CLIMATE CHANGE

THE GREAT DEBATESession 8

MORE CLIMATE VARIABILITYENSO

• 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 conditionsover 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 ofcorals showsannual banding

Record of fluorescent bandsfrom 1922 to 1984in coral core taken from Pandora Reef

Intensity of fluorescence bands is shown on the right hand side and record of flood events of the BurdekinRiver on the left hand side

Particularly intense fluorescence bands aremarked in the centre of thediagram

Annual run-off reconstructed for Burdekin River from coral fluorescence. It shows a very wet periodfrom 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 uraniumHow much of these elements is includeddepends on the temperatureIt 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 showrelationshipsof traceelements totemperature

Quarterly measurementsover eight years show close agreement between SST, strontiumratios and oxygenisotope ratios

The last two can beused as proxy data forchanges 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 PandoraReef. Intensity of fluorescence indicated by shading.Black bands indicate major floods which affect the seasonal patterns of the isotope records