Much of Southern Australia is currently experiencing a prolonged hot spell. You can't really call it a heatwave because there are few extreme temperatures, but we're unquestionably in the middle of a much-longer-than-usual spell of well-above-normal temperatures.
- Sydney is now 34 days into a spell where the maximum daily temperature has been 26 degrees or more. The old record was 19 days. The mark of 26 degrees is particularly significant because the average daily maximum in January, the hottest month, is just 25.9 degrees.
- After reaching 33 degrees on Wednesday afternoon, Canberra has now had nine straight days of 30 degrees or more. This has never happened before in any month. The March average maximum is 24.5C
- And Melbourne just had its warmest ever March night, exceeding the old record minimum of 26.5 by at least two degrees. Something pretty interesting happened afterwards. Melbourne sweltered under a warm north-westerly airflow all night. It was still 28 degrees at 9 am. But then a cooler south westerly change blew in on Wednesday morning, meaning the daily maximum topped out in the mid 20s.
As we all know, when extreme heatwaves and bushfires strike, experts remind us that this is the harsh face of climate change in action. But this week has been different. This week is warmer than usual but not in an insane ice-cream-sales-through-the-roof kind of way. Are warm spells like this a climate change thing too?
The answer is yes. It is also yes, yes and yes. To understand why, here's Professor Will Steffen of the Climate Council.
"Climate change comes in various ways," Steffen told The Huffington Post Australia. "It comes in temperature spikes where heat waves are more intense and the hottest day is becoming hotter. But at the same time, the effects are more prolonged."
So Professor Steffen, you're talking about conditions like we're seeing in southern Australia over the last week or two?
"Yes. Climate change can produce a more even period of heat like we've seen in Canberra the last nine days."
Steffen is just about the best-credentialled climate scientist in Australia. He also has a real gift for communicating the science in a way that makes sense. We asked him to explain how we can be assured climate change is real for those who may still be sceptical. Here's what he said.
"There are three ways I look at it.
"One. The basic physics. There is an enormous amount more heat in the atmosphere than normal. This is going to play out one way or the other [whether it's extreme heat spikes or prolonged heat events like the current one]. You just can’t get rid of that heat.
"Two. The so called bell-shaped curves. You find that if you change averages by small amounts, you will get a disproportionate increase in extremes. Let's say a cricket batsman increases his average from 45 to 50. He is going to get a lot more 60s and 70s and maybe the odd century for that to happen. He will also have a disproportionate decrease in low scores like ducks, fives and tens, just as we've seen a decrease in record cold temperatures.
"Three. The modelling. (Eminent Melbourne-based climate scientist) David Karoly has run models on the changing climate with and without extra greenhouse gases. He found that 2013, which was the hottest year on record in Australia, was virtually impossible without climate change. He ran the same model 13,000 times, and found it was highly, highly unlikely you'd get even close to the record temperatures of 2013 without the (current levels of) increased greenhouse gases."
In summary, the increased heat in the atmosphere is affecting our climate in all sorts of ways. Sometimes it makes extremely hot days even hotter, and other times -- like right now -- it is turning what would otherwise be unremarkable weather spells into prolonged hot periods.
"We’ve always had summers, but we don’t have summers like this," Steffen said. "You don’t have a week of weather between 33 and 36 degrees in Canberra in March.
"There are 100 years of Bureau of Meteorology records so on average you'd expect to see heat records set once every 100 years if there was no underlying trend. But we're seeing records tumble five or ten times more frequently than before."
Records have been set across Australia this summer. Adelaide and Perth both endured their longest sequence of days over 40. This is precisely what Steffen is talking about. More records, more frequently in more locations.
But he's also talking about the changes we barely notice. To emphasise this point, here's a really revealing set of data. It's the monthly meteorological data set for Sydney in January. Why Sydney in January? Because Sydney had well above average temperatures in January despite a series of factors which ordinarily would have produced the exact opposite.
Sydney was cloudy and wet in January. The city had 250mm of rain, which was two-and-a-half times the monthly average. The rain fell on 13 separate days, which was also well above normal.
Wet weather typically shaves at least a couple of degrees off daily maximum temperatures. Also, Sydney had no days above 40 to bump up the averages in January, and just six days in the 30s.
All of the above adds up -- or so you'd assume -- to a cooler January. So what happened? Sydney's average daily maximum was 27.4, which was 1.5 degrees above normal. Let's say that again for emphasis like they do on talkback radio shows.
Everything we know about weather says it should have been cooler than normal than Sydney in January. Yet it was significantly warmer.
Why? Because as Will Steffen and countless experts like him have told us, the overall atmosphere is much, much warmer than normal. And it's humans making that happen.