The human role in our ‘angry’ hot summer

The numbers are in: human-generated emissions increased the odds of our record-breaking summer. Lazellion/Flickr

Today we released a study that shows quantitatively that anthropogenic climate change substantially increased the likelihood of the record-breaking Australian summer of 2013. Indeed, human influences on the climate system increased the chances of our record hot, “angry” summer by more than five times.

Average temperatures across the globe are now 0.8°C warmer than a century ago. This shift in the average climate can lead to substantial changes in the frequency and severity of extreme climate events.

Globally, many of the record-breaking heatwaves and extreme summer temperatures occurring elsewhere have been linked to anthropogenic influences. Our latest analysis of the 2013 extreme Australian summer also demonstrates a strong human influence on the record temperatures.

We started this study with over 20 of the latest generation of climate models being used in an international initiative that undertakes standardised model experiments. Of these, only the nine models that were best able to capture the observed variations and included runs with just natural climate influences were included in our analyses.

Probability distribution of average temperature variations across Australia in summer from observations (dashed line) and climate model simulations (solid line) for 1910-2005. The vertical lines mark the temperature departures for 1998 summer (the second hottest) and 2013 (the hottest) summer across Australia/ Lewis & Karoly
As above, but showing the shift in the probability distribution for 2006-2020 from climate model simulations including increasing greenhouse gases and other human influences on climate. Lewis & Karoly

Using these remaining nine state-of-the-art global climate models, we investigated changes in the probability of extreme Australian summer temperatures due to human influences.

We compared temperature probabilities in a suite of model simulations of the climate including only natural climate influences (volcanoes and solar radiation changes) with a parallel set of model simulations including natural and anthropogenic climate influences, such as greenhouse gases.

The results were clear. When anthropogenic influences were included in the model simulations, the probability of warmer summer temperatures like 2013 were far higher. Indeed, it was very likely (with 90% confidence) that human influences increased the odds of extreme summers by at least five times.

The recent record summer was also notable because it occurred at a time when El Niño-Southern Oscillation (ENSO) conditions were neutral to weakly La Niña. This typically produces cooler temperatures in Australia. Previously, six of the eight hottest Australian summers occurred during El Niño years. Our research shows that natural ENSO variations are unlikely to explain the record 2013 heat.

The model experiments also show that these types of extreme Australian summers will become more severe and more frequent in the future, with further global warming. Extreme summers occur eight times more frequently in the climate model simulations that include human influences, such as greenhouse gases, compared to the simulations with only natural climate variations.

It is now virtually certain that the frequency and severity of hot days will increase. Previous research has also shown that the number and duration of summer heatwaves in Australia has increased over the last 50 years and that heatwaves are occurring earlier in spring and later in autumn. In some places, the first heatwave of the season is now occurring up to a month earlier than 100 years ago.

During our hot summer of 2013, temperature records were broken on daily through to monthly timescales, as well as for the entire summer. Although we are rugging up for winter now, on January 7 this year, we experienced our hottest day on record for the entire continent, as we sweltered under a “dome of heat”. The recent record hot summer was also bookended by heatwaves in November 2012 and much of the first half of March.

What will these more severe and more frequent hot summers of the future feel like? Future, angrier summers will likely be characterised by hotter days, and longer and more frequent heatwaves, also covering large parts of the continent.

It is clear that human influences have loaded the climate dice and substantially increased the odds that we will roll long, hot summers. The risk will increase even further for future summers.

Our study demonstrates the substantial human influence on extreme Australian summers when we warm the continent by 0.9°C . Unless concerted global efforts are implemented soon, the long-held goal of remaining below 2°C of global warming will very soon become unachievable. Warming of up to 4-5°C is possible by the end of the century, and warming of this magnitude would make our recent record-breaking angry summer seem very mild-mannered indeed.

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