The Conversation; Dr Barrie Pittock, Honorary Fellow, Marine and Atmospheric Research at CSIRO

Recent wet weather and flooding across eastern Australia has caused many to ask, wasn’t climate change supposed to cause more droughts, not floods? Critics of climate change science have suggested that the recent observed flooding shows the science, and especially the climate modelling, must be wrong. This is a misreading of what climate change scientists have repeatedly stated.

Such misunderstandings would not necessarily matter. But confusion has arisen from these claims, especially among decision-makers from flood-affected areas. These people have a strong interest and responsibility to get the story right in order to make the right policy decisions.

A long history of predicting increased risk of both floods and droughts

Climate scientists have been saying for a long time that one of the effects of climate change is likely to be more intense rainfall and flooding. In 1989, at a presentation to the Prime Minister’s Science Council, Dr Graeme Pearman of CSIRO summarised a scenario of climate change for Australia in 2030. He said there would be:

• higher summer rainfall over northern Australia and extending further south.
• possibly drier winters in southern Australia
• more intense rainfall.

CSIRO’s Climate Impact Group, in a report to the Northern Territory Government in 1992-93 wrote: “The above results suggest that wet season runoff may increase substantially by 2030 … the results strongly suggest that flood frequencies may increase significantly. If so, this would have a major impact on the urban environment … built infrastructure, agriculture and hydrological planning, including the management of tailings dams and other sources of water pollution.”

Other reports to state governments contained similar conclusions. For example, CSIRO warned the NSW government in 1997-98 of “extreme daily rainfall intensity and frequency increases in many regions, particularly in summer and autumn”.

The Intergovernmental Panel on Climate Change (IPCC) has made similar remarks regarding the increase in heavy rain events for Australia and other regions in its reports starting in 1990.

The models get better, but the results remain similar

In the 1980s and early 1990s climate modelling was in its infancy. Scenarios of climate changes due to the enhanced greenhouse effect were based additionally on our understanding of basic physical laws, plus some reference to situations in the past with similar climatic variations.

There has been a gradual improvement of climate models and increase in computing power, going from simple energy balance models in one or two dimensions to fully three-dimensional models with multi-layer oceans and atmospheres. This has included more detailed physics such as ocean currents and atmospheric circulation, clouds, and interactions with the biosphere.

Despite all these improvements, the general conclusions on a global scale have not changed significantly, although they now give more regional detail.

As the climate models improved – and as they were tested against real observed climatic variations such as the annual cycle and El Niňo-La Niňa cycles – they became more useful for making predictions about specific regions.

The models are still not perfect but are increasingly reliable if used within their acknowledged limitations. They enable us to establish broad directions and magnitudes of change, with quantified degrees of uncertainty suitable for making decisions about how to quantify risk.

What makes it rain?

For Australia, several considerations will determine the actual regional changes in a warmer climate. The first is the dominant synoptic weather patterns that lead to the natural distribution of floods and droughts in Australia.

In the north, high-intensity rainfall events are produced by:

• convective storms over land when there are onshore winds containing lots of moisture
• onshore winds crossing over mountain ranges, such as the trade winds blowing westward over the Queensland coast
• onshore westerlies associated with weather patterns in the Indian Ocean
• tropical cyclones right across northern Australia
• the summer monsoon which affects all of northern Australia, but sometimes penetrates further south in the form of tropical lows.

These are predominantly summer half-year phenomena and all will likely become more intense as a result of global warming. This is simply because of more available moisture content in warmer air that has passed over warmer oceans, leading to more intense convection.

In the south, however, wet weather is dominated by low pressure systems and associated cold fronts which come from turbulence associated with the mid-latitude jet streams and low pressure belt. Again, a warmer world will likely make rainfall from these systems more intense.

However, models and the paleo-climate record indicate that these systems will be located further polewards in general in a warmer world. This has been observed in recent decades. These systems produce rain over southern Australia predominantly in the colder months, so winter rains in southern Australia are likely to be less frequent, with heavier rain occurring further south.

This was summarised in a CSIRO-BoM set of projections in 2007:

“Models show an increase in daily precipitation intensity but also in the number of dry days. Extreme daily precipitation tends to increase in many areas but not in the south in winter and spring when there is a strong decrease in mean precipitation.”

Droughts are also likly to be more intense

In situations where there are no rain-bearing systems – such as in winter in northern Australia and summer in southern Australia – warmer conditions and a greater water-holding capacity of the air will lead to more rapid evaporation. This will lead to a more rapid onset of droughts, which will become more intense.

The combination of heavier rains followed by more rapid onset of drought will likely lead to lots of drying fuel for bushfires, which are likely to become more frequent and intense.

Another important factor is the behaviour of the El Niňo and La Niňa phenomena. These are hemispheric-scale systems that lead respectively to wetter or drier conditions over northern and eastern Australia.

Evidence from paleoclimatic records is that they both existed during past warmer and cooler periods. But how they might change under enhanced global warming is still uncertain. Despite this uncertainty, when they do occur their effects on rainfall will be magnified by the warmer conditions, as we have seen in the last decade.

A Special Climate Statement from BoM in February 2012 summarised the real observed situation in 2010-2011 thus: “It was notable … that the bulk of the above average rainfall of the past two years fell during the northern wet season … with tropical influenced weather systems bringing monsoonal-like rainfall to much of the continent. By contrast, dry conditions persisted during the 2010 and 2011 April to June period (the start of the early winter rainfall period) across southern Australia. … The high 2010 and 2011 rainfall was therefore not associated with winter-time storm systems, and did not represent a return to normal conditions over the southern Australian winter season. In this way, the recent trend of rainfall reductions in autumn and winter was not reversed by the back-to-back La Niňa events.”

So, basic physics and climate models both suggest that a warmer Earth will likely see both more intense droughts and floods over Australia, with some regional differences. The succession of events in the last decade or so is consistent with this prognosis.

What we should do about it, by way of prevention or adaptation, is a subject for further discussion. It requires appropriate risk management.

For further information, please see the author’s 1992 and 1993 papers investigating climate change impacts on rainfall.