The Conversation
- Xubin Zeng
Author Xubin Zeng is Professor of Atmospheric Sciences and Director of the Climate Dynamics and Hydrometeorolgy Center, University of Arizona. |
Extreme rainfall and flooding left paths of destruction through communities
around the world this summer.
The latest was in Tennessee, where preliminary
data shows a record-shattering
17 inches of rain
fell in 24 hours, turning creeks into rivers that
flooded hundreds of homes
and
killed at least 18 people.
A lot of people are asking: Was it climate change? Answering that question isn’t
so simple.
There has always been extreme weather, but human-caused global warming
can increase extreme weather’s frequency and severity. For example, research shows that human activities such as burning fossil
fuels are
unequivocally warming the planet, and we know from basic physics that
warm air can hold more moisture.
A decade ago, scientists weren’t able to confidently connect any individual
weather event to climate change, even though the broader climate change trends
were clear. Today, attribution
studies can show
whether extreme events were affected by climate change and whether they can be
explained by natural variability alone. With rapid advances from research and
increasing computing power,
extreme event attribution
has become a burgeoning new branch of climate science.
The
latest attribution study, released Aug. 23, 2021, looked at the rainfall from the European storm that
killed more than 220 people when floods swept through Germany, Belgium,
Luxembourg and the Netherlands in July 2021.
A team of climate scientists with the group
World Weather Attribution
analyzed the record-breaking storm, dubbed Bernd, focusing on two of the most
severely affected areas. Their analysis found that human-induced climate change
made a storm of that severity between 1.2 and 9 times more likely than it would
have been in a world 1.2 degrees Celsius (2.1 F) cooler. The planet
has warmed just over 1 C
since the industrial era began.
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Parts of Tennessee saw about 17 inches of rainfall in 24 hours in
late August, shattering the state’s previous record. AP Photo/John Amis
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Similar studies haven’t yet been conducted on the Tennessee storm, but they
likely will be.
So, how do scientists figure this out? As
an atmospheric scientist, I have been involved in attribution studies. Here’s how the process works:
How do attribution studies work?
Attribution studies usually involve four steps.
The first step is to define the event’s magnitude and frequency based on
observational data. For example, the July rainfall in Germany and Belgium
broke records by large margins. The scientists determined that in today’s climate, a storm like that would
occur on average every 400 years in the wider region.
The second step is to use computers to run climate models and compare those
models’ results with observational data. To have confidence in a climate model’s
results, the model needs to be able to realistically simulate such extreme
events in the past and accurately represent the physical factors that help these
events occur.
The third step is to define the baseline environment without climate change –
essentially create a virtual world of Earth as it would be if no human
activities had warmed the planet. Then run the same climate models again.
The differences between the second and third steps represent the impact of
human-caused climate change. The last step is to quantify these differences in
the magnitude and frequency of the extreme event, using statistical methods.
For instance, we analyzed how
Hurricane Harvey in August
2017 and a unique weather pattern interacted with each other to produce the
record-breaking rainstorm in Texas. Two attribution studies found that
human-caused climate change
increased the probability
of such an event by roughly a factor of three, and increased Harvey’s rainfall
by 15%.
Another study determined that the western North American extreme heat in late
June 2021 would have been
virtually impossible
without human-caused climate change.
|
The extreme heat wave in the Pacific Northwest in June 2021 sent
temperatures more than 27 F (15 C) above normal in some areas. NASA Earth Observatory
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How good are attribution studies?
The accuracy of attribution studies is affected by uncertainties associated with
each of the above four steps.
Some types
of events lend themselves to attribution studies better than others. For
instance, among long-term measurements, temperature data is most reliable. We
understand how human-caused climate change affects heat waves
better than other extreme events. Climate models are also usually skillful in simulating heat waves.
Even for heat waves, the impact of human-caused climate change on the magnitude
and frequency could be quite different, such as the case of the extraordinary
heat wave across western Russia
in 2010. Climate change was found to have had minimal impact on the magnitude
but substantial impact on the frequency.
There can also be legitimate differences in the methods underpinning different
attribution studies.
However, people can make decisions for the future without knowing everything
with certainty. Even when planning a backyard barbecue, one does not have to
have all the weather information.
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