First, let’s look at some of the evidence
that points to an impact. In a November
2018 article in Nature, Patricola and We-
hner found that “relative to pre-industrial
conditions, climate change so far has en-
hanced the average and extreme rainfall
of hurricanes Katrina, Irma and Maria”
and that “future anthropogenic warming
would robustly increase the wind speed
and rainfall of 11 of 13 intense tropical
The scientists, from the Climate and
Ecosystem Sciences Division at Berke-
ley Lab, calculate that recent hurricanes
dropped 10 to 30 percent more rain than
similar storms in pre-industrial times;
and they predict the rising trend will con-
tinue. That may be the case — other ac-
ademic papers draw similar conclusions.
Another piece of evidence is the
fact that the world’s oceans are getting
warmer, (with ocean heat fueling tropical cyclones) and so is the atmosphere.
Warmer air also holds more moisture:
each additional 1C causes a 7% increase
in moisture retention.
In another Nature article last year,
James Kossin showed that the land speed
of cyclones between 1940 and 2016 has
gone down 10% globally, and 16% for
North American hurricanes. The longer a
hurricane remains over land, the higher
the total precipitation.
Meanwhile, atmospheric blocks — areas of high pressure that cause hurricanes
to remain stationary for long periods and
therefore dump more rain — are on the increase, possibly driven by climate change.
All of the evidence seems to show
that climate change and global warming might be producing wetter cyclones;
however, this may also be a mix of interannual variability and long-term cycles.
Maybe wetter. Maybe not.
Harvey was indeed the wettest ever
hurricane, but you have to go back all
the way to 1950 to get the third-wettest
ever for the continental U.S., which was
Hurricane Easy. The second wettest was
in 1978, and the ninth back in 1960. So,
we’re having to criss-cross backwards and
forwards through the decades in order to
rank these storms.
Before 1960, measurement was relatively rudimentary, and much less widespread — so, including pre-war events
in the ‘wettest-ever’ table is problematic.
Meanwhile, recent evidence does not
constitute a proverbial smoking gun.
Whilst Florence last year is the 8th on this
list, it is hard to judge the recent evidence
regarding the top of the list where Harvey
and Florence are the third pair of consecutive years in the top 10 list, as it was previously with 1978 Amelia-1979 Claudette
and 1997 Danny-1998 Georges. There
are roughly two decades between pairs
of extreme wet hurricanes on the top 10
list (top 8 actually). Again, is this climate
change and global warming driven or just
Factors for flooding due to hurricane
precipitation other than climate change
impact insurance claims. Urbanization is
one. Serious historical U.S. flood events
now seldom used to calibrate models,
date back to the 1920s. Cities were a lot
smaller then compared to modern me-
tropolises, and flood defenses in place to-
day were not as widespread. Green areas
and natural features have been replaced
by concrete and asphalt.
These hard surfaces absorb much less
of the total precipitation, run-off waters
that cause flood damage. Constructed
drainage helps, but its effectiveness is
not equivalent to that of natural features.
Run-off over a smooth surface is also
much faster than over grass or forest.
More run-off at higher velocities equals
As stated earlier, last year saw the introduction of the first generation of U.S.
flood models. It is not clear to what extent developers accounted for urbanization and increasing wetness during their
initial development, but these changes
should be considered as model evolution
continues. Indeed, recent events have set
model revisions in motion.
Of the five U.S. flood vendor models
available in 2018 — developed by AIR,