Five new studies examine how greenhouse gas emissions helped Australia break multiple temperature records and increase the risk of extreme heat

Australia had a hot one in 2013 – a real record breaker.



We started off with a heat wave that made January 2013 the hottest month on record that was part of the hottest summer on record that then became the hottest year on record.

Meteorologists and climatologists looking at records tend to express things by “anomalies” – how far above or below the long-term average is a particular temperature.

September 2013 had all the anomalous bells and whistles you could muster, managing to break above the long term average by 2.75C – a departure greater than any other month on record going back to 1910.

But how much of the temperature rise from that remarkable year was actually down to the extra CO2 in the atmosphere that’s caused the planet to continue to build up heat? Did we leave our dirty fossil fuel fingerprints anywhere?

Here’s one answer (actually there are several, but we’ll start with one).

Climate scientists Dr Sophie Lewis, of the Australian National University, and Professor David Karoly, of the University of Melbourne, ran two groups of computer models for a study into Australia’s scorching 2013.

In one group, they included the extra carbon dioxide in the atmosphere being added at the rates they are now. In another group of models, they left out the human contribution.

They found that on average, the computer models with current levels of carbon dioxide managed to reproduce the temperatures comparable to that scorching year of 2013 every six years.

For the computer models without the added greenhouse gas emissions, they got a year as hot as 2013 only once in more than 12,000 years.



So was it global warming what done it? I asked Dr Lewis.



Instead of focusing on blame, it is more useful to understand the contributing factors to an event, such as natural variability and greenhouse warming. In the case of Australia’s record hot year, anthropogenic influences were a big contributor, to the point that the temperatures we experienced would have been virtually impossible without greenhouse gases. This doesn’t mean that natural variability isn’t important too. There is one line of thought that all the weather and climate events we have experienced have occurred in a hotter, wetter environment caused by greenhouse gases. Because of this, we play a part in everything. Another is the old stock statement that because many factors interact in complex systems, we can’t separate out human influences from natural climate variability. I don’t think either is particularly useful for helping to understand the extremes we are experiencing. Statements like “no single event can be blamed on global warming” aren’t useful. They prevent us from understanding how climate change is influencing what we experience.

Lewis’ study was one of 22 in a special edition of the Bulletin of the American Meteorological Society that looked at potential human influences on 16 extreme weather events across the globe.

Five of the 22 studies looked at Australia’s record heat of 2013.

One study, led by scientists at the US Government’s National Oceanic and Atmospheric Administration, looked at Australia’s record-breaking temperature “anomaly” in 2013 for average temperatures – that’s 1.72C.

Then they ran a series of computer models. They then sliced up the 1.72C into chunks and found that just under half of the temperature increase (0.81C) was down to human emissions.

Another study led by Australia’s Bureau of Meteorology looked in particular at that hot September where the maximum temperatures were 3.32C above the long-term average (what they were between 1961 and 1990).

Human influences played “an important role”, the study said, and up to 15 per cent of the extra heat was down to the warming trend since the early 1980s.

Another study found that the risk of Australia experiencing more heat waves had now tripled when compared with an atmosphere without the added greenhouse gases.

Finally, the fifth study looked at the relationship between drought and maximum temperatures. Before 2013, the hottest year in terms of the maximum temperatures was 2002.



The study found that the risk of Australia getting a year of maximum temperatures hotter than 2002 is 23 times greater than it was in the late 19th century. There was an at least seven-fold increase in the risk of Australia getting a combination of extreme heat and drought occurring at the same time.

But hold on there.



I’ve just realised that in the course of all this, I have forgotten to add those stock phrases about scientists not being able to separate human influences in the climate from all that natural variability.

I also neglected the one about human-caused climate change being something we should worry about in the future.

My sincere apologies.