Climate Central - Andrea Thompson
When climate scientists examine whether the warming of the Earth has
made extreme weather events such as heatwaves or downpours more likely,
they generally do it
on a case-by-case basis. But a group led by Stanford climate scientist
Noah Diffenbaugh has aimed to develop a more global, comprehensive approach to investigating how climate change has impacted such extremes.
With a new framework they developed, Diffenbaugh’s team found that
heat records were made both more likely and more severe for about 80
percent of the area of the globe with good observational data. For
precipitation records, that percentage was about half.
 |
| Residents who refused to be evacuated sit on makeshift boats during evacuation operations of the Villeneuve-Trillage suburb of Paris on June 3, 2016. Credit: REUTERS/Christian Hartmann |
The team also examined a few particular events, finding, for example,
that warming was clearly linked to the record-low summer Arctic sea ice
extent of 2012.
Given the findings of previous so-called attribution studies as well
as long-term warming trends, those results aren’t surprising, but they
do show how much human-caused global warming has affected weather
extremes already, the study authors and outside experts said.
And while several outside researchers quibbled with some aspects of
the study, they said it provided a new tool that could help researchers
more easily and uniformly probe what ingredients of a particular extreme
event exhibit a climate change signal.
“The overall message — that changes in extremes worldwide can be
attributed to human-induced climate change — is not new, but this paper
adds another piece of relevant evidence to bolster that conclusion,”
Peter Stott, a UK Met Office climatologist who conducted the 2003 study that kicked off the attribution sub-field, said in an email.
The idea behind extreme event attribution studies is to gain a better
handle on how warming is changing the risk of different types of
extreme weather in different areas. Because extremes have some of the
biggest impacts on people, infrastructure and the economy, understanding
how those risks are changing can help government officials and
businesses better plan for the future.
Most of these studies, though, are generally case studies of specific
events, often ones that happen in scientists’ backyards. While
informative, they lead to what scientists call “selection bias,” meaning
they aren’t taking in the full scope of how warming is affecting
extreme weather.
Diffenbaugh and his colleagues, who have done several attribution case studies, particularly on the
California drought,
sought to get a broader view by using existing attribution methods to
look at particular climate measures across a broader swath of the
planet. These included the hottest day, hottest month, driest year and
the wettest five-day period.
The results, detailed Monday in the journal
Proceedings of the National Academy of Sciences, show that heat records in 80 percent of the study area were more likely affected by climate change than not, Diffenbaugh said.
This suggests that the world is not quite at the point where every
single record-setting heat event has a discernable climate change
influence, “but we are getting close,” he said.
For both the driest year and wettest five-day period, “about half the
area exhibits an influence of global warming, and that is substantial,”
even though it is less than for heat, Diffenbaugh said.
The higher percentage for extreme heat makes sense given the clearer
line between warming and temperature; that extreme heat events are
expected to occur more often and be more severe is one of the more
robust outcomes of warming.
On the other hand, “precipitation is just a noisier quantity,” making
it harder to pick out the climate change signal in some areas,
Adam Sobel, a Columbia University climate scientist who wasn’t involved in the study, said in an email.
But that “doesn't mean the influence isn't there — all we can say is
that it hasn't clearly risen above the noise, but the noise is large so
it is reasonable to expect that it will emerge in time,” he said.
The biggest influence from climate change was seen on heat and dry
extremes in the tropics, “a combination that poses real risks for
vulnerable communities and ecosystems,” Diffenbaugh said in a statement.
 |
| Sydneysiders take refuge from sweltering conditions alongside apartments at Sydney's North Cronulla Beach during a heatwave along Australia's east coast on Feb. 11, 2017. Credit: REUTERS/Jason Reed |
The downside to the approach the team used is that the measures they
used aren’t always the most relevant for the actual impacts on the
ground, which is what people most care about and what attribution case
studies try to address, Friederike Otto, of Oxford University's
Environmental Change Institute, said. Otto, who works with Climate Central’s own
real-time attribution effort, also would’ve liked to see the study use more than one climate model.
While the new approach is useful “to gain confidence in real-time
attribution,” allowing teams to place what they find in a larger
context, “it doesn’t replace the actual attribution study in any way,”
she said.
Diffenbaugh agreed and said that the team is working to develop ways
to use their approach to look at the climate influence on particular
impacts, such as the relationship between high temperatures and crop
yields or coral bleaching.
He also said that his team’s framework can better help scientists
look at how climate change is impacting the various ingredients that
combine to cause extreme events, rather than focusing on just one aspect
as many have to-date. For example, they found that warming had made a
certain atmospheric pattern that led to a deadly heatwave in Russia in
2010 more common and more severe.
Conversely, while previous studies showed that changes in such atmospheric patterns made a major downpour and
flooding event in Boulder, Colo., in 2013 less likely, the warming and moistening of the atmosphere would increase its likelihood.
The hope is that the framework is a step toward doing more real-time
attribution studies and making analyses more consistent from study to
study. Stott, who is working on a similar effort, said that this study
does help move things in that direction.
This approach is “one brick in the wall and there are a lot of really
smart people working hard on different aspects of this,” Diffenbaugh
said. “We’re building a strong foundation for being able to ask these
questions and answer them in a scientifically valid way.”
Links
