This week the Australian Greens established a Senate Inquiry on extreme weather and emergency preparedness. The inquiry will be conducted by the Environment and Communications Reference Committee. Details and instructions on making submissions can be found here.
Over the coming weeks I’ll be addressing each of the terms of reference on this blog, gathering and summarising relevant literature and providing my own opinion. I’ll also add some context around past state and federal inquiries and other activities of a similar nature and outline why I don’t believe this inquiry will make a significant difference in the preparedness of our emergency services. Submissions are due by 18 January 2013 and the committee is due to report on 20 March 2013 (I expect that the deadline will probably need to be extended).
Today I’ll examine the first ToR. Warning – heavy science content.
(a) recent trends on the frequency of extreme weather events, including but not limited to drought, bushfires, heatwaves, floods and storm surges;
The reason why we’re concerned about the impact of climate change on extreme weather is that extreme weather does bad stuff to society. If a tropical cyclone impacts a stretch of uninhabited coastline it receives little coverage.
A recent string of high profile disasters have many asking if climate change is involved. Is there anything unusual about the recent disasters and are disasters in Australia getting worse? Although there have been different findings globally Crompton and McAneney examined disaster losses in Australia since 1967. They found that population growth, increasing wealth and development accounted for all increases in insured losses in Australia over the last 40 years. The impact of climate change on disasters cannot yet be detected in this data.
This is actually what you would expect. Firstly, disaster loss data is incredibly noisy with the highest loss years experiencing around 50 times the loss of the lowest loss years. Secondly, when compared against the drivers of growth in population and wealth the modest warming experienced to date would be difficult to separate out. The corollary to this is that if we could detect a signal in disaster loss data with only modest warming having occurred, predicted warming (assuming only limited climate action) of up to 4C this century should have us very, very worried.
Rather than look at disaster losses it is better to examine data on the extreme weather phenomena themselves. These data is somewhat less noisy and doesn’t have to contend with competing drivers like population growth.
Looking at the phenomena themselves presents just as many challenges however. For many phenomena quality data and good coverage may only extend back 30 or 40 years. This timescale can make it difficult to separate out the effects of multi-decadal natural cycles or make inferences about very rare events such as 1% Annual Exceedance Probability floods. So climate change could still be having an impact on a particular extreme weather phenomenon, but if the impact is small we might not yet be able to detect it. Again this uncertainty isn’t evidence of absence, as with the disaster losses if we could currently detect changes in rare events we should be very, very worried.
Keeping these issues in mind Australian scientists have been able to say quite a lot with reasonable confidence about the current impact of climate change on extreme weather in Australia. The most recent publication that examines multiple extreme weather hazards is Australian Climate and Weather Extremes: Past, Present and Future, published by the Department of Climate Change in 2008. It covers the issues quite well but as research in this field is fast progressing, I’m going to go hazard by hazard and cite any more up to date sources:
The number of weak and moderate tropical cyclones appears to have decreased although this could be due to changes in the observation system. On the other hand the number of intense tropical cyclones appears to have increased. An increase in intense tropical cyclones is consistent with warmer ocean temperatures under climate change, however more research is needed to understand the drivers behind these changes. There is ongoing discussion in the scientific literature on trends in tropical cyclones in the Australian region.
Measuring the frequency of bush fires and coming up with long-term trends is challenging due to the large proportion of human induced bush fires. The closest proxy is to look at bushfire weather which is reflected in the Forest Fire Danger Index (FFDI). Researchers at the Bushfire CRC have examined trends in FFDI across southeastern Australia. They found that the median FFDI (a measure of the overall strength of a fire season) has increased across the study area, although this increase was only significant in South Australia and inland areas of NSW and Victoria. Evidence was also found of changes in seasonality with trends differing over spring, summer and autumn in different areas. Also examined was the sum of FFDI over a fire season, which measures both the season strength and length. Significant increases were found across most of the study area (except for some coastal stations and Canberra) with the largest trends being in inland areas.
As temperature is one of the key variables in FFDI and temperature has been increasing with global warming it is likely that at least part of these trends towards more severe bush fire weather can be linked to climate change.
Consistent with a trend in average temperatures the number of hot days, hot nights and record hot temperatures have been increasing. More importantly mean minimum temperatures and the number of hot nights are increasing and the number of cold nights and cold extremes are decreasing. Heatwaves are most dangerous when temperatures fail to drop to comfortable levels overnight. Also record hot temperatures are now two to four times as likely as record lows
How all this impacts heatwaves is complex. A heatwave is prolonged hot temperatures, so pure temperature records don’t give the full picture. Deo et.al. investigated heatwave conditions across Australia. Their study didn’t examine the frequency of heatwaves, but the average apparent temperature (which takes into account relative humidity) of the worst 3-day heat wave each year. They found that heatwave severity is increasing across eastern and southeastern Australia where the severity increased by 1-4 ºC per decade. No statistically significant trend was found in western and northern Australia.
As with changes in bush fire weather climate change is likely behind at least part of this increase.
A drought is more than just a lack of rain, the extreme dryness that characterises drought is also contributed to by increased evaporation. Lacking detailed and long timescale measurements of soil moisture, examination of trends in Australian drought conditions look at rainfall and proxies for evaporation, like temperature.
Records show that recent Australian droughts have been no worse in terms of lack of rain, than droughts earlier in the 20th century. However increased temperatures during recent droughts has increased evaporation. Increased temperatures could be increasing the severity of drought, even with no decline in rainfall. Studies of various drought indices have had mixed results in showing any trends. For example over the period 1950-2008 Dai found that eastern Australia became drier and western Australia wetter in terms of drought conditions.
However drought doesn’t necessarily paint the full picture when it comes to agriculture and the environment. There has been a long term trend towards drier average conditions over much of eastern Australia and southwest Western Australia with fewer very wet years and deficiencies in autumn and early winter rainfall. Although some of these changes are in part due to natural variability, anthropogenic climate change is also a factor. This pattern makes it more difficult for drought-impacted areas to recover, and in particular store water for periods of drought.
Most analysis of flooding trends has focussed on rainfall, particularly daily rainfall. A recent discussion paper prepared for the review of Australian Rainfall and Runoff summarised the latest research. At the daily timescale they found no statistically significant increase in daily extreme rainfall, although there there is some evidence to suggest that extreme rainfall is decreasing where average rainfall is decreasing. On the other hand there is evidence for increases in extreme sub-daily and sub-hourly rainfall, having possible consequences for flash flooding.
Looking at flooding records is more complex as changes to catchments over time from land use and infrastructure like dams and levees can also cause changes to flood frequency and severity. A Project Report, also for Australian Rainfall and Runoff found a downward trend in annual maximum flood peaks across the country, particularly in southeast and southwestern Australia. Upwards trends were observed in northwestern Australia with mixed results in Queensland. However they caution that the short record for many of the stations examined means that the trend observed may be due to decadal climate variability rather than anthropogenic climate change. Further research is needed.
Storm Surge, Extreme waves and Coastal Erosion
Storm surge in the tropics is mainly associated with tropical cyclones and can reach heights of several metres, whereas in the south of the continent they are caused by low pressure systems and don’t reach as high. With a rise in sea level averaging 1.2mm/year over 1920-2000 we would expect the frequency of a given extreme sea level event to increase. As extreme sea levels are less variable in the south of the continent the frequency rise should be greater here. In the two locations with a long timescale quality dataset (Fort Denison in Sydney and Fremantle) scientists have found that extreme sea level recurrence intervals were around 3 times shorter after 1950 than before. They also found some evidence that extreme sea level events were rising faster than mean sea level.
Storm surges are often concurrent with heavy seas. This combination can lead to coastal erosion damaging coastal infrastructure, property and beach amenity. Onshore wave data is limited so investigations have examined various proxies for it. In northern Australia extreme waves are strongly associated with tropical cyclones so any change in their frequency will also impact waves. An increase in the frequency of large wave events has been observed in southern Australia, although this is most likely linked with trends in the Southern Annular Mode. Further investigation of wave climate is needed.
Thunderstorms and Hail
Thunderstorms can produce extreme winds, very heavy rainfall, large hail and tornadoes. Increases in population and the development of technology like weather radar have made the detection of these phenomena much easier. This makes determining long term trends very difficult. An increase in the number of thunder days has been observed over southeastern Australia, although its unclear what the source of this increase is.
An increase in monitoring has been reflected by a massive increase in the number of hailstorms reported over NSW. At the same time however the reported frequency of hailstorms over Sydney has decreased.
Other approaches to investigating these phenomena use downscaling, examining atmospheric conditions likely to give rise to them. One study found that the conditions where cool season tornadoes are likely to form have increased.
Investigations of trends in extreme wind speeds in Australia are probably the least developed of all climate variables. Extreme winds that aren’t associated with tropical cyclones or thunderstorms can come from a range of weather systems (for example east coast lows in coastal NSW) further complicating the detection and attribution of any trend.
The only area where any investigation has occurred is Bass Strait, where some changes in wind speed (as measured by pressure gradient) have been observed. Exactly what this means for extreme wind speed, or whether it can be generalised to the rest of Australia is unclear.
Understanding of trends in extreme weather is hampered by a lack of long timescale, consistent, wide coverage and quality data on these phenomena. I want to make it very clear that this should not be taken as evidence that climate change is not occurring or that we have less to be concerned about. The statistics of rare events would make detection of climate change signals in extreme weather difficult, even with a perfect data set. Detection of climate change trends now in these phenomena with only 0.8 ºC of warming would have very concerning implications for future warming.
The most complete and accurate records are for temperature, and this is the climate variable most closely linked to global warming. Thus the clearest trends and highest confidence have been observed in extreme weather most closely linked to temperature: heatwaves and bushfire weather. Although some trends have been observed in other extreme weather, notably tropical cyclones, the confidence in these trends is lower and their attribution to climate change unclear.
- The scientific investigation of trends in extreme weather is rapidly evolving and involves many different weather variables and study techniques. This makes it extremely difficult for non-experts to get a clear picture of the current science. A recent paper by NOAA scientists and other US experts published in the Bulletin of the American Meteorological Society surveyed the current literature and gave clear information on the current picture of the science in this area (see, for example, graph on page 54). The Bureau of Meteorology, CSIRO and Geoscience Australia should collaborate to release a similar report summarising both the observation of any trends in extreme weather and the understanding of whether they are related to anthropogenic climate change. This report should be regularly updated when new research is published.
- Further research on historical climatology of extreme weather is needed. In particular
investigation of extreme winds (and the synoptic phenomena that cause them), floods, extreme waves, thunderstorms and tropical cyclones is needed.