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Devastation following Cyclone Pam.
Graham Crumb/ Commons
22 June 2015 16:30

Climate change attribution studies are asking the wrong questions, study says

Robert McSweeney


Robert McSweeney

22.06.2015 | 4:30pm
AttributionClimate change attribution studies are asking the wrong questions, study says

Scientists are calling for a rethink in the way we seek to understand how climate change affects extreme weather.

The latest in so-called attribution studies is to study each individual event by itself, looking for how climate change may have made it stronger or more likely.

But a new paper says the methods used in many of these studies underestimate the influence of climate change, and suggests a new approach to identify the “true likelihood of human influence”.

Single-event attribution

One of the first studies to attribute a single extreme weather event to climate change was published just over a decade ago. Researchers showed that climate change had doubled the chances of the record heatwave Europe experienced in 2003.

In the years that followed, many more studies have aimed to provide answers on how climate change is affecting our most brutal weather.

But while scientists have been able to attribute events caused by temperature extremes, linking other extreme events like storms and heavy rainfall events has proved more difficult, says a new paper in Nature Climate Change.

Canicule _Europe _2003

Difference in temperature for 20 July to 20 August 2003 compared to long-term average. Source: Reto Stockli and Robert Simmon (NASA).

In our chaotic weather system, the complex dynamics of the atmosphere mean the size and path of a storm or heavy rainfall event has a large element of chance, the authors say. This can make it tricky to identify where climate change fits in.

But rather than analysing the wind patterns that bring a storm to an area, scientists should be looking at how the impact of that storm has been boosted by temperature changes –  known as thermodynamic effects.

Temperature increases mean more moisture evaporates into the atmosphere and more ice melts into our warming oceans, raising their levels. These are changes that scientists can be confident of, the authors say, and so should be the basis for attribution studies – rather than looking at changes to circulation patterns in the atmosphere.

Boulder floods

The paper highlights a few examples of where this approach could have been taken. One is the floods in Boulder in the US, which took 10 lives and caused over $2bn of damage in September 2013.

According to a study published in the latest Bulletin of the American Meteorological Society (BAMS) special issue on attribution, climate change did not contribute to the extreme five-day rainfall event that caused the floods.

But the authors of the new paper dispute this result. While it may be that climate change didn’t influence the specific weather system that brought the rain, it is likely to have contributed to the sheer volume of moisture in the atmosphere, the paper says.

In short, the study didn’t ask the right questions, lead author Prof Kevin Trenberth from the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, tells Carbon Brief:

“Tracking where the moisture came from in flood situations is always valuable to do, but is not done often enough.”

But not all studies overlook this factor. A study after the UK floods in the 2013-14 winter took such an approach. The researchers found that climate change made the floods 25% more likely because of the extra moisture that winter storms could carry. You can read more in a guest article written for Carbon Brief by co-author Prof Richard Allan.

Flooding -in -boulder

High school fields in Boulder on 13 September 2013. Credit: littlegreenfroggy

We shouldn’t admit defeat

Not all scientists agree that attribution studies need a change of focus.

Dr Peter Stott, who leads the Climate Monitoring and Attribution team at the Met Office and wasn’t involved in the paper, says we shouldn’t admit defeat and ignore the impacts of climate change on circulation patterns. He tells Carbon Brief:

“We’d be failing in our mission to society if we didn’t consider both dynamic and thermodynamic effects, given it is both that influence the probability and magnitude of extreme climate events.”

Through initiatives like the BAMS special issue and the new EUropean CLimate and weather Events: Interpretation and Attribution (EUCLEIA) project, scientists are already looking at both, says Stott.

And the research will only get better, he says:

“As our modelling capability improves and our understanding of the dynamical causes of extreme events develops, the potential for making holistic event attribution statements that consider all facets of the event – and not just one particular aspect – will likewise improve.”

Real and present

Attribution studies tend to start from an assumption that climate change didn’t have an impact on an extreme event. This means they can underestimate the impact of climate change, the paper says:

“Because climate change is real and present, it is not a question as whether it is playing a role, but what the role is.”

In order to implement this new approach, scientists should simulate the extreme event in climate models, and then factor in what we know about how climate change has affected the heat and moisture in the climate system, the authors say.

But this is an approach that many attribution scientists have considered and rejected, says Dr Friederike Otto, a senior research at the University of Oxford, who also wasn’t involved in the study:

“The fundamental problem with this suggested setup is that it virtually guarantees you will find a positive role for human influence in all the most extreme weather events.”

If an extreme event is dependent on many different factors, and you take one away, you won’t get such an extreme event, she says:

“So you end up with the conclusion that human influence made a particular winter snow event worse, even though it is also – at the same time – reducing the risk of extreme winter snow events.”

This could be confusing when trying to communicate attribution to the public, Otto says. But it’s clearer communication that the paper’s authors are aiming for, says Trenberth:

“I hope that this [paper] will cause a sea change in how these studies are done, and bring much greater clarity to the messages to the public.”

So it seems the discussions around how best to get across to the public how climate change is affecting extreme weather will rumble on for a bit longer. And what might seem like the simplest approach for communication purposes to some, others might see as a missed opportunity to communicate the most important progress in science.

Main image: Devastation following Cyclone Pam.

Trenberth, K.E. et al. (2015) Attribution of climate extreme events, Nature Climate Change, doi:10.1038/nclimate2657

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