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In climate science, linking a given climate phenomenon or weather event to a variety of external factors (greenhouse gases, solar variability, El Niño) is called attribution. ... To picture this, let's use an oft-used analogy: baseball. ... extreme ...
Linking Extreme Weather to Climate Change
One of the most
frequent questions climate scientists are asked after an extreme
weather event such as a drought or hurricane is: “Was it caused by
climate change?” For a long time, scientists would usually say that no
single event could be attributed to climate change with any degree of
confidence. That is no longer the case.
This originally appeared on the Climate CIRCulator.
In
climate science, linking a given climate phenomenon or weather event to
a variety of external factors (greenhouse gases, solar variability, El
Niño) is called attribution. A still up-and-coming field, attribution of
extreme weather events has nonetheless developed by leaps and bounds in
recent years. For proof one need look no further than a recent report focusing on attribution of extreme weather events from the National Academies of Sciences.
A GOES-13 infrared satellite image of Tropical Storm Isaac provided by the U.S. Naval Research Laboratory in Monterey, Calif.
The
report is the result of an analysis of the field by an expert panel of
10 members, including OCCRI Director Dr. Philip Mote. In their report,
the panel gives readers a kind of guided tour of the history and
development of attributing extreme events, and what can and cannot be
said when attempting to attribute an extreme event to anthropogenic
climate change.
The
history goes something like this: The first attempt to attribute an
extreme event to climate change came in 2004. The event under scrutiny
was the 2003 European heat wave, a devastating episode that’s estimated
to have killed tens of thousands of people, mostly elderly individuals
caught by surprise in housing without air-conditioning. Since the
publication of that initial analysis, the field of attribution has taken
off, with arguably the largest launch occurring between 2012 and 2015
when the number of submitted studies concerning attribution grew by a
factor of five, according to the report’s authors.
Not
surprisingly, as the number of studies tackling the problem of
attribution has grown, so too has the field’s sophistication. Which
brings us to the report’s next big highlight: Why can some events be
attributed to climate change with more confidence than others? To
picture this, let’s use an oft-used analogy: baseball.
In
recent decades, professional baseball, that great American pastime, has
been plagued by a number of doping scandals involving anabolic steroids
and other performance-enhancing drugs. In tandem with the drug use has
been an extreme increase in the number of home runs, a rise far
surpassing what could be expected from past performance. Understandably,
many suspect a connection between the performance-enhancing drugs and
the athletes’ enhanced performance. The problem is that while the
overall number of home runs can be correlated to the rise of drug use,
attributing individual home runs to drug use is impossible. In fact, you
can’t say for certain if a particular home run was due to steroids.
Instead what you get is a number representing the probability that a
given home run occurred in the presence versus the absence of steroids.
This, in a nutshell, is a problem of attribution.
Now
take the same idea and consider climate change and extreme event
attribution. Here the steroids are carbon dioxide (CO2) and other
greenhouse gases, while home runs represent extreme events. To
paraphrase Jerry Meehl of the National Center for Atmospheric Research,
we now have extreme events occurring in a climate that is on steroids.
By
trapping heat from the sun, CO2 adds energy to the climate system,
energy that — given what we now know about the mechanisms of climate,
which is quite a lot — should increase the number of extreme events,
such as heat waves and droughts. As with home runs, attributing
individual extreme events to a single cause is difficult, even if the
overall trend is clear.
As
with steroids and home runs, what you get when you try to ascertain
anthropogenic climate change’s fingerprints on a given extreme event is a
number representing the probability that a given event is more or less
likely to have occurred given the presence or absence of anthropogenic
climate change. What’s more, note the report’s authors, these
probabilities occur on a spectrum that ranges from events that can be
attributed to climate change with a high degree of confidence to events
whose connection to climate change can be less confidently attributed.
How
extreme events fall on the spectrum is pretty straightforward: events
with a direct link to temperature increases can be attributed with a
higher degree of confidence because there exists a pretty
straightforward and well-understood link between increased CO2 levels
and rising temperatures. So it’s not surprising that extreme heat
events, such as heat waves, can be linked to climate change with a high
degree of confidence. But as that initial connection to rising
temperatures weakens as other compounding factors are added, so too does
the confidence level.
What
results is a spectrum that stretches from extreme heat and cold events
(whose frequency is directly influenced by shifts in the temperature
distribution) on one end of the spectrum, followed by droughts (which
can result from warm winters, a lack precipitation, or both) and extreme
rainfalls (warm air holds more moisture, but factors other than just a
direct temperature link are also key), and on down the line to the low
confidence end of the spectrum which includes wildfires, cyclones, and
severe convective storms, which can produce hail storms and tornadoes.
Regardless,
this spectrum, however intuitive it might seem, represents the extent
to which the science of climate attribution has grown in its
sophistication. This means that when the question “Was that event caused
by climate change?” comes up, climate researchers no longer have to
give the old noncommittal answer. Instead, as the report’s authors put
it, they can say:
“In
many cases, it is now often possible to make and defend quantitative
statements about the extent to which human-induced climate change (or
another causal factor, such as a specific mode of natural variability)
has influenced either the magnitude or the probability of occurrence of
specific types of events or event classes.”
In
other words, we are that much closer — if not exactly, then at least
probabilistically — to saying what’s been hitting so many balls out of
the park.This originally appeared on the Climate CIRCulator.
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