3
Extreme Weather Event (continued)
There is a much lower level of scientific understanding con-
cerning changes in extreme event types that are not linked
so directly to thermodynamic aspects of climate change and
are controlled by regional circulation patterns. Although the
most intense tropical cyclones are expected to become more
intense in a warmer, moister atmosphere, enormous damage is
caused by cyclones below maximum strength, and there is little
understanding of how these may change. Similarly, whereas
certain conditions conducive to drought are expected to
worsen (increased snowmelt during winter, increased surface
evaporation during summer), the main driver of drought is pre-
cipitation deficit, and this is determined largely by the location
of storm tracks and monsoon circulations. Moreover, drought
is strongly affected by non-climate factors such as land use and
water demand. For these classes of extreme event types, it may
be that changes in preferred location lead to the largest climate
impacts.
Extreme Event Attribution
Given that climate change is expected to affect extreme
weather events, a natural question to ask whenever an event
occurs is what role climate change might have played in the
particular event. Because every extreme weather event is
unique, the question is not simple to answer, and the overall
subject is a highly area of active scientific research. There are
two main ways in which such a question of extreme weather
attribution is framed. One main way asks whether events of
the measured intensity are becoming more frequent because
of climate change. Because the approach is inherently proba-
bilistic, it is most appropriate when it is possible to group
similar kinds of events together (for example, heat waves over
a continental region). The other main way asks whether the
particular event was made more intense or impactful because
of climate change. This approach is most appropriate when
the event is singular (for example, a freak weather event), or
when the impacts involve complex non-climate factors spe-
cific to the particular event's location. The ability to attribute
an extreme weather event to climate change is limited by the
scientific understanding of the overall effect of climate change
on that event type. This understanding, in turn, depends on the
understanding of the relevant physical mechanisms, the length
and quality of the observational record, and the capability of
climate models to represent the event type, all of which vary
greatly among different event types (Fig. 2).
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Fig. 2: Schematic depiction of the assessment of overall confidence in capabilities for
attribution of specific events, by event type, vs. the understanding of the effect of climate
change on that event type, in general. A position below the 1:1 line indicates the potential
for improvement by technical means alone, but this potential is limited by the level of
physical understanding.
(Reproduced from National Academy of Sciences Report on Attribution of Extreme Weather
Events in the Context of Climate Change, March 2016, https://doi.org/10.17226/21852)
