Melbourne City Sunset.

Explainer – What are heatwaves?

By Kevin Hennessy, CSIRO

Last week, South-eastern Australia roasted under another heatwave. Adelaide had 5 days between 42 and 45°C, Melbourne had 4 days over 41°C (a new record) and Canberra had 5 days over 37°C.

The cause was a blocking high pressure system in the Tasman Sea which funnelled hot winds over the region. This air had been cooking for a while – it was hotter than 39°C when it moved from WA into northern SA on 10 January, then into western NSW on 11 January, southern SA and western Victoria on 13 January, and the rest of Victoria and Canberra on 14 January (Figure 1).

This heatwave followed on from the heat of 2013, which was Australia’s warmest year on record since measurements began in 1910 (Figure 2). Over the past decade, the number of extreme daytime heat records in Australia outnumbered extreme cold records by almost 3 to 1. These increasing temperatures are part of a global warming trend.

According to the Intergovernmental Panel on Climate Change (IPCC), it is extremely likely that human influence has been the dominant cause of the observed global warming since the mid-20th century, due to increases in greenhouse gases. An increasing number of studies have partly attributed the rise in extreme heat events to this increase in greenhouse gases.

Sequence of temperature maps from the Bureau of Meteorology showing the movement of hot air from western to eastern Australia from 11-17 January 2014.

Figure 1. Sequence of temperature maps from the Bureau of Meteorology showing the movement of hot air from western to eastern Australia from 11-17 January 2014.

Annual Mean Temperature Anomalies for Australia 1910-2010. BoM.

Figure 2. Annual mean temperature anomalies for Australia (compared with 1961–1990 average). The black line shows the 10-year moving average.

Next time it will be worse

In the future, heatwaves are very likely to be longer and occur more often, according to the IPCC. CSIRO and the Bureau of Meteorology have estimated that the annual average number of days over 35°C may increase from 9 days at present in Melbourne to 12-26 days by 2070 (Table 1). Adelaide will go from 17 days over 35°C to 24-47 days by 2070. While Perth will go from 28 days over 35°C to 36-76 by 2070 and Canberra will go from 5 days to 8-26 days by 2070.

  Current average days per year over 35°C Medium emissions (2030) Low emissions (2070) High emissions (2070)
Perth 28 36-43 36-46 48-76
Alice Springs 90 102-118 112-138 132-182
Sydney 3.5 4.1-5.1 4.5-6.6 6-12
Melbourne 9 11-13 12-17 15-26
Brisbane 1 1.5-2.5 2.1-4.6 4.0-20.6
Canberra 5 7-10 8-14 12-26
Adelaide 17 21-26 24-31 29-47
Hobart 1.4 1.6-1.8 1.7-2.0 2.0-3.4
Table 1. Current and projected annual-average number of days over 35°C at selected Australian cities. The current period is defined as 1971-2000. Projections are for 30-year periods centred on 2030 and 2070, for low (B1), medium (A1B) and high (A1FI) IPCC greenhouse gas and aerosol emissions scenarios. Projections for medium emissions in 2030 are very similar to those for low and high emissions.

Heatwaves cost us

Heatwaves have significant impacts for health, infrastructure, ecosystems and agriculture, and these are likely to grow in the future.

For example, the heatwave in southeast Australia in late January 2009 caused 374 excess deaths. This heatwave set up conditions for the February 2009 bushfires in Victoria which caused 173 deaths and about $4.4 billion in damage.

Since 1994, more than 30,000 flying foxes died in heatwaves at sites along the east coast of Australia. Over 3,500 flying foxes were killed in January 2002 along the New South Wales coast due to extreme heat. While in January and February 2009, nearly 5,000 flying foxes died at Yarra Bend Park in Melbourne.

Agricultural productivity and profits decline when livestock suffer heat stress. Extreme temperatures also reduce crop yield and quality.

Blackouts and infrastructure breakdown

Heatwaves can lead to black-outs due to increased energy consumption for air conditioning. For example, during the January 2009 Victorian heatwave, electricity demand soared and the Basslink electricity cable between Tasmania and Victoria had to be shut down for safety reasons. Around 500,000 people were without power, breaking all previous records for Victoria. Water consumption also increases during heatwaves.

Extremely hot days can cause transport delays due to fires, buckling train lines and air conditioning faults. On 30 January 2009, approximately one quarter of Melbourne train services did not run due to buckled train lines and equipment faults. Financial losses from heatwaves can run into many millions of dollars.

Protecting ourselves

In the short term, we need improved early warning systems and better preparation of communities, emergency services, health and medical services, transport and energy services. We also need to educate the community on how they can reduce their exposure to heat stress, including making sure they stay cool and drink lots of water, head to shopping centres to take advantage of the air-conditioning, and never leave children or pets alone in a car.

Benefits of future proofing our cities and farms

In the longer term, we can build more resilient cities by increasing vegetation to reduce urban heat islands, providing incentives for better designed homes that are cooler and more energy-efficient, and improving management of peak demand loading on the electricity grid.

Our cites will need to be built differently using improved engineering design standards, with less heat-sensitive materials and better maintenance routines for essential services.

We also need to implement emergency response plans that encourage adaptability through collaboration across agencies.

In agriculture, we can develop and select more heat-tolerant crops, and provide shade and water for livestock.

These and other actions are included in the National Strategy for Disaster Resilience, and some are already being implemented. Our challenge is to identify and overcome the remaining barriers to prevention, preparedness, response and recovery.




  • Graham Lovell

    The emphasis on the need to adapt to a warmer climate in this article is helpful.

    It is difficult to know what the future climate effects of higher greenhouse gas (GHG) concentrations in the atmosphere will be in exact terms, since we are in uncharged territory here. Mostly the IPCC predictions have been in the right direction, but generally overstated. This has been partly due to the failure of their models, but most significantly due to the fact that they have been quite unable to predict the actual levels of GHG concentrations in the atmosphere over even a short time frame. Fair enough, this is not their area of special expertise. Nevertheless, it is a devastasting critique of the IPCC approach.

    Apart from a category failure to predict GHGs even approximately correctly, the main problem with the IPCC reports is that they are entirely based on theoretical models of atmospheric behaviour at higher levels of GHGs. There has been no publised attempt to test their models with appropriate historical statistical analysis. When I attempted to do this, I was told by serious climate scientists to go away. From one journal I was told that this did not add anything to the debate.

    Unfortunately, climate scientists are shooting themselves in the foot in the PR war. While this is not the case in this article, by overstating the consequences at this time when the effects are relatively minor, they are reducing the political will to implement the changes required.

  • Kevin Hennessy

    The IPCC assesses peer-reviewed literature, however it doesn’t conduct research or develop climate models.

    Each IPCC Working Group 1 report has included a chapter on climate model evaluation, e.g. Chapter 9 of the Fifth Assessment Report –

    The models have strengths and weaknesses in particular aspects, and continue to be the most useful tool for assessing causes of past climate variability and future climate change.

    The IPCC Summary for Policymakers (2013)
    states: “The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (e.g., 1998 to 2012). The observed reduction in surface warming trend over the period 1998 to 2012 as compared to the period 1951 to 2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from natural internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. There is medium confidence that natural internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of natural internal variability. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (dominated by the effects of aerosols)”.

    In regards to your comment – “unable to predict the actual levels of GHG concentrations in the atmosphere over even a short time frame” – if the time frame you refer to is 1990-2010, the IPCC Special Report on Emissions Scenarios (SRES) provided projections for this period and they have been reasonably accurate.
    See Figure 1 the journal paper by Peters et al (2012) at

    • Graham Lovell

      Your comment: “The IPCC doesn’t develop climate models,” is rather pedantic. They use the models developed by others and make them their own. I have heard your complaint before: it adds nothing to the debate.

      How can mode are not even very good at reproducing what has happened in the past be “the most useful tool” for predicting the future?

      As for the bulk of your answer, which is based on a long extract from the IPCC report, includes the “hope” that the half of the warming that has gone into the oceans is eventually going to re-emerge. Where is the evidence for this.

      The predictions made by the IPCC on the “business-as-usual” case have not been “reasonably accurate”. Since the IPCC appears to work on the assumption that nothing has been done, rather than the truth that the world has taken action, it is fair to compare their predictions with the actuals for 2010: CO2 – prediction 397 ppm actual 390 ppm; Methane – pred 2223 ppt actual 1795 ppt; nitrous oxide – pred 321 ppt actual 324 ppt. In regard to CO2, it is tracking Scenario C, which involves substantial action on greenhouse emissions levels. In regard to Methane, despite the dire predictions of it suddenly jumping up (to 2223 ppt in 2010, it has only increased from 1710 ppt to 1794 ppt.

      On the basis of the actual numbers, your cited article contains a blatant lie: “The latest carbon dioxide emissions continue to track the high end of emission scenarios.” They don’t. They track Scenario C, which is the “taking serious action but not going overboard” approach of the IPCC 1990 report.

      Is ignoring the facts, refusing to look at alternative approaches, and finally lying about the evidence, the best path forward for scientific debate about the effect of higher levels of greenhouse gases on global average temperatures?

      • Claire Manson

        Hi Graham,
        We have been tracking for over a decade the most carbon intense pathway explored by the IPCC. They all confirm the same. For a quick check, just see figure 3 in Friedlingstein’s article (Friedlingstein, P. Persistent growth of CO2 emissions and implications for reaching climate targets. Published online in Nature Geoscience on 21st September 2014) and Figure 1 in Fuss’s article (Fuss,N. Betting on negative emissions. Nature Climate Change, Vol 4, Oct 2004. Available at: To download the datasets and a full set of analysis/figures, please check The Global Carbon Project’s website