Bushfires in Southeast Australia

Over the Christmas break this year, I hopped onto the plane and headed down under, towards Australia! During the trip, I visited the Blue Mountains, just west of Sydney, some parts of which were recently ravaged by major bushfires in October. While bushfires may not be as widely researched and discussed as the other types of natural disasters that I have brought up in the previous posts, it has nonetheless been a recurring phenomenon in Australia for millions of years. As such, I will briefly touch on the impacts of climate change on bushfires in Australia, particular the southeastern region, in this post.

Bushfires on Blue Mountain in October 2013. Credits: AFP

Southeastern Australia is one the top 3 fire prone regions in the world, together with southern California and southern France. In the past century, bushfires have destroyed thousands of homes and claimed hundreds of lives including the infamous Black Friday fires in 1939 and Ash Wednesday fires in 1983. One of the driving factors that have caused southeastern Australia to be particularly vulnerable to bushfires is its climate – hot, dry summers and mild, wet winters. The precipitation received during winter and spring allows fuel (the vegetation) to grow, while the dry summers favour the development of bushfires (Lucas et al. 2007). Moreover, periods of drought have exacerbated the dry conditions and fire risks in the region. Over the past decade, it has been observed that temperatures of the region have shifted towards higher temperatures, while rainfall has declined below the 1961-1990 mean (Murphy and Timbal 2008). The extended period of dry conditions have contributed to the large fires that burned with little control in 2006/07.  

Eucaplyptus trees that are commonly found in Australia. Credits: Joon Ting

Another factor that has been blamed for the bushfires is the predominant type of vegetation that lines the landscape of Australia – eucalypts. There are more than 800 endemic species found in Australia and forms the main diet for koalas. These trees are highly flammable as they contain oil, which gives them their distinct spicy fragrance. During periods of dry and windy conditions, their flammable oil can cause small fires to develop into huge firestorms very rapidly. Yet, these trees are extremely fire resistant themselves and tend to survive the bushfires, allowing for the regeneration of the eucalyptus forest after the fire. Hence, they tend to be naturally selected in regions prone to bushfires including Australia and California, where they continue to dominate the landscape. As such, it seems inevitable that southeast Australia experiences such frequent bushfires.

Eucaplytus trees that cover the Blue Mountains. Credits: Joon Ting

It has already been projected that southeast Australia will become hotter and drier under climate change (Suppiah et al. 2004). Modelling studies have been further carried out to determine how this projected change in climate will affect the fire risks of the region. The Forest Fire Danger Index (FFDI) has been used to quantify the fire risks and is calculated based on observations of temperature, relative humidity and wind speed. Modelling studies carried out by Hennessy et al. (2005) on 17 sites in southeast Australia have suggested that the combined frequencies of days with very high and extreme FFDI rating are likely to increase by 4-25% by 2020 and 15-70% by 2050. This corresponds with more recent studies by Lucas et al. (2007) that show that the increase in annual cumulative FFDI is generally 0-4% in the low scenarios and 0-10% in the high scenarios by 2020, and 0-8% in the low scenarios and 10-30% in the high scenarios by 2050.

Nonetheless, there is still large uncertainty in these studies given that much of the climate of southeast Australia is dominated by interannual and interdecadal variability that is influenced by complex systems including ENSO and Southern Hemisphere Annular Mode (SAM). The evolution of these systems under future climate change is still not fully understood thus it is difficult to ascertain how fire weather and risks will change in future when such variability is taken into account (Lucas et al. 2007).

Droughts and climate change

With the past few posts looking at floods, let’s now move on to the next category of natural disasters – droughts. Droughts are generally defined as ‘a period of abnormally dry weather long enough to cause a serious hydrological imbalance’ (IPCC 2012). Droughts have been a relatively common phenomenon in several regions including Australia, Sahel, North America, and even the United Kingdom (though they are generally much less severe than those experienced in the other countries mentioned). In fact, since 2012, North America has been hit with one of its worst droughts, with 81% of the contiguous United States being covered with at least abnormally dry conditions (DO) at its peak on July 17 2012 (Fig.1). Currently, the situation has improved significantly although the western parts of the US are still affected by the droughts (Fig.2).

Figure 1 US drought monitor report for July 17 2012. 81% of the contiguous US is affected by at least abnormally dry conditions. Source: http://droughtmonitor.unl.edu/

Figure 2 US drought monitor report for December 10 2013. Most of the western part of the US is still affected by the drought. Source: http://droughtmonitor.unl.edu/

When discussing about droughts, it is important to note that there are different types of droughts – meteorological, hydrological and agricultural (Fig.3). The definitions for each category of droughts are taken from Mishra and Singh (2010):

• Meteorological – lack of precipitation over a region for a period of time.
• Hydrological – a period with inadequate surface and subsurface water resources for established water uses of a given water resources management system.
• Agricultural – a period with declining soil moisture and consequent crop failure. 

Figure 3 Links between the different types of droughts and the main drivers for the droughts. Source: IPCC SREX report (2012)

However, out of all the atmospheric hazards, droughts are at the moment the least well-understood and predictable (Mishra and Singh 2010). There are still large uncertainties involved regarding observed global-scale trends in droughts as well as how anthropogenic climate change will affect droughts in future (IPCC 2012).

Firstly, there has been a lack of consensus with regards to the observed trends in drought over the recent past. This is mainly due to the fact that there are few direct observations of drought-related variables available for global analysis and hence drought indices, which attempt to integrate precipitation, temperature and other variables, are used instead to infer the changes in drought conditions. One of the most prominent index used is the Palmer Drought Severity Index (PDSI) (Palmer 1965) that measures the cumulative departure in surface water balance. Based on this index, Dai et al. (2004) have suggested that globally, very dry areas have doubled in extent since 1970. However, such studies based on the PSDI have been criticised greatly due to the fact that the PSDI relies on a temperature-based method of calculation of potential evaporation (PE), known as the Thornthwaite equation. Sheffield et al. (2012) argued that temperature-based PE tend to overestimate the extent of drought as it does not factor in the effects of radiation, vapour-pressure deficit and wind speed. By using a physically-based estimate of PE based on the Penman-Monteith equation instead, Sheffield et al. (2012) showed that there has actually been little change in drought over the past 60 years (Fig.4). As such, due to the lack of direct observations of drought, it has been difficult to determine the global trends of droughts. The IPCC SREX report (2012) could at best conclude that there is only a medium confidence that some regions of the world have experienced more intense and longer droughts since 1950s.

Figure 4 A) PSDI calculated using the Thornthwaite PE method is shown in blue and using the Penman-Monteith method is shown in red. B) Area in drought (PSDI < -3). The shading shows the range derived from uncertainties in precipitation (both) and radiation (Penman-Montieth only). Source: Sheffield et al. (2012)

Secondly, there are also large uncertainties involved with our understanding of how climate change will affect droughts in the future. There have been preliminary studies based on model simulations that suggest that aridity increases and because severe by the 2060s over most of Africa, southern Europe, Middle East, most of Americas, Australia and Southeast Asia, while central and northern Eurasia, Alaska and northern Canada and India become increasingly wetter (Fig.5) (Dai 2011). Such results broadly correspond to the modelling results by Hirabayashi (2008) that suggest that drought frequency from 2070 to 2100 increases over North and South America, central and southern Africa, the Middle East, southern Asia and central and western Australia. Meanwhile, the eastern part of Russia shows significant decreases in drought days due to increases in precipitation and flood flows. 

Figure 5 Mean annual self-calibrated PDSI (sc-PDSI) using the Penman-Monteith PE method for years 2060-2069 and 2090-2099. These values were calculated using the 22-model ensemble-mean surface air temperature, precipitation, humidity, net radiation and wind speed from the IPCC AR4 using the SRES A1B 21st century simulations. Red to pink areas are extremely dry conditions while blue colors indicate wet areas relative to the 1950-1979 mean. Source: Dai 2011. 

However, these projections do not consider the impacts of changes in large-scale atmospheric and ocean circulations under climate change as there is insufficient knowledge of how these have an impact on droughts. Moreover, it is also not known how the behaviour of plant transpiration, growth and water use efficiencies will change under increasing CO₂ emissions, which would consequently affect soil moisture storages (IPCC 2012). Therefore, unfortunately, it is still hard to determine how trends in droughts will evolve in the future under climate change.

Documentary: Earth Under Water by National Geographic

As a follow up to the previous post on coastal flooding, here's a documentary produced by National Geographic which looks at the impacts of predicted future sea level rise on different cities and regions around the world. They also explore the ways that societies have been trying to adapt to such changes by beefing up on their sea defenses.