Heat, Heat Waves and Angry Australian Summers (and Years)

I recently wrote a short article about the summer of 2012-13 in Australia. It got some reaction from one person in particular who was claiming the record heat couldn't be attributed to global warming, he postulated that it was caused by the Pacific Decadal Oscillation (eventually having some comments moved).

Rum Runner's challenge

He also posed a challenge. Rum Runner wrote: (October 9, 2014 at 2:26 AM)

@Lou "Why do you think I run this blog?"
A sense of empowerment I suppose. On other people's blogs you'd have your ass handed to you on a plate in an open debate. But here you can just *pop* delete any responses that are a bit too challenging for you.

I assume that by Lou he meant Sou. Here are the comments Rum Runner would have been talking about.

Now since Rum Runner regards the prestigious Bulletin of the American Meteorological Society as "political advocacy", and says that therefore doesn't take his information from there; and since he doesn't seem to have recognised as Australians, the scientists who wrote the papers I referred him to, I'll admit it's not much of a challenge.

Still, I thought it worthwhile doing two things. First, reporting some of the latest scientific findings about the weather Australia experienced in 2012 and 2013. And secondly, briefly touching on the Pacific Decadal Oscillation (PDO) - what it is and what is known about it and its effects on weather - in Australia and elsewhere. (I've written about the PDO a couple of times - here and here.) This article is about the former. I'll be writing a separate article on the PDO in the near future.

Australia looks forward to still hotter from the extra CO2

There are four papers in the BAMS supplement relating to the years 2012 and 2013.

The first one is: "The role of anthropogenic forcing in the record 2013 Australia-wide annual and spring temperatures" by Sophie C Lewis and David J Karoly.

What they did was investigate the roles of anthropogenic climate change and natural variability in regard to the record-breaking heat of 2013.

Here is a chart showing just how much hotter 2013 was, compared to past years. Compare it not just to recent years, but to just how cool the hot, dry continent of Australia was prior to the 1960s:

Source: Bureau of Meteorology

In 2013, the average annual temperature across all Australia was 1.53°C higher than the 1911-1940 mean. The previous record was set in 2005, when the average temperature was 1.36°C above the 1911-1940 mean.

The way the scientists went about their analysis was to use multiple simulations of CMIP5, selecting models using pre-determined criteria including data availability and representation of observed Australian temperature variability. They estimated anthropogenic forcing factors by looking at the suite of CMIP5 simulations with different climate forcing factors. They considered 65 realisations of CMIP5 historical from 1911 to 2005, representing a mix of natural and anthropogenic forcings. They also considered 35 realisations of RCP8.5 experiment, which is representative of global greenhouse gas emissions from 2005 to the present. The paper states:

The probabilities of Australian annual, spring, and September temperatures were calculated for experiments incorporating anthropogenic and natural forcings, and they were compared to a parallel set of experiments including only natural forcings. The long control runs with no changes in external forcing factors (piControl) completed for each participating CMIP5 model provide an estimate of the unforced variability of the climate.

The paper itself goes into a lot of detail about all the experiments conducted and more. The comparisons are fascinating, if worrisome. The results included:

• on the high emissions pathway (RCP8.5), once every six years annual mean temperature anomalies are higher than the second hottest year on record in Australia
• with only natural forcings, only one year in 6,795 years was higher than the second hottest year on record
• in the historical experiment (historicalNAT), the second hottest year on record was entirely outside the bounds of the simulated natural climate variability.

The conclusion included the statement that:

Hence, temperature anomalies as extreme as those observed in 2005 occur only once in over 12,300 years of model simulations without anthropogenic forcings, and the resulting FAR [fraction of attributable risk] value is essentially equal to one....

...Indeed, further analysis of Australia’s 2013 record annual Tmean also demonstrates anomalies were largely outside the modeled natural variability, with the attributable risk to anthropogenic forcing essentially 100%. 

That's strong stuff. I hope Tony Abbott and his advisers are taking note. I don't expect Maurice Newman will even read it, let alone allow it to permeate his consciousness.

Australia faces the risk of more frequent and more intense heat waves

Whereas the above paper was mainly about the full year of 2013, another paper in the BAMS Supplement focused just on the Australian summer of 2012-13. This had the rather long title: "Increased simulated risk of the hot Australian Summer of 2012/13 due to anthropogenic activity as measured by heat wave frequency and intensity." The title explains the focus of the research and its approach.

The Bureau of Meteorology reports the extraordinary temperatures in January 2013:

The January-averaged maximum temperature was the highest on record for Australia as a whole. The national anomaly was +2.28 °C, a substantial increase on the previous record of +2.17 °C in January 1932. All States and the Northern Territory recorded positive monthly anomalies for maxima with four also recording anomalies in the top six records: Northern Territory +2.52 °C, highest on record; Queensland +2.62 °C, second highest on record; New South Wales +3.66 °C, third highest on record; and South Australia +2.89 °C, sixth highest on record.  

Thing is, it was hot almost everywhere for a very long time.  As Perkins et al report it:

The Australian summer of 2012/13 was the warmest since records began in 1910 (Bureau of Meteorology 2013a). The season was characterized by the hottest month on record (January), where the continental mean temperature reached 36.9°C. Averaged nationally, the last four months of 2012 were 1.61°C higher than the long-term mean. Rainfall was below average for much of the country since July 2012. 

This analysis included an examination of heat waves using heat wave characteristics:

While focusing specifically on seasonal heat wave measures, such an analysis also allows for the assessment of whether changes in risk are consistent for heat wave magnitude and frequency, thus providing important information for adaptation and impacts groups.

The study calculated heat waves using accepted definitions. It investigated the effects of human activity by comparing observations with a pre-industrial control (289 years long), historical observations and RCP8.5 experiments, using different models. They analysed two heat wave characteristics:

• the total number of heat waves, representing the frequency, and 
• the peak amplitude (the hottest heat wave day), representing intensity.

What they discovered was that the risk of frequency increases faster than the risk of intensity:

The summer of 2012/13 experienced an unprecedented number of heat waves; however, the peak intensity was not particularly unusual (see Table 10.1)....

...A striking result is that during 1955–2012 and 1984–2012, the risk of the summer of 2012/13 having such a high heat wave frequency anomaly increases faster than heat wave intensity. During the latter period, the risk of experiencing a summer heat wave number (intensity) greater than that of 2012/13 increases by almost three-fold (two-fold) compared to a world with no anthropogenic forcing. This corresponds to a reduction in return periods to ~33 and 3 years, respectively, compared to 1955–2012. It is also an interesting and important result that even though the 2012/13 summer heat wave intensity was much less “extreme” than heat wave frequency (see corresponding return periods in Table 10.1), human activity has clearly increased the risk of both characteristics occurring. Thus, there is a calculable human influence on the hot Australian summer of 2012/13. 

The scientists conclude that the risk of both frequency and intensity of heat waves is increasing, with the risk of frequency increasing faster than that of intensity:

When isolating 1984–2012, the 2012/13 heat wave frequency increased three-fold due to human activity, while heat wave intensity increased two-fold, compared to a climate with no anthropogenic forcings.

Drought enhances heat

This article is already quite long, so I'll skip any detailed discussion of the other related papers in the September 2014 BAMS supplement. They are worth reading. One of the other papers explains the factors underlying Australia's hottest September on record so far (2013), which was attributed to:

...a combination of a strongly anomalous atmospheric circulation pattern, background warming, and dry and warm antecedent land-surface conditions.

Another was about the record heat of 2013 in the context of extreme drought. It found that:

...the extreme heat was made much more likely by important contributions from both the anthropogenic warming of the climate and the very dry conditions over the inland eastern region of the continent. The combination of these factors increased the probability of 2013 being Australia’s hottest year on record.

Extreme heat waves are becoming less rare

Heat waves are happening more frequently now. The Bureau of Meteorology issued a Special Climate Statement describing the heatwave this last summer, in January 2014, which you can download here. I'll copy some extracts from the introduction:

One of the most significant multi-day heatwaves on record affected southeast Australia over the period from 13 to 18 January 2014. A dome of very hot air developed over Western Australia in the second week of January, setting a number of records in that state, before moving eastwards to be over the southeast of the continent....

...The major area affected by the heatwave consisted of Victoria, Tasmania (particularly the western half), southern New South Wales away from the coast, and the southern half of South Australia. Over most parts of this region, it ranked alongside the heatwaves of January-February 2009, January 1939 and (from the limited information available) January 1908 as the most significant multi-day heatwaves on record. While peak temperatures mostly fell short of those observed in 2009 and 1939, extreme heat persisted for a longer period than it did in those heatwaves over some areas, particularly near-coastal regions of Victoria and South Australia (including Melbourne and Adelaide).

Numerous records were broken for extended periods of heat. Most notably, state-average data reveal that Victoria had its hottest four-day period on record, for both maximum and daily mean temperature. In both cases these surpassed records set in 2009, while for three-day periods the 2014 heatwave ranked second behind that of 2009. These two heatwaves, both of which have occurred in the last five years, stand ahead of any others recorded on a statewide basis....

---

Lewis, S.C. and D.J. Karoly. (2014). "The role of anthropogenic forcing in the record 2013 Australia-wide annual and spring temperatures", Bulletin of the American Meteorological Society, [in Explaining Extreme Events of 2013 from a Climate Perspective]

Perkins, S.E., S.C. Lewis, A.D. King and L.V. Alexander "Increased simulated risk of the hot Australian Summer of 2012/13 due to anthropogenic activity as measured by heat wave frequency and intensity." Bulletin of the American Meteorological Society, [in Explaining Extreme Events of 2013 from a Climate Perspective]