07.11.2015 17:02

decades, scientists have said that
more CO means higher
temperatures, longer dry spells, and
bigger storms. But ask them
whether global warming caused a
Midwest heatwave, the California
drought, or a New York hurricane,
and they’ll explain ad nauseam how
hard it is to untangle whether any
single weather event is due to
natural variation or climate change.
Hard, but not impossible. A new
NOAA report released November 5
looked at how (or how not) climate
change affected 28 different global
events in 2014. Each study attempts
to quantify how much climate
change affected the duration,
geographic extent, and severity of
the weather event in question. But
some numbers were easier to
crunch than others. For some
historically well-recorded events—
like heat waves—scientists can
quickly and reliably calculate the
impact of climate change. Take a
rare and complex storm like a
tropical cyclone … and, well, things
get complicated.
Yesterday, 50 years ago, Lyndon B.
Johnson received a warning about
the effects of climate change, the
first of many issued to US
presidents. Since that day, climate
study has been focused on the
large-scale effects, current and
predicted, of greenhouse gases.
Those predictions—higher
temperatures, stronger storms—are
familiar now. But strangely missing
was any formalized way to look at
climate change in the opposite
direction: to identify a particularly
nasty weather event and ask
whether it was caused by human
Event attribution began with a 2003
Nature paper that explored
whether deductive tools from
epidemiology could be applied to
events like heat waves. “In
epidemiology they look at things
like, to what extent does cigarette
smoking increase the chance of
getting lung cancer,” says Heidi
Cullen, chief scientist at Climate
Central (Disclosure: This author
interned for Climate Central briefly
in 2013), and co-author of one of
the papers in the report. In 2004
came the first real example of an
event attribution study (also
published in Nature). “They found
that climate change likely doubled
or quadrupled the European heat
wave of 2003,” says Cullen.
That methodology was pretty
simple. Set up two models: one of
the world without fossil fuels, the
other one chuggin’ and burnin’.
Then you run the scenario
thousands, millions of times in each
model, and eventually get a
statistical likelihood for the heat
wave happening in the world
without humans versus the world
as it exists today. In fact, it’s so
simple that some people in the
field think these heatwave-climate
change connections could soon be as
rote as your local meteorological
forecast. That makes sense:
Warmer temperatures are what
atmospheric greenhouse gases do
But drawing the connection
between warmer temperatures and
more complex weather events like
drought, flooding, hurricanes—or
even the expansion of the Antarctic
ice sheet—is way more difficult.
“For events like that, it’s not clear
that we have an established
agreement in the scientific
community,” says Stephanie
Herring, a NOAA climate scientist
and editor of the report. The
problem is complexity. Or maybe
the problems are complexities.
Anyway, it’s confusing.