Markus Stowasser: The period from late November 2010 to mid January 2011 was extremely wet in most regions of eastern Australia. Six major rain events affected large parts of the eastern states during this period. This resulted in widespread flooding on many rivers, culminating in severe flooding in Brisbane and nearby areas during the second week of January. This flooding, in terms of extent, impact and severity, was amongst the most significant in Australia's recorded history.
Flooding in Australia
Mr. Stowasser, can you explain the reasons for the extreme flooding in Australia?
It was the wettest December on record for Queensland and for eastern Australia as a whole. This was followed by an extremely rainy spring, the wettest on record for some regions, meaning many catchments were already humid before the flooding rain. The rains of late 2010 have taken place during a strong La Niña event in the Pacific Ocean - one of the four strongest events of the last century. Previous strong La Niña events, such as those of 1973/74 and 1955, have also been associated with widespread and severe flooding in eastern Australia.
Markus Stowasser: "This flooding, in terms of extent, impact and severity, was amongst the most significant in Australia's recorded history"
What is La Niña?
Stowasser: La Niña is part of a natural climate oscillation in the tropical Pacific. It oscillates between the warm El Niño phase and the cold La Niña phase. La Niña is a purely natural event and occurs every few years as part of this natural oscillation. The effect of this is to contain the warmest waters in the western Pacific which then drives a stronger than normal monsoon. Hence, during La Niña, the rainfall that normally falls out over the Pacific shifts west over Indonesia and indeed northern and eastern parts of Australia. So the fact that recently there have been lots of floodings in Queensland is very consistent with the occurrence of near record La Niña this year.
Climate change predictions show a clear intensification of extreme rainfall events in a warmer world
Is there are link between La Niña and severe tropical cyclone Yasi?
Stowasser: With wind speeds of around 230 km/h Yasi was a strong Category 4 system on the Saffir-Simpson Scale (see below further information) and one of the strongest and largest tropical cyclones to affect Queensland. The strongest storm to hit Australia since records began in 1970 was cyclone Monica 2006 which made landfall in the Northern Territory with a wind speed of 287 km/h, the equivalent of a Category 5 storm. La Niña events historically bring an increase in cyclones during the Australian storm season from November to April. The current strong La Niña help to drive the recently observed record ocean temperatures around Australia. These warm sea surface temperatures are one important ingredient to fuel such a strong system like Yasi by providing abundant heat and moisture.
How are natural catastrophe risks calculated and why is the estimation of flood risks so complex compared to other hazards?
Stowasser: Modern catastrophe models combine insights from natural science, e.g. weather prediction models, with databases of insured values and the vulnerability of properties to evaluate the probability of a financial loss. Sophisticated flood risk models are very complex because they have to incorporate several components: First of all a weather model that predicts where how much rain falls. Then you need a flood model that distributes the water on the land surface and into the rivers depending on the soil conditions. Furthermore, to calculate the final flood plains, you need a model that takes into account the existing flood defenses and the probability that these defenses will fail.
Can the current Australian events be attributed to climate change?
Stowasser: It's very hard to attribute any particular extreme event to climate change. La Niña, El Niño cycles have been going on for a very long time, they're natural cycles. They're part of a natural oscillation in the Pacific and indeed, when one looks at climate models running into the future with increasing levels of greenhouse gases, then there are no consistent changes in the El Niño, La Niña cycle.
Do you think that floods and storms will take place more frequently in the future?
Stowasser: Up-to now it is very hard to distinguish between the natural climate variability and a climate change trend. However, climate change predictions show a clear intensification of extreme rainfall events in a warmer world. This is entirely consistent with our understanding of the physics of the atmosphere in which warmer air can hold more moisture.
So, although climate change is very unlikely to have been solely responsible for recent extreme weather, it is likely that climate change is loading the dice and shortening the odds of heavy rainfall events around the globe in the future. A lot of climate models show an increase in the intensity of the strongest tropical cyclones along with a decrease in the frequency in some regions of the world. However, since current climate models are still too coarse to depict all the details of these complex storm systems, the response of tropical cyclones in a global warming scenario is still a hot topic in the scientific community.
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