Is it still true to say you can’t point to any single extreme weather event and claim you can’t link it to human-caused climate change?
Plenty of people seem to think this is still the case. But a rapidly evolving field of climate science suggests that it’s not.
Take Australia’s prime minister, Malcolm Turnbull, for example, who was touring Tasmania after the devastating flooding there in June.
Turnbull pointed out that “larger and more frequent storms” were predicted by climate scientists, but then followed up with that stock standard caveat:
But you cannot attribute any particular storm to global warming, so let’s be quite clear about that. The same scientists would agree with that point.
 But in fact, climate scientists are finding ways to examine the influence of increasing levels of greenhouse gases in the atmosphere on extreme events.
This is a field of science called attribution research. Dr Andrew King, at the University of Melbourne, has been involved with several attribution studies.
So is it time to throw out that old stock response that you can’t blame climate change for any single event? He says:
I would reframe the question – has climate change altered the likelihood of an event happening, like a flood in Louisiana or a heatwave in Melbourne? We can usually say with those types of events that climate change has increased the likelihood of an event happening.
When extreme weather events do strike, questions about human influences are coming up more and more. Some scientists want to be able to respond quickly with more relevant answers, before the news cycle turns elsewhere.
For example, King joined colleagues to look at the record warm sea temperatures that caused the mass bleaching of corals on the Great Barrier Reef last summer. The results were out in April while the reef’s plight was still making headlines.
“We found that the warm sea temperatures were made at least 175 times more likely because of climate change,” King says.
To carry out the research, King looked at two sets of climate models. One was set up to reproduce the levels of greenhouse gases in the atmosphere that we have now and the other had those human influences removed.
In short, the models showed the kind of conditions that eventually killed about a quarter of the corals on the reef are now expected to come around once every four years. But in a world without the extra greenhouse gases, you might expect to see those ocean temperatures once every 1,000 years, if at all.
The reef research is about to be submitted to a journal and so the results could change. But King says the methods being used had been peer-reviewed.
King also looked at the heatwave that had world leaders sweating during the November 2014 G20 summit in Brisbane. Getting a 38C day in November was “at least 44% more likely” thanks to climate change, his study found.
From torrential downpours to record ocean temperatures, heatwaves and the monotonous breaking of monthly and yearly global temperature records, more and more studies are finding a distinct human fingerprint on events.
There have been a bunch of attribution studies looking at heat records in Australia. A study of its consecutive run of record warm springs in 2013 and 2014found it would have been almost impossible without all that extra carbon dioxide in the atmosphere.
Another study found that without the added greenhouse gases, Australia’s record hot 2013 would only have come along once every 12,000 years. But now, thanks chiefly to the burning of fossil fuels since the industrial revolution, we might expect a record breaker every six years.
Dr Sophie Lewis, at Australian National University, was involved in both those studies. She says climate attribution is a “fast-evolving field” thanks to quicker computers, better collaborations and established methods.
But like King she’s not a fan of simply asking if climate change “caused” something, or was “to blame” for particular events. She says:
A better question is to ask if climate change has influenced a particular event. That’s an important distinction.
An example she gave is 2015 – the planet’s warmest year on record that coincided with an El Niño climate system.
El Niños are natural events that tends to deliver hotter temperatures, but they are happening over the top of human-induced warming that pushes temperatures to record-breaking levels. Lewis says:
We know that both natural and human-caused climate change have impacts on events and we don’t want to lose that complexity. People do understand that the environment and the climate system are complicated.

Speed counts

Getting results out faster gives the media, the public and policymakers more informed answers soon after events hit.
In early August, for example, Louisiana was struck by torrential rains that caused severe flooding – killing 13 people and damaging about 60,000 homes. Less than a month later, a team of scientists concluded that human-caused climate change had probably doubled the chances of Louisiana being hit by downpours like that.
The lead author of that study, Dr Karin van der Wiel, of Princeton University and the US government’s National Oceanic and Atmospheric Administration, told me:
Extreme events have always happened and this could have happened a hundred years ago. But it would have been much more rare.
Van der Wiel says researchers managed to carry out the analysis quickly because the data from the climate models was readily available and they had good had rainfall data for the area.
The Louisiana study has been submitted to a journal where the peer-review process happens out in the open so, again, the conclusions could change.
Now, some scientists are uneasy about research being publicised before it has been through peer review, for obvious reasons. What if, for example, there’s a mistake in the analysis that completely changes the conclusions?
Both Van Der Wiel and King say they can have a degree of confidence in their results because the methods have already been tested. I’ll leave you with King’s thoughts on this.
As far as I see it, one purpose of event attribution is to communicate to the public and policy makers that climate change is altering how extreme events are occurring – both their frequency and how bad they are.
That’s why we’re moving towards doing this work more quickly. If we can better inform the debate, then that provides some useful information that’s grounded in science, when often there has been just speculation.
www.theguardian.com

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