3 November 2018 | NewScientist | 23debate about whether ocean
warming through climate change
is creating clear trends.
But the evidence is growing.
The number of storms with wind
speeds of 200 km/h or more has
doubled since 1980 and those
with winds of 250 km/h and up
have tripled. “Modelling and
theory says we should expect
to see the strongest hurricanes
get stronger,” says Masters.
“By gosh, we certainly have seen
some very strong hurricanes the
last two years.”
Worrying trends
Modelling suggests these
maximum wind speeds will rise
still further as the world warms.
If we don’t limit carbon emissions,
by 2100 we could be seeing storms
with speeds topping 370 km/h,
says Timothy Hall at the NASA
Goddard Institute for Space
Studies in New York. That’s worse
even than it sounds because
a wind’s destructive power is
proportional to its speed cubed.
Flooding is often a storm’s
most severe impact. A warmer
atmosphere contains more
moisture, meaning more rain –
or snow – can fall when conditions
are right. Each degree of warming
could result in 5 to 2o per cent
more rainfall, says Sobel, resulting
in a greater risk of inland flooding
when storms make landfall.
A trend yet to be confirmed is
that hurricanes also seem to be
moving more slowly as the world
warms, dumping more rain in
one place. Hurricane Florence
this year and Hurricane Harvey
last year may be examples.
Storm surges are certainly
getting higher because of global
warming: average sea level has
already risen around 0.2 metres,
and could rise another 3 metres
by 2100. A storm surge’s size also
depends on wind strength, the
area of water it blows over and
how long it blows for. Bigger,
stronger and slower-moving
storms might pile up higher
surges on top of rising seas,
although this trend remains
uncertain, says Sobel.
The warming of the oceans
also means that the region where
storms can reach peak strength
is expanding out from the tropics.
In the northern hemisphere,
it is migrating 50 kilometres
northwards each decade.
In the southern hemisphere,
it is moving south.
This raises the risk of places at
higher latitudes, such as Europe,
being hit by major storms. By the
time storms get that far north or
south, they may technically be
extratropical storms, poweredby the temperature difference
between warm and cold air
masses. “But it doesn’t really
matter much how it’s classified
if it does a lot of damage,”
says Masters. “This has major
implications for the design of
infrastructure.”
Perhaps the most disturbing
feature of Hurricane Michael
was how it intensified rapidly
just before landfall. Such storms
are likely to become increasingly
common as the planet warms,
according to modelling done
in 2016 by Kerry Emanuel at
the Massachusetts Institute ofTechnology. The rate at which
tropical storms can intensify rises
exponentially as the maximum
potential strength, which depends
on ocean heat, goes up.
“That’s very concerning
because it doesn’t give people
time to prepare,” says Masters.
Forecasters won’t be able to help
much: although weather models
are now great at predicting the
paths storms will take, they are
much less adept at predicting
changes in intensity. These are
thought to depend on small-scale
features of storms far beyond the
power of models to capture.
We are also unsure of the effects
a warming world might have on
typical storm tracks, or how many
storms will form. Earlier models
had suggested there would be
fewer storms overall, with agreater number of strong ones,
but some more recent models
have started showing an overall
rise in numbers.
Either way, we need to adapt.
The main reason why the costs
of tropical storms have soared in
recent decades is that ever more
people are living in the danger
zones. Countries should be
discouraging development in
places at risk, says Sobel.
And if people insist on living in
harm’s way, they should prepare
for the worst. The photographs
of places like Mexico Beach show
both the immense destructive
power of hurricanes, and that
some buildings, like the now-
iconic Sand Palace, can withstand
it if they are designed to.
While the devil is still in the
detail, there seems to be no doubt
that tropical storms are set to
become ever more dangerous as
the world warms – and besides
limiting further warming, now
is the time to prepare. “Is this the
future?” says Masters. “I think it
very well could be.” ■“The region where
hurricanes reach their
peak strength is expanding
out from the tropics”For daily news stories, visit newscientist.com/news
Tropical storms have set a series
of disturbing records over recent
years – in wind speed, intensity
and geographical range.OCTOBER 2018: Hurricane Leslie
Kept its hurricane status until it was
within 300 kilometres of Portugal.
No hurricane had been recorded in
this region of the Atlantic since
modern weather records began.SEPTEMBER 2018: Typhoon Jebi
Its storm surge flooded Kansai airport,
Japan’s third biggest, shutting it
down for 17 days. The airport was
supposedly built to withstand surges.SEPTEMBER 2018: Hurricane Florence
Dumped a record amount of rain in
the Carolinas and caused extensive
flooding, just a month before
Hurricane Michael hit Florida
(see main story).SEPTEMBER 2018: Storm Zorba
Caused extensive flooding and wind
damage in Greece and Turkey. This
was a peculiarly intense “medicane”,
a sort of hybrid storm, with a
hurricane-like eye, that forms when
extratropical storms push cold air over
the Mediterranean and the system
starts deriving energy from the
warmer waters below it. Medicanes
remain rare, but a study last year
concluded that the risk will double
if warming continues unabated.OCTOBER 2017: Hurricane Ophelia
Was literally off the charts used by
the US National Hurricane Center,
forming further to the north-east
than any on record. With peak winds
of more than 180 km/h, it weakened
into an extratropical storm before
striking Ireland, where it caused
extensive damage.SEPTEMBER 2017: Hurricane Irma
Maintained wind speeds of more than
300 km/h for 37 hours, longer than
any other storm, and left a trail of
destruction across the Caribbean.AUGUST 2017: Hurricane Harvey
Vying with Hurricane Katrina, which
hit New Orleans in 2005, for the title
of costliest hurricane on record, it
dumped more rain on the US than any
other hurricane, causing extensive
flooding around Houston, Texas.FEBRUARY 2016: Cyclone Winston
The strongest tropical storm on
record in the southern hemisphere,
with winds of over 280 km/h.
It killed 44 people in Fiji.OCTOBER 2015: Hurricane Patricia
A Pacific hurricane, it became the
strongest tropical storm ever recorded
in terms of wind speeds, increasing
from 135 km/h to a peak of 345 km/h
in just 24 hours. It was small in extent,
however, weakening before hitting a
sparsely populated part of Mexico.TRAILS OF DESTRUCTION