The cause of recent Australian heat waves

Figure 1: Highest daily maximum temperatures during the first two weeks of January 2013. Source: Australian Bureau of Meteorology

The cause of recent Australian heat waves 

Australia started 2013 with a record-breaking heat wave. Much of the country experienced extremely high temperatures that lasted more than two weeks. The map below shows that over 70% of the continent recorded temperatures above 42°C, with temperatures exceeding 48°C at a number of locations. On 7 January 2013, Australia experienced its hottest day on record with a national average maximum temperature of 40.3°C.

The heat wave had significant impact on Australia’s health, roads and railway tracks, agriculture and the environment. The question that many people are asking is whether this level of extreme heat is linked to climate change. While Australia has always experienced heat waves, this particular event was very unusual as the extreme temperatures were widespread and persisted for a long time.

In New Zealand, the weather condition making the headlines in early 2013 was drought. Over the period January to March 2013 the North Island experienced an average of almost 80 days without rain, far more than ever recorded previously. The figure below illustrates the situation on 17 March 2013, approximately the peak of the drought. It shows the soil moisture deficit, (defined as incoming rainfall minus outgoing evapotranspiration). Red shading indicates the very driest areas – soil moisture deficits greater than 130mm.



Figure 2: Soil moisture deficit across New Zealand on 17 March 2013, probably the point in the drought when soils were at their driest. Circled areas indicate sample locations analysed in the NIWA (2013) report from which the figure is taken.

A warming climate can change the frequency, intensity and duration of extreme hot weather. The figure below shows this relationship for temperature. For drought, we might envisage an analogous picture but with an index that measures drought such as the total number of dry days in a given period.

When the average temperature is increased by a small amount, the effect on the tail ends of the temperature range (the ‘extremes’) is pronounced. This means that the likelihood of extreme cold weather goes down while the likelihood of extreme hot weather goes up. In addition, new record hot weather and heat waves occur after the shift in the climate to a warmer state.



Figure 3: The effect of changes in temperature distribution on extremes. Source: Australian Climate Commission (closed by the Australian government in October 2013)
It may never be possible to say that any specific extreme weather event was caused by human-induced climate change alone, in the sense that, without climate change, that event would not have happened. We can, however, ask the question how the odds of getting an extremely hot summer, or extremely severe drought, have changed due to man-made climate change: have past greenhouse gas emissions and other forms of pollution “loaded the weather dice” towards (or perhaps even away from) events of this nature?

The challenge is that, on any measure, it appears these are relatively unusual events, however the dice have been loaded, and all experiments to date suggest that the impact of any loading will be subtle. So, in effect, we need to find out if we are getting “too many double-sixes” with the weather dice. The chance of a double six with a fair pair of dice is only one in thirty six, so to check if we are getting slightly too many of them, we need to roll the dice hundreds of times (unfortunately, for the weather dice, we can’t work out how the odds of a rare event have changed simply by looking at more frequent events, like a single six). So, in effect, to pin down the role of human influence on climate in 2013, we need to roll the weather dice again thousands of times.

Weather@home 2013: the cause of recent Australian heat waves