Better Planning for Extreme Floods Possible: A Note from Stewart Franks
DESPITE increased understanding of a number of different climate processes and their impact on a range of different timescales, this knowledge is not being used to inform planning and decision making. This is because long-term climate risk is often viewed only in statistical terms.
For instance, engineering techniques for estimating flood risk, where records exist, are largely based on simple statistics of their historic occurrence rather than on any real understanding of the processes that actually cause them.
In essence, if we have 100 years of flood record, then the largest flood measured represents, more or less, the hundred year flood level. This hundred year flood level is probably the most important of hydrological statistics in terms of its use in planning management. It is the yardstick by which decisions are made.
The problem with a simple statistical representation of risk is that it implies a static climate – the expected flood risk is equally likely in any year, irrespective of the actual climate processes that may or may not be dominating at that particular time. If this were the case, we would expect to see an equal spread of floods throughout our historical records. In Australia and in fact many other parts of the world, this is not so.
The observed history of Australian flood risk demonstrates that this fundamental assumption is invalid – climate is not static – Australia regularly experiences epochs lasting between 20 to 40 years where extreme floods cluster only to then be succeeded by a similarly long period where droughts dominate and extreme floods are rare occurrences.
In Australia, between 1910 and 1945, long-term and persistent drought dominated the Australian climate – very few floods were experienced – this was largely due to the dominance of El Nino events.
Around 1945, we experienced a major change in climate from this previously El Nino dominated regime to one dominated by La Nina events. This led to as much as a three-fold rise in the average annual maximum flood. The dominance of La Nina and associated southward shift in the location of the Inter-Tropical Convergence Zone meant that rainfall was frequent, and frequently extreme.
Around 1975 we had another major climate shift – a marked change in the Pacific that is associated with a return to El Nino event dominance. Once again, Australia suffered repeated droughts and again very few floods.
Since 2001, we’ve had seven years of drought, initiated and then sustained by El Nino conditions, but more recently, the last couple of years have seen a shift towards marked La Nina activity.
Last summer was a classified La Nina event, whilst this summer we saw a late rush of La Nina activity. Consequently we’ve seen an 18-month period of disastrous storms and floods, including the recent widespread flooding.
This recent La Nina activity has not broken the long-term drought everywhere but has been useful in many places where the impacts of ENSO are strong and largely reliable – typically those with summer dominated rainfall regimes – but also with typically devastating consequences.
The double-edged regional impact of La Nina aside, the recent La Nina activity has not made much of an impression in other parts of Australia, notably Victoria and South Australia. Indeed, the recent tragic bushfires are most likely due in large part to the ongoing impact of the unrelenting drought.
Long-term drought in these regions was the primary cause of the initial high bushfire risk leading to the dreadful bushfires – low rainfall means dry combustible fuel. Soil moisture feedbacks mean that when soil moisture is low, the air temperature is hotter, the humidity is lower and the air column itself is more unstable. These conditions are all amenable to increasing extreme bushfire risk.
And so we find ourselves witnessing the iconic extremes of fire and flood devastating different regions of Australia simultaneously. Dorothea would have wept – she might have seen it before us but we are seeing it worse.
Many might even be tempted to say that the current perverse circumstance of deadly bushfires in one part of Australia and widespread flooding in others is a further sign of CO2-driven climate change.
However, an alternative understanding would be that whilst southern states continue to be in drought with the attendant bushfire risk, other regions of Australia may have started to experience a natural change in climate that would be more consistent with their historical instrumental evidence of flood and drought risk, despite increasing atmospheric CO2 concentrations.
The historical record of climate variability suggests that we should expect a return to a 20-40 year period where La Nina dominates the climate of at least eastern Australia once more. The observation that many regions of Australia routinely experience multi-decadal variability of flood and drought, suggest that we should expect a return to major widespread flooding on a regular inter-annual basis, and for entirely natural reasons.
There are other known climate mechanisms relevant to Australia that also display marked multi-decadal variability that only add to the difficulty of climate prediction in different parts of Australia. It is however increasingly recognised that different observed and studied climate modes appear to have definable interactions.
Further studies of the interactions between climate modes may yield additional insights into the possibilities and limitations of prediction and estimation of risk, both now and into the future. They might also inform us on the true relevance of atmospheric CO2 concentrations in a naturally varying climate.
And yet at present Australia’s historic variability does not figure in typical estimates of climate risk for planning purposes. Nor though should we pay much heed to those that may loosely call themselves climate change scientists and who make alarmist claims for the future climte. They should acknowledge that we never could predict the future climate of 10, 20, 50 or 100 years time, at least not with any credibility. It is no different now that we fear CO2.
If we cannot prevent climate-related disasters we can at least prepare for them. The recent tragedies of both fire and flood have lessons to teach. We should engage ourselves in developing more effective measures to reduce our vulnerability to the extreme vagaries of climate variability. More prescribed burns and more flood infrastructure would seem appropriate.
Whether we can predict climate on multiple timescales usefully or not, we should certainly be preparing ourselves for both ends of the spectrum of climate extremes at all times. I would argue that in a country as vulnerable as Australia, this should be happening irrespective of any consideration of possible impacts of increased CO2.
After all, if we fail to cope with natural climate variability what hope have we for coping with any supposedly catastrophic human-induced climate change?
The text is republished from ABC Unleashed with permission from the author.
Margaret Menzel provided the photograph of the drown cattle taken at Cowan, North Queensland, in February 2009.