Transcript U3A-ClimChange08 7593KB Nov 08 2012

CLIMATE CHANGE
THE GREAT DEBATE
Session 8
MORE CLIMATE VARIABILITY
ENSO
• ENSO stands for El Nino – Southern
Oscillation
• Short term cyclic climate changes are
generated by periodic changes in the
circulation of surface waters in the
Pacific Ocean
• It effects are worldwide but are
particularly strong around the Pacific
including Australia
SOUTHERN OSCILLATION
• It was discovered by Sir Gilbert Walker
early in the 20th Century
• He showed that there was a negative
teleconnection in climate between SE
Asia and Northern Australia on the one
hand and the Eastern Pacific on the
other
• When one area had high pressure the
other had low pressure. High pressure
was associated with drought and low
pressure with heavy rain
SOUTHERN OSCILLATION
INDEX (SOI)
• The index was defined as the sea level
ratio between the average air pressure
at Tahiti and the average air pressure at
Darwin
• A negative ratio means that Darwin is in
drought and Tahiti has wet conditions
When the ratio is positive it is the other
way round
Map shows areas most strong affected by the
ENSO phenomenon. Weak effect on Tasmania
EL NIÑO PHENOMENON
• It occurs off the coast of Peru and is
related to the Southern Oscillation
• Normally there is a cold ocean current
(Humboldt Current) flowing north along
the coast. It provides rich fishing
grounds, especially for anchovia
• From time to time cold water is
replaced by warm water from the west
causing the collapse of the fisheries
EL NIÑO PHENOMENON
• The change often happens around
Christmas. The name is Spanish for the
Christ Child
• The warm water creates instability and
can cause heavy rain in a normally very
dry coastal area in Peru
• Six months later NE Australia enters a
period of extreme drought because of
low SST’s for the western Pacific
ocean
LA NINJA PHENOMENON
• This is the opposite extreme with warm
surface water off the Queensland coast
and cool surface water over eastern
Pacific
• NE Australia will be wetand the eastern
Pacific will be dry
• The ENSO cycle is very irregular but
has an average five year interval. It is
very difficult to predict for more than
six months in advance
It shows the weather pattern associated with the
La Ninja extreme. Low pressure and wet conditions
over NE Australia and drought in the Eastern Pacific
SOI is based on the relative air pressures at Darwin
and Tahiti. Blue peaks indicate wet periods in NE
Australia and red peaks indicate drought periods
EXTENDING THE RECORD
• The instrumental record of climate
change in NE Australia is only a short
one
• Scientists have turned to proxy records
to obtain a much longer record
• Some corals, such as species of
Porites, form massive colonies that
grow continuously for periods of up to
one thousand years
• They also grow quite fast
CORALS
• Corals can be drilled underwater to
obtain a core which is cut in half to
expose the annual growth layers
• Peter Isdale of AIMS at Townsville
discovered that some coral cores
contained layers that fluoresced under
UV light. These bands contained
organic compounds that entered the
Barrier Reef when the Burdekin River
was flooding
• They provide a record of La Ninja
conditions in NE Australia
X-ray picture of
cross-sections of
corals shows
annual banding
Record of fluorescent bands
from 1922 to 1984
in coral core taken from
Pandora Reef
Intensity of fluorescence
bands is shown on the right
hand side and record of
flood events of the Burdekin
River on the left hand side
Particularly intense
fluorescence bands are
marked in the centre of the
diagram
Annual run-off reconstructed for Burdekin River
from coral fluorescence. It shows a very wet period
from 1650 to 1760 – a dominance of La Ninja
conditions during the Maunder Minimum
TRACE ELEMENTS IN CORAL
• Coral is made up of the mineral aragonite
(CaCO3). It is deposited by coral polyps
and incorporates small quantities of
beryllium, magnesium, strontium and
uranium
How much of these elements is included
depends on the temperature
It means that the coral record can be used
as a palaeo thermometer. It can provide a
detailed record of seasonal temperature
variations from year to year
Graphs show
relationships
of trace
elements to
temperature
Quarterly measurements
over eight years show
close agreement
between SST, strontium
ratios and oxygen
isotope ratios
The last two can be
used as proxy data for
changes in SST
ISOTOPE RATIOS
• Oxygen and carbon isotope ratios
O18/O16 and C13/C12 can also be used
as proxy data to provide more
environmental data
• Oxygen isotope ratios, like some trace
elements, can also be used as proxy
data for past temperature variations
Oxygen and carbon isotope records from Pandora
Reef. Intensity of fluorescence indicated by shading.
Black bands indicate major floods which affect the
seasonal patterns of the isotope records