Tuesday, January 7, 2014

Australia's hottest year was no freak event: humans caused it

Sou | 5:16 PM Go to the first of 7 comments. Add a comment

By Sophie Lewis, University of Melbourne and David Karoly, University of Melbourne

The Bureau of Meteorology has confirmed that 2013 was the hottest year in Australia since records began in 1910.

Unusual heat was a persistent feature throughout the year. For the continent as a whole, we experienced our hottest day on record on January 7. Then January was the hottest month on record, and the 2012-13 summer was the hottest recorded for the nation.

The nation-wide temperature record set for the month of September exceeded the previous record by more than a degree. This was the largest temperature anomaly for any month yet recorded.

Averaged across all of Australia, the temperature for 2013 was 1.2C above the 1961-1990 average, and well above the previous record hot year of 2005 of 1.03C above average.

What caused these extreme temperatures? Climate scientists have a problem: because climate deals with averages and trends, we can’t attribute specific records to a particular cause.

But our research has made significant headway in identifying the causes of climate events, by calculating how much various factors increase the risk of extreme climate events occurring. And we have found sobering results.

We previously analysed the role human-caused climate change played in recent extremes across Australia.

For various record-breaking 2013 Australian temperatures, we investigated the contributing factors to temperature extremes using a suite of state-of-the-art global climate models. The models simulated well the natural variability of Australian temperatures.

Using this approach, we calculated the probability of hot Australian temperatures in model experiments. These incorporated human (changes in greenhouse gases, aerosols and ozone) and natural (solar radiation changes and volcanic) factors. We compared these probabilities to those calculated for a parallel set of experiments that include only natural factors. In this way, natural and human climate influences can be separated.

In our previous studies, we then applied an approach (known as Fraction of Attributable Risk) widely used in health and population studies to quantify the contribution of a risk factor to the occurrence of a disease. Health studies, for example, can quantify how much smoking increases the risk of lung cancer.

Using the climate models, the Fraction of Attributable Risk (FAR) shows how much the risk of extreme temperatures increases thanks to human influences.

In our earlier study of our record hot Australian summer of 2012-13, we found that it was very likely (with 90% confidence) that human influences increased the odds of extreme summers such as 2012-13 by at least five times.

In August 2013, Australia broke the record for the hottest 12-month period. The odds of this occurring increased again from the hottest summer. We found that human influence increased the odds of setting this new record by at least 100 times.

Recent extreme temperatures are exceeding previous records by increasingly large margins. The chance of reaching these extreme temperatures from natural climate variations alone is becoming increasingly unlikely. When we considered the 12-month record at the end of August, it was nearly impossible for this temperature extreme to occur from natural climate variations alone in these model experiments.

We have just completed a preliminary investigation of contributing factors for the record Australian temperature in the 2013 calendar year.

In the model experiments, it is impossible to reach such a temperature record due to natural climate variations alone. In climate model simulations with only natural factors, none of the nearly 13,000 model years analysed exceed the previous hottest year recorded back in 2005.

Australian annual temperature changes (relative to 1911-1940 average) for observations (dashed black) and model simulations with natural influences only (green) and with both human and natural influences (red). The grey plumes indicate the range of values simulated across nine global climate models used. Average Australian temperature anomalies are indicated for 2013 and the previous hottest year on record in 2005. David Karoly & Sophie Lewis.

In contrast, in model simulations including both natural and human factors, such as increasing greenhouse gases, record temperatures occur approximately once in every ten years during the period 2006 to 2020. (On a mathematical note, as there is no instance in which the record hot yearly temperature occurred without human contributions, the FAR value is one.)

Probabilities of annual average temperatures for Australia from climate model simulations including natural influences only (green) and both natural and human climate influences (red) for model years 2006-2020. The vertical lines show the temperature anomalies observed in 2013 and in 2005 (the previous hottest year observed). David Karoly & Sophie Lewis

Clearly both natural climate variability and global warming from humans contribute to recent temperature records. Natural variability always plays a major role in the occurrence of weather and climate extremes. But in the case of our recent hottest year on record, human-caused global warming made a crucial contribution to our extreme temperatures.

Our extensive catalogue of 2013 record-breaking events in Australia occurred in a global context of increasing temperatures that must be considered. Globally, 2013 will likely rank as the 6th hottest year recorded.

So to return to our question, what caused the 2013 record hot year across Australia? Simply put, our climate has changed due to human activities. Recent extremes, such as this hot year, are occurring well outside the bounds of natural climate variations alone.

Sophie Lewis is a Research Fellow at the University of Melbourne node of the ARC Centre of Excellence for Climate System Science.

David Karoly receives funding from the Australian Research Council Centre of Excellence for Climate System Science and the Australian Antarctic Division. He is a member of the Climate Change Authority and the Wentworth Group of Concerned Scientists.

The Conversation
This article was originally published at The Conversation. Read the original article.


  1. I'm sure that most of my scientific colleagues who work with systems that exhibit central tendency will look at that shift in the bell curve in the second graphic and feel a clenching in their bellies.

    It bodes grimly indeed.

    Bernard J.

  2. It is odd that the temperature was so high yet all the coral reefs recovered?

    1. So check out Europe.


    2. "All the coral reefs recovered"?!

      References to evidence, please.

      Bernard J.

  3. Will Abbott cheer the biblical bat scenes?

    Will the world evaluate how terribly easily CAGW can wipe out entire species, entire ecosystems in a matter of a day from that clear warning? Nope. 'That's just Australia, it always gets hot there' is the Dutch sound :)

  4. Anonymous points us to "Europe", using a piece by Dominik Jung, who actually only looks at a few high-altitude stations mainly in the middle Alps Northern face in Winter. I checked the trend for two of the stations in GISTEMP, and in Winter it is indeed negative for 1991-2012. But guess what the trend is for the annual temperatures in the same period? If you guessed "negative", you're wrong.

    I thus ventured a little bit further and checked for some other lower-lying stations nearby:
    1. Klagenfurt: Winter and annual trend = up
    2. Zurich: slightly negative in Winter, much more up annually
    3. Geneva, same story as Zurich
    4. Innsbruck: same as Zurich and Geneva
    I guess we now know how to rate the abilities of Dominik Jung to mislead...

    1. That Dominik Jung is a climate revisionist. One of the younger 'usual suspects', another waste suiciding his career.


Instead of commenting as "Anonymous", please comment using "Name/URL" and your name, initials or pseudonym or whatever. You can leave the "URL" box blank. This isn't mandatory. You can also sign in using your Google ID, Wordpress ID etc as indicated. NOTE: Some Wordpress users are having trouble signing in. If that's you, try signing in using Name/URL. Details here.

Click here to read the HotWhopper comment policy.