
mountain glaciers elsewhere
• This led to a rapid rise in sea level until it
reached its present level about 6,500 years
ago. This had a big impact on coastal areas
due to inundation and changing coastal
currents causing localized climate changes
• Globally precipitation was much higher than
it had been during the Last Glacial
SEA LEVEL CHANGES
• They caused many continental areas to
be separated as islands
• Tasmania and New Guinea were
separated from the Australian mainland
• Britain and Ireland were separated from
Europe
• The land bridge between East Asia and
Alaska was flooded
• Populations of humans, plants and
animals became isolated
BLACK SEA CATASTROPHY
• During the Last Glacial the Black Sea
had been cut off from the
Mediterranean and had become a freshwater lake – the Euxine Lake
• About 7500 years ago the rising Mediterranean flooded back through the
Bosporus into the lake causing a very
rapid rise in water levels (80 meters)
creating the Black Sea
• Human populations around the lake
were displaced and the event probably
gave rise to the story of Noah’s flood
HOLOCENE MAXIMUM
• Minor climate changes are not
necessarily in phase between the two
hemispheres
• In the Northern Hemisphere a warm
period lasted from approx. 8000 to 4000
years ago. The Arctic was up to 3°C
warmer than today with evidence from
Greenland, Iceland and Canada
• The Sahara had a much wetter climate
with rock paintings of animals and
people found in areas that are now
desert and have been for the last 4000
years
TASMANIAN HOLOCENE
CLIMATE CHANGES
• In eastern Tasmania the climate was
mild and wet during the early Holocene
with evergreen beech (myrtle) growing
in the Derwent Valley
• From 4500 to 3000 years ago the
climate was dry and windy, fires were
frequent and fossil dunes were being
re-activated by the wind
MIDDLE EAST
• Early civilizations in the middle East
were based on irrigation agriculture
and their fortunes were strongly
affected by periods of drought
associated with climate cooling. Early
cities in the Indus valley were
abandoned about 4000 years ago
• Millennia later the Mayan Empire in
Central America also reliant on
irrigation agriculture collapsed in about
900 AD
ROMAN WARMING
• The Mediterranean climate warmed
between 250 BC and 450 AD and gave
rise to the flowering of the Roman
Empire. Grapes were drown in Britain
and olive trees in the Rhine Valley.
• The climate was wetter as well as
warmer and agriculture thrived. North
Africa became a breadbasket for the
Romans
THE DARK AGES
• It was a time of sudden cooling that
lasted from 530 to 900 AD. It caused
food shortages which led to famine,
war and disease
• The Black Sea froze in 800, 801 and 829
AD and ice formed on the Nile River
• There were large scale migrations of
people. By 590 AD an estimated 25
million people in Europe had died of
the plague
MEDIEVAL WARMING
• This occurred from 900 to 1300 AD.
Summers were warmer and longer and
crops were plentiful. The amount of
agricultural land increased and
extended to higher altitudes and
latitudes.
• In the Middle East the Muslim Empire
reached its zenith and the Vikings
prospered in Scandinavia and
colonized Iceland and Greenland and
even reached North America
LITTLE ICE AGE
• It lasted from 1300 to 1850 AD. It is a
misnomer because it was not an ice
age or even a glacial but simply
another period of unusually cool
climatic conditions. Glaciers advanced
all over the world.
• The Vikings were cut off from their
colonies in Greenland and Iceland by
sea ice and the Greenland population
died out between 1420 and 1500 AD.
Iceland’s population suffered famine
Little Ice Age
temperature
fluctuations
(estimated) in
Iceland from
950 to 1980 AD
LITTLE ICE AGE
• After a brief warming during the first
half of the 16th Century, the second
phase from 1550 to 1850 AD was even
colder than the first phase.
• Temperatures in Europe were lower
than at any other time since the Last
Glacial. Much agricultural land at
higher latitudes and higher altitudes
was abandoned.
• Glacial ice in the Alps reached its
maximum extent in the first half of the
19th Century
Holland in the Little Ice Age – a 17th Century
Painting by Hendrik Averkamp
Little Ice Age winter scene in Holland painted
by Van der Keer. Source: Brian John in “The
Winters of the World”
Most recent temperature reconstruction is in
dark blue, the oldest is in red
RECENT WARMING
• Since 1850 world climates have
warmed significantly and most glaciers
are retreating rapidly
• The climate is now similar to what it
was in the Roman and Medieval warm
periods but probably not quite as warm
as it was in the early Holocene
• The warming is generally blamed on
the increase in greenhouse gases in
the atmosphere but we must be aware
that the recent warming is not
unprecedented
Global Warming from 1880 to 2003
Athabasca Glacier in retreat, Canadian Rockies
Mueller Glacier in retreat, New Zealand
Retreating Hooker Glacier and meltwater lake