New York Times - Henry Fountain
 |
| Scientists attribute several extreme weather
events to climate change. Clockwise from top left: a 2013 drought in New
Zealand; fires in Los Angeles last month; a 2014 heat wave in
Australia; flooding southeast of Paris in June. The goal of the research
is to get sound scientific analysis to the public to help counter
misinformation, deliberate or otherwise, about an event.
Credit
Clockwise from top left: Christine Cornege/New Zealand
Herald, via A.P.; Gene Blevins/Reuters; Daniel Munoz/Getty Images;
Jeremy Lempin/European Pressphoto Agency |
When days of heavy rain in late May caused deadly
river flooding in France and Germany, Geert Jan van Oldenborgh got to work.
Dr. van Oldenborgh is not an emergency responder or a disaster manager, but a climate researcher with the
Royal Netherlands Meteorological Institute.
With several colleagues around the world, he took on the task of
answering a question about the floods, one that arises these days
whenever extreme weather occurs: Is
climate change to blame?
For
years, most meteorologists and climate scientists would answer that
question with a disclaimer, one that was repeated so often it became
like a mantra: It is not possible to attribute individual weather events
like storms, heat waves or droughts to climate change.
But
increasingly over the past decade, researchers have been trying to do
just that, aided by better computer models, more weather data and, above
all, improved understanding of the science of a changing climate.
Attribution
studies, as they are called, can take many months, in large part
because of the time needed to run computer models. Now scientists like
Dr. van Oldenborgh, who is part of a group called
World Weather Attribution, are trying to do such studies much more quickly, as close to the event as possible.
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Scientists did not attribute
climate change to other weather events, including ice floes in
Antarctica in 2014, left, and bush fires in Malaysia in 2014, right.
Credit
Left: DeAgostini/Getty Images. Right: Mohd Rasfan/Agence France-Presse — Getty Images |
"Scientific
teams are taking on the challenge of doing this kind of analysis rather
rapidly," said Peter A. Stott, who leads the climate attribution group
at the
Met Office, Britain's weather agency.
The
goal is to get sound scientific analysis to the public to help counter
misinformation, deliberate or otherwise, about an event.
"It's
worthwhile to give the best scientific evidence at the time, rather
than not saying anything and letting others say things that are not
related to what really happened," said Friederike Otto, a researcher at
the
University of Oxford who works with Dr. van Oldenborgh.
In the case of the European floods, World Weather Attribution, which is coordinated by
Climate Central,
a climate-change research organization based in Princeton, N.J.,
released a report less than two weeks after the Seine and other rivers
overtopped their banks. The group
concluded that climate change had made the French flooding more likely, but could not draw a conclusion about the flooding in Germany.
"In
the French case, we had five almost independent measures, and they all
agreed," Dr. van Oldenborgh said. "With Germany, we only had two, and
they disagreed."
 |
The scientists were uncertain about the link between climate change and recent flooding in Germany.
Credit
Sven Hoppe/European Pressphoto Agency |
Climate scientists
have said for decades
that global warming should lead to an increase in extreme weather like
heat waves and droughts. Because more water evaporates from the oceans
and warmer air holds more moisture, climate change should also lead to
more intense and frequent storms.
Studies have shown that these effects are occurring on a broad scale. The
National Climate Assessment, for instance, notes that
heavy downpours have increased across most of the United States in the last 25 years.
But
analyzing individual events is problematic, largely for two reasons:
Weather is naturally variable, even without climate change, and global
warming may be only one of several factors influencing a particular
event. Since reliable data is required, studies are also less likely to
be undertaken for events in countries that lack much data-collecting
infrastructure, or where governments do not share data widely.
Dr. Stott, of the Met Office, was the lead researcher for an early attribution study, a 2004
paper in Nature that linked a deadly
2003 heat wave in Europe
to human-caused increases in greenhouse gas concentrations in the
atmosphere. Since then the pace of such studies has increased; last
year, a
publication
of the American Meteorological Society, edited by Dr. Stott and others,
had 32 studies of 28 events, covering all seven continents.
Not
all of them found a link to climate change. A study of water shortages
in southeastern Brazil during a dry period in late 2014 and early 2015,
for instance, found that the shortages were most likely driven by
increasing population and water use than by climate change.
But
other analyses — of a 2013 heat wave in Argentina, extreme rainfall in
the Cévennes Mountains in France in 2014 and an extremely hot spring in
South Korea that year — found a connection.
David W. Titley, a professor of meteorology at Pennsylvania State University who was chairman of a National Academies
committee
that looked at developments in the field of climate-change attribution,
said that at this point studies of heat waves and other
extreme-temperature events appeared to produce the most reliable
assessments.
Studies
of extreme rainfall are considered less reliable in finding links to
climate change, and studies of events like wildfires and severe
thunderstorms even less reliable.
Still, Dr. Titley said, such studies are worth doing, as long as certain conditions are met.
"There
are very legitimate reasons why people want to do this rapidly," he
said. "But they need to state very clearly what the assumptions are,
what the methods are, what the confidence level is," he said.
"This is still not like predicting what time is sunrise for New York City tomorrow," he added.
Attribution
studies usually involve running climate models many times over. Because
no model is a perfect representation of reality, varying them slightly
for each run and averaging the results give scientists more confidence
in their accuracy.
One
set of runs simulates the climate as it actually is, incorporating the
effects of greenhouse gas emissions, while the other set simulates the
climate as if that human influence had never happened.
Researchers
then compare historical data, as well as data from the actual event,
such as rainfall or temperatures, to the different model results to
assess any climate-change impact. The analysis is usually given in terms
of probabilities, or increased or reduced likelihood, that climate
change had an effect.
Rather
than running models after an event, researchers like Dr. van Oldenborgh
and Dr. Otto shorten the process by using models that have already been
run.
"The only way we can do this rapid attribution is by precooking everything that we can," Dr. van Oldenborgh said.
The
process starts with emails among members of the group, usually followed
by a Skype session to discuss whether a specific event is worthy of
study. (One group member is in Australia, so arranging a conference call
can be tricky.)
At
least one member of the group also has to have time to do the work.
"You have to put everything aside for a week" said Dr. van Oldenborgh,
who did most of the work on the flood analysis because the other
researchers were at a conference.
A
crucial part of the task is to define the event — what happened and
what meteorological variable is best to study it. In the case of the
European study, the models the researchers used simulate rainfall, not
flooding. So they used rainfall as the variable.
"We can't look at flooding, only extreme precipitation," Dr. Otto said. "Where did it rain, how much did it rain?"
But
they also consulted with a hydrologist who understood how rainfall
affects the river systems involved, Dr. van Oldenborgh said, "just to
make sure we didn't do something stupid."
Among the models they use is one that Dr. Otto's group at Oxford, the
Environmental Change Institute, runs regularly, using the personal computers of a large number of volunteers, a project called
climateprediction.net. While the model is a global one, regional results can be extracted and used for the rapid analysis.
Dr.
van Oldenborgh said that the European flooding analysis was a good
illustration of the need for transparency, because while the researchers
were confident in their findings for France, they acknowledged that
they could not draw conclusions regarding Germany.
"We
have to make sure we're open-minded enough to conclude that, although
we've spent a lot of time on this, we can't conclude anything," he said.
When
the initial analysis is completed and released to the public — Climate
Central helps with the communication process — the work is still not
over. Although the hope is that such studies will eventually become so
routine that there is no need to publish the findings in a scientific
journal, for now, at least, a research paper has to be written and
submitted for peer review. So for the flooding analysis, that was
another week's work for Dr. van Oldenborgh.
"The
worst problem is that you're pretty tired and want to take a break and
put your feet up," he said. "We haven't got a solution for that yet."
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