«We did not start from scratch»
The IPCC (Intergovernmental Panel on Climate Change) published the first special report on the risks of extreme events and disasters (SREX). Together with the Australian climate scientist Neville Nicholls, ETH-Zurich professor Sonia Seneviratne was coordinating lead author of one of the nine chapters. She talks about her work and the report in an interview.
The new IPCC SREX special report evaluates and summarises scientific literature on changes in weather and climate extremes, and assesses the consequences of such changes in the context of disaster risk reduction and climate change adaptation. The chapter that Sonia Seneviratne coordinated provides the physical scientific basis regarding the observed and projected changes in climate extremes. It is the first IPCC report exclusively devoted to extreme events.
Ms Seneviratne, as coordinating lead author you
supervised a chapter of this special report. The full title of the report is
Managing the Risks of Extreme Events and Disasters to Advance Climate Change
Adaptation. Writing a chapter within such a report sounds like a
challenging task.
Although the
topic of our chapter is related to my research, it was indeed a considerable
additional work load. It may have partly been a disadvantage while I was
working on it as I had less time for my own research. But in the long run, it
was very useful to me as a researcher because I now have a much better overview
of the entire field and can see where there are gaps and which areas are most
in need of further research. Of course, it is also an honour to have been given
this task.
How long did the project take?
The project
began in 2009. In all, we worked on this special report for two and half years.
The chapter you supervised is entitled Changes
in Climate Extremes and their Impact on the Natural Physical Environment.
What does it mean?
The whole report was coordinated by IPCC Working Group 1, which addresses
the physical science basis on climate change, and IPCC Working Group 2, which addresses
climate change impacts on ecosystems and society, as well as climate change
adaptation. Our chapter was the only one by Working Group 1 in the report. The
term “natural physical environment” highlights that our chapter addresses the
purely physical aspects, including physical impacts, but not impacts on
ecosystems and society.
What are the purely physical aspects of climate
extremes?
We
distinguished three separate categories of events relevant to disasters: first,
weather and climate extremes in temperature, precipitation and wind; then,
phenomena that are linked to these occurrences: tropical and extratropical cyclones,
as well as El Niño and other large-scale modes of variability; finally, impacts
on the physical environment, such as droughts, floods, coastal impacts, or
impacts in mountainous environment.
You and the other authors of the chapter considered
around 1,100 publications on extreme events. How did you know where to start?
We did not
start from scratch. There were previous reports by the IPCC that had addressed
aspects of this topic. But this is the first IPCC report exclusively devoted to
extreme events. Previously, the topic was divided into different reports and,
in the Fourth Assessment Report, the AR4, into different chapters. We began
with this material. Of the 1,100 publications, however, over seventy percent were
published after the AR4, in other words since mid-2006. Even while we were
compiling the report, new relevant literature kept on being published. We
compared the new publications with the AR4 assessment and that of an IPCC technical
paper from 2008 on the topic “climate change and water”. We checked what was
new or what was no longer correct. We had for instance a significant amount of
new literature documenting more extensive analyses on the topics of droughts
and tropical cyclones.
How did you approach the evaluation of
publications on the specific extreme events?
The evaluation process became very systematic compared to the AR4, and we
used the new IPCC uncertainty guidance for the assessments. As a first step, we
determined the confidence we had in the data basis available for the considered
extreme events. For instance, if for a given extreme we had only few
publications based on a limited number of datasets or if the process
understanding still showed considerable gaps, the level of confidence was
assessed as being low. If the quality of the underlying data basis was good
enough for us to be able to evaluate the sign of the change in a given extreme but
not sufficient to make quantitative assessments regarding the probability of
change, then the level of confidence was assessed as medium. As an example, drought
is an important topic, but we assessed the level of confidence in the projections
as only medium given that there are not many relevant observations and that the
involved processes are complex. It is also difficult to assess how good the
models are in cases where there are strong natural fluctuations and few events for
the analyses, such as for El Niño. Quantitative statements on the probability of a change are only provided
in cases where we assessed that we had high confidence in the underlying
evidence.
Can you give an example?
Based on the
available evidence, we can state that a future increase in heat extremes on the
global scale is virtually certain, that is that it will occur with a degree of certainty
of ninety-nine to 100 percent.
You even provided assessments of observed and
projected changes in extremes at the regional scale.
Yes, we provided
such assessments for temperature extremes, heavy precipitation events and
droughts for twenty-six regions. It is the first time that such detailed
regional information on extreme events has been compiled. We also provided
global-scale assessments for all considered types of extremes, which are
summarized in another table.
How did you deal with contradicting studies?
We outlined
the different views on the given topics and provided an objective synthesis on
the available literature.
What are the underlying messages of your
chapter?
The main message is that there are already changes in extremes that we
can observe. How sure we are with regard to these changes depends strongly on
the considered extremes and regions. For instance, we assess it as very likely
that increases in hot extremes have been observed on the global scale. On the
other hand, cold extremes have become less frequent. In the case of heavy
precipitation events, we identify more regions with increases than decreases
globally, hence an overall tendency for increases in these events at the global
scale, but at the level of single regions it is often more difficult to
identify a signal.
Why is that?
We have to
determine the presence of a signal from statistical analyses. The
identification of the signal depends on the signal to noise ratio. If the noise
is large, for instance because there is a strong natural variability, it is
harder to see a signal. The signal-to-noise ratio is generally larger at global
than regional scale, because the regions present different types of background
noise that cancel out when all the data is averaged on the global scale. Finally,
there are also extreme events for which the data is still limited and for which
we are thus still unable to assess whether they are influenced by the
increasing greenhouse gas concentrations.
Were there some surprises?
No, with a few
exceptions. Globally speaking, it is clear that there are changes for some
extremes that are caused by humans, especially for temperature and heavy precipitation,
and that we expect an increase of these trends for the future. But there were
also some changes compared to the AR4 assessments. On the one hand, we have now
in the SREX more detailed assessment for aspects that were not previously
treated, for instance more detailed regional assessments as well as partly
quantitative assessments on the magnitude of changes for temperature and
precipitation extremes. These analyses reveal large regional differences in the
data basis, the observed trends and the projected changes in extremes. On the
other hand, the latest literature has also revealed a greater level of uncertainty
for certain extremes than was discernable at the time of the AR4. This is
especially the case for droughts and tropical cyclones. Nevertheless, we can
also provide assessments for these extremes – for instance, with medium confidence,
that some large regions, including Central Europe and the Mediterranean, will
be at a greater risk of droughts in future.
Apart from the two coordinating lead authors,
twelve lead authors and twenty-eight contributing authors worked on the
chapter. Was there a clear division of responsibilities?
I was
extremely lucky that we were two coordinating lead authors and that Neville
Nicholls already had experience as lead author on previous IPCC reports. The
main structure and title of the chapter were provided by the IPCC based on a
scoping meeting for the report. We could provide some suggestions for the
choice of some lead authors given their scientific expertise and country of origin.
In particular, we had experts on specific extremes or specific methods in
climate research.
The chapters of the special report were reviewed
by other scientists and government representatives for quality check. How did
that work?
We had first an informal review of the zero-order draft of the report. The
first-order draft was then officially reviewed by scientific experts. Following
this, the second-order draft was reviewed by scientists and government representatives.
There were also separate reviews of the Summary for Policymakers, in which
material from our executive summary was included. We received a total of ca. 5,000
comments, not all on content, as a few of them also pointed out typos. However,
we had to answer them all in writing. All the authors from our team had to do
this for their respective contributions. But in the end Neville Nicholls and I
had to read through and check them all to make sure everything had been
answered correctly. This represented a large amount of work, but it was a valuable
process to ensure the quality of our chapter..
Is quality assurance new within the IPCC and a
response to errors that were found in the AR4?
No, these
were also the quality standards at the time of the AR4. But we were
particularly careful in this context to make sure that there were no mistakes.
Links and References
The
IPCC-report
Link to
Sonia Seneviratne’s homepage
Information about the
Centre of Climate System Modelling (C2SM)
Link to the ETH-Zurich climate blog
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