Saturday, March 12, 2016

In-Depth:  the Scientific Challenge of Extreme Weather Attribution

Working out whether human activity is supercharging extreme events, such as floods, storms, droughts and heatwaves, is one of the youngest branches of climate science.  But it’s moving at breakneck pace.

So much so, that the US National Academy of Sciences has fast-tracked a report, published Friday, taking stock of the science and where it’s heading.

Event attribution is the field of science that asks if extreme weather around the world would look any different if we could replay the last 200 years or so, without human-caused greenhouse gases.

Today’s report is an overview, rather than a showcase for new results. And at about 150 pages long, it’s not a light read.  But its weightiness is apt for a topic that has come to underpin climate conversations everywhere from flooding in the UK to climate change adaptation.

Carbon Brief has been speaking to key scientists in the world of attribution about how far the science has come, experimenting with communicating the nuances, and the thorny issue of making results public at lightning speed, often before peer review.

One thing is for sure, Dr Heidi Cullen, chief scientist at Climate Central and a contributor to today’s report, tells Carbon Brief:
“The days of saying no single weather event can be linked to climate change are over. For many extreme weather events, the link is now strong.”
‘A universal talking point’
Storms, droughts, heavy rain, heatwaves and other extreme weather events are of huge interest to society because of their often disastrous consequences for people and property.

As Prof Ted Shepherd, professor of climate science at the University of Reading in the UK, explains in a recent commentary in the journal Current Climate Change Reports:
“Just as weather is a topic of daily conversation, extreme weather events…provide a universal talking point.”
A woman and a dog are rescued from flood waters in Chennai, India, 04 December 2015. (Credit: © BABU/epa/Corbis) Click to Enlarge.
Prof Myles Allen, head of the climate research programme at Oxford University’s Environmental Change Institute, first proposed the idea of extreme weather attribution in a 2003 paper.  His basic premise was that scientists could use climate models to work out what proportion, if any, of a given extreme weather event could be pinned on human activity.

This is not the same as asking whether climate change “caused” an event to occur.  Today’s report calls this framing a “poorly formed (or ill-posed) question that rarely has a scientifically satisfactory answer”.  And as Shepherd explains in his review paper:
“If a weather or climate event is truly extreme in the present climate…it requires unusual meteorological conditions, which means that climate change is at most only a contributing factor.”
The point of extreme event attribution is to work out how big that contribution is and how it compares to other factors, such as natural fluctuations in the atmosphere and oceans.

Read more at In-Depth:  the Scientific Challenge of Extreme Weather Attribution

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