A few weeks
ago New York City was hit by hurricane Sandy. This hurricane caused much damage
in the North East of the United States. It caused billions in damage and took
the life of 131 persons in the US alone according to Reuters. But more than
lives or damage it caused fear. A deadly
hurricane was able to go all the way up north to the densely populated east
coast and hit the most important city in the country. Most hurricanes stay much further south and make landfall in the countries
and states around the Caribbean Sea and the Gulf of Mexico. It is rare that a
hurricane goes all the way up to New York City, but it did happen in the past.
There were other deadly hurricanes that have made landfall close to New York
City. According to the New York City Office of emergency management there
were deadly and destructive hurricanes
in 1821 and 1938. Still people are scared and ask how it could happen. Was is climate
change or just a single event without further consequences.
Months
before the hurricane made headlines three
scientists published an article in Nature in which they simulated the
likelihood of hurricanes making landfall
around New York City and causing storm surges . When the hurricane Sandy actually hit New York
City and caused large scale flooding this article was often cited in the press.
I will talk about this article and give the highlights.
This article
was published in February 2012 in Nature Climate Change and is called: Physically
based assessment of hurricane surge threat under climate change. It is written
by Ning Lin, Kerry Emanuel, Michael Oppenheimer and Erik Vanmarcke.
As said
before this is a case study about New York City and examines how much chance
there is that New York City suffers from a hurricane and large scale storm
surge takes place. Of course they have to simulate this. But doing this is not
an easy task and especially when you also try to determine how much influence
climate change has on it. How much storm surge there is during a hurricane
depends on many complex factors. Their
aim is to determine if climate change has any influence on storm surges. To
answer this question they use different models with names such as CNMR-CM3, GFDL-CM2.0
or ECHAM. They use four climate models to predict the hurricanes and two
hydrodynamic models for the storm surges. As measure point they use Battery on the
southern tip of Manhattan. In the beginning of their article they state : Associated with extreme winds, rainfall and
storm surges, tropical cyclones present major hazards for coastal areas. Moreover,
tropical cyclones respond to climate change. They already think that
hurricanes and climate change are influencing each other . To support this claim they refer to three articles about
climate change and hurricanes. And now they want to find out if storm surges
self are also affected. To compare storm surges in situations with and without
climate change they simulate hurricanes in the present and in the future. In the
present they simulate 5,000 New York region storms based on observed climate
between 1981 and 2000. For the future they also simulate 5,000 storms but based
on the AB1 emission scenario of the IPCC. According to the writers the models don’t give
a consistent answer to the question if there will be hurricanes due to climate
change. This is even before they start measuring storm surges. The models ranges from a 15% decrease to a
290% increase. The hydrodynamic models are used for estimating the height of
the storm surge. All the climate and hydrodynamic models they use have
different assumptions and different ways of measuring. The writers give
additional information about the models in a 14 page long document http://www.nature.com/nclimate/journal/v2/n6/extref/nclimate1389-s1.pdf
. Most models have also their own internet page, for example http://chl.erdc.usace.army.mil/chl.aspx?p=s&a=Links;126
about the Advanced Circulation Model.
Then they
let the models do their work. According to the used models climate change does
have an influence on storm surges. In their article they show the two worst
case scenario’s for New York City when faced with climate change and storm
surges.
These two
worst case scenarios are creating a surge of 4,75 meter and 4,57 meter. It is
very unlikely that such storm surges can occur in the temporally climate around
New York City. The highest storm surge in recent history was around 3.8 meter
according to the writers. They also tried to measure the effect wave set-up,
the influence of the moon on storm surges and using another model to be sure
that they can exclude all the other possibilities for the extreme high storm
surges. In total they generated more
than 45,000 different events with all their models according to a summary on the website of Princeton University https://wws-edit.princeton.edu/webmedia/power-point/R-Brief-Lin-Oppenheimer.pdf
With all
the data they did statistical analyses. This is the boring bit of the article
that never made headlines. First of all they presume that the annual number of
storms Poisson distributed is. This means that you know the annual number of
storms in New York City but you don’t know when the storms exactly will strike.
They also estimate the probability
density function and other difficult
mathematical functions. At the end of the article is the methods section in
which they explain it all.
With these
calculations they make graphs about storm surges in the future. They do this per
model because every model has its own predictions.
T stands
for years. 10^1 resembles storm surges
that are so high that they only occurs once in 10 years. 10^2 storm surges are
even higher and occurs only once per 100 years and so on.
This graph
above shows the height in meters of storm tide and surge when there is no
climate change. But they say that even without climate change New York City is
already in danger. Because Manhattan lays only 1,5 meter above sea level.
The black
line is the height without the climate change, the other lines with. As you can
see the blue and red lines are clearly higher than the black one and heavy
storm surges will occur more often according to these models . But as you can
see with the GFDL, after 10,000 years
the ‘’normal’’ has almost closed the gap again. There are differences between
the models. But despite these differences the authors still think that their
claim can hold.
The difference
between with and without climate change is quite big. A storm surge so large that without climate change it only
occurred once every 500 years now suddenly occurs in one model every 25 years
and even the most conservative model says that every 240 years such a storm
surge could happen instead of 500 years. And that is worse than New York City has ever faced. As some sort of a conclusion you could say that the authors
argue that Climate change have quite a large influence on storm surges. And for places like New York City this poses
a great danger.
After reading
this article you can say that there are certainly a lot of arguments for the
hypothesis of the authors. But they focus almost solely on hurricanes, the Netherlands for example has never seen a
hurricane but during a storm surge in 1953 more than 1,800 people died. The
authors say against this that most storm surges occur during hurricanes. But that
argument makes the whole article very USA minded. More research is certainly needed to improve
the models. But one thing that hurricane
Sandy did was putting climate change back on the agenda. A Google search on ‘’Hurricane
Sandy Climate Change’’ has 84,2 million
hits.
For everyone
who didn’t saw Hurricane Sandy in the new here is a short impression.
Lin, N, Emanuel, K Oppenheimer, M
and Vanmarcke, E (2012) Physically based
assessment of hurricane surge threat under climate change. Nature Climate
Change Volume 2. Page 462-467
Summary of the article Physically
based assessment of hurricane surge threat under climate change on the
Princeton University website
https://wws-edit.princeton.edu/webmedia/power-point/R-Brief-Lin-Oppenheimer.pdf accessed on 18-11-2012
Reuters Press
Release about Hurricane Sandy http://www.reuters.com/article/2012/11/16/us-storm-sandy-deaths-idUSBRE8AF0ZX20121116
accessed on 18-11-2012
New York
City Office of emergency management about Hurricanes in the past around New
York City http://www.nyc.gov/html/oem/html/hazards/storms_hurricanehistory.shtml
accessed on 19-11-2012
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