Attribution of 2013’s extreme events

Attribution? What is attribution and why should I be interested?

Attribution in the climate science world is defined as “the process of evaluating the relative contributions of multiple causal factors to a change or event with an assignment of statistical confidence” (Intergovernmental Panel on Climate Change 5th Assessment Report 2013, Chapter 10, page 872). As the IPCC notes, attribution is complex and combines statistical analysis of many factors with a physical understanding of their causes. It almost always involves sophisticated computer modelling. Attribution in this post basically involves the determination of whether human causes such as burning fossil fuels contributed to extreme events related to climate change (anthropogenic climate change), or whether natural climate variability may currently be adequate to explain observed extreme events.

The hard science

A recent American Meteorological Society report, Explaining Extreme Events of 2013 from a Climate Perspective, looked at attribution of numerous extreme  events which occurred in 2013 across the Earth. Five papers dealt with Australian extreme events, especially the very hot and sustained temperatures across the 2013 calendar year including the remarkable 2012-13 summer.


1Jan-14Jan 2013

Highest maximum temperatures, 1 January – 14 January 2013.
Note very large areas of 46 degrees Celsius and above.
Copyright Commonwealth of Australia 2013, Australian Bureau of Meteolology.


The collected works report the investigations of research groups into 16 extreme events. Their findings indicate that anthropogenic climate change greatly increased the risk of extreme heat events. The role that anthropogenic climate change played in other types of extreme events such as droughts, heavy rain falls, and storms is presently less clear, and this may have a great deal to do with the chaotic nature of such events which are extremely difficult to model (being chaotic they involve a sensitive dependence on initial conditions, they are deterministic, and they are nonlinear).

The five Australian papers all conclude that there is strong evidence for the influence of anthropogenic climate change on the high temperatures of 2013 (references in full below):

  1. Knutson and coworkers state that the extremely warm year “was largely attributable to human forcing of the climate system” (page S26);
  2. Lewis and Karoly note that “anthropogenic climate change has caused a very large increase in the liklihood of extreme events such as the record Australia-wide average temperatures in September, spring, and the 2013 calendar year” (page S31);
  3. Perkins and coworkers conclude that “human activiy has increased the risk of experiencing the hot Australian summer of 2012/13, as measured by simulated heat wave frequency and intensity, by two- and three-fold, respectively” (page S34);
  4. Arblaster and coworkers state that ” a multistep attribution process suggests that anthropogenic climate change played an important role in the record Australian maximumtemperatures in September 2013″ (page S41); and
  5. King and coworkers conclude that “the record heat of 2013 across inland eastern Australia was caused by a combination of anthropogenic warming and extreme drought” (page S41),  that “there has been no significant change in meteorological droughts in this region related to anthropogenic climate change” (page s44), but that “in a warming climate, with increasing evaporation and reduced soil moisture, droughts may become more severe” (page S44).

Understandable summaries

Our Climate Council has written several approachable reports on the 2013 heat, including Off the Charts: 2013 was Australia’s Hottest Year and Heatwaves: Hotter, Longer, More Often. The Council notes that the heat of 2013 was remarkably high and prolonged across Australia and many previous weather records were broken, including:

  1. 2013 set the record for the hottest year since records began in 1910;
  2. the country experienced the hottest summer and warmest spring on record;
  3. as well as the hottest January and warmest September on record;
  4. and the hottest summer day and warmest winter day on record

The Climate Council has produced a great little infographic showing these (and other) new records.



Credit: Climate Council 2014, all rights reserved. Click to embiggen.


The complete extreme event report can be downloaded from the AMS website: Herring, S. C., M. P. Hoerling, T. C. Peterson, and P. A. Stott, Eds., 2014: Explaining Extreme Events of 2013 from a Climate PerspectiveBulletin of the American Meteorological Society, 95 (9), S1–S96.

The five Australian articles are:

1. Knutson, TR, Zeng, F, and Wittenberg, AT, 2014. Multimodal assessment of extreme annual-mean warm anomlies during 2013 over regions of Australia and the western tropical Pacific. In Explaining Extremes of 2013 from a Climate Perspective, Bull. Amer. Meteor. Soc., 95 (9), S26–S30.

2. Lewis, SC, and Karoly, DJ, 2014. The role of anthropogenic forcing in the record 2013 Australia-wide annual and spring temperatures. In Explaining Extremes of 2013 from a Climate Perspective, Bull. Amer. Meteor. Soc., 95 (9), S31–S34.

3. Perkins, SE, Lewis, SC, King, AD, and Alexander, LV, 2014. Increased simulated risk of the hot Australian summer of 2012/13 due to anthropogenic activity as measured by heat wave frequency and intensity. In Explaining Extremes of 2013 from a Climate Perspective, Bull. Amer. Meteor. Soc., 95 (9), S34–S37.

4. Arblaster, JM, Lim, E-P, Hendon, HH, Trewin, BC, Wheeler, MC, Liu, G, and Braganza, K, 2014. Understanding Australia’s hottest September on record. In Explaining Extremes of 2013 from a Climate Perspective, Bull. Amer. Meteor. Soc., 95 (9), S37–S41.

5. King, AD, Karoly, DJ, Donat, MG, and Alexander, LV, 2014. Climate change turns Australia’s 2013 big dry into a year of record-breaking heat. In Explaining Extremes of 2013 from a Climate Perspective, Bull. Amer. Meteor. Soc., 95 (9), S41-S45.