Until now scientists have been cagey about linking extreme weather events such as this summer's heatwave to climate change. An emerging field is changing all that
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| Patrik Stollarz/AFP/Getty Images |
In Japan, more than 22,000 people were taken to hospital with heat stroke as the country recorded its highest-ever temperature of 41.1 degrees Celsius. In California, Portugal and as far north as the Arctic Circle huge wildfires, encouraged by months of unusually dry conditions, followed the searing heat.
For years, climatologists asked to explain these kind of extreme events have fallen back on a well-worn phrase. “It’s impossible to attribute a single weather event to climate change,” the refrain goes. And they’re right. Weather is by its very nature unpredictable – extreme events will always happen in one place or another, regardless of global temperature levels, and they’re not necessarily tied to one particular cause.
For Friederike Otto, deputy director of the Environmental Change Institute (ECI) at the University of Oxford, this response has its drawbacks. “If scientists don’t answer, someone else gives an answer and it’s usually people who aren’t interested in the size and have their own agenda,” she says. Instead, Otto wondered if scientists could start saying whether climate change had made certain extreme weather events more or less likely.
Now Otto is right at the heart of a growing scientific movement called extreme event attribution. Her aim? To be able to point to an extreme weather event and use climate modelling to say whether that same event would have been more or less likely to happen in a world where humans hadn’t caused global temperatures to rise by a whole degree over the last 120 years.
Up until a few years ago, it wasn’t possible to draw that link with any degree of accuracy, Otto says. But in 2004, Pete Stott at the UK Met Office published a paper in the scientific journal Nature showing that climate change had at least doubled the risk of the 2003 European heatwave that killed tens of thousands of people.
Twelve years later the Bulletin of the American Meteorological Society dedicated an entire issue to the new field of extreme event attribution. In the introduction, its editors argued that it was now possible to detect the effects of climate change on some events with high confidence. “That was really the first time we could say that we can attribute events to anthropogenic climate change,” Otto says.
In late 2014, Otto helped set up the World Weather Attribution (WWA) initiative – a collaboration between the ECI, the Netherlands-based Red Cross Red Crescent Climate Centre and Royal Netherlands Meteorological Institute. The aim of the project wasn’t just to draw a link between extreme events and climate change, but to provide this analysis in real-time so they’d have answers while the extreme weather event was actually happening.
In July, while most of Europe was still sweltering under unusually high temperatures, Otto released her analysis of this year’s heatwave. She looked at temperature measurements in seven locations in Ireland, Denmark, the Netherlands, Norway, Sweden and Finland and used different climate models to estimate how likely it was that those same temperatures would occur in a world without climate change.
To do this, Otto ran hundreds of simulations on a pair of climate models. These are the same kind of models used for weather reports that take into account variables like rainfall, wind, temperature and air pressure. The only variable changed in the two models is the concentration of greenhouse gases in the atmosphere, as these are main drivers of climate change.
One model represents our present-day atmosphere while the other models the kind of world we’d be living in if the concentration of greenhouse gases hadn’t ramped up post-1900. By simulating hundreds of years of weather using these two models, Otto and her colleagues are able to compare what the weather might be like in a world with or without climate change.
When it comes to the northern European heatwave, the analysis was unequivocal. Climate change caused by humans has made these kinds of temperatures at least twice as likely to happen. In other words, if we were living in a world where humans hadn’t heated up the planet by a whole degree, this summer heatwave would have been half as likely.
In Copenhagen, Denmark, which saw temperatures of just below 31 degrees Celsius, climate change made the likelihood of those temperatures occurring five times higher. In Linköping, Sweden, it was six times higher. This is weather attribution in action, in real-time. “What climate change can do, and in the case of this heatwave what it has done, is change the likelihood of an event occuring,” she says.
Although the language of likelihood and chance may sound imprecise, this is a great deal more certain than anything researchers would have been able to identify a decade. Otto compares her analyses to studies into smoking and cancer. Although it’s impossible to definitively say that an individual case of lung cancer was caused by smoking, we can draw a direct link between smoking and the likelihood of someone developing cancer.
So far, Otto and her colleagues have analysed a raft of extreme weather events including extreme rainfall in Japan in early July 2018 that killed 200 people and the unusually cold winter across North America last year. And the analyses don’t always turn up a link between climate change and extreme weather events. In a study into the 2015 Ethiopian drought that affected nearly 10 million people across the country, Otto and her colleagues found no influence of climate change.
And finding examples where climate change isn’t behind an extreme weather event is just as worthwhile as finding cases where it is. Otto’s hope is that individuals and governments use her analyses to futureproof themselves against the impact of climate change.
“One of the main motivations behind this project is that climate change is already happening today and it’s having an impact on our day to day life,” she says. In the UK, extreme event attribution has already been used to help assess the risk of flooding in local areas, and Otto expects this kind of analysis to be used more widely as the field becomes more established.
“For a large part of the world it’s still a very new science,” she says. But this emerging field could help governments to start making their decisions on what might happen on the future instead of thinking about what has happened in the past. “If you have a changing climate and you only look at the past you will not get the right answer,” Otto says.
At the moment the WWA’s analyses compare a world with no warming to a world with one degree of warming, but Otto also runs models that look at how the weather will change if the world warms by a further one degree, as is projected to happen by the end of this century. If that happens, the temperatures in Copenhagen we saw this summer will be four times more likely to happen in the future.
And it’s these extreme weather events that we should be paying attention to – not just the headline figures of global temperature increases. “Global mean temperature doesn’t kill anyone,” Otto says. “It’s extreme events that kill people.”
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