Useful climate science helps humanity adapt to natural
weather patterns and plan for future extremes. Minimizing risk would be a wise
course of action, but too often humans have ignored nature’s indicators of high
flood risk and continue to build on flood plains that inevitably place them in
harms way. By the 1950s most private companies in America got out of the flood
insurance business due to heavy losses from natural flooding. In 1968 the US
government unwisely decided to subsidize flood insurance and created the
National Flood Insurance Program. One unintended consequence was subsidies
encouraged people to continue building in flood plains. As a consequence, by
2014 the program was $24 billion in debt. Recent legislation has attempted to
raise flood insurance rates to better reflect the real risks. But raising
insurance rates to reflect real risks could force many homeowners into
foreclosure and thus any meaningful solution creates a political nightmare.
Whatever the political solution, accurate risk assessments require hydrologists
and climate scientists to determine the frequency of major flood producing
storms over hundreds of years. In Attribution
of Extreme Climate Events (henceforth Trenberth 2015) Trenberth
suggests extreme storms are more frequent due to global warming. But from a
perspective of several centuries, we know flood risks due to hurricanes were
greater during the cooler climate of the Little Ice Age (LIA). So how valid and
useful is the science of Trenberth 2015?
Textbooks
published years before the landfall of Hurricanes Katrina or Sandy prophetically
warned, “New Orleans lies below sea level. Should hurricane-driven floods top
or break the protecting levees, the city would be inundated with seawater.” And
“Large parts of Long Island, New York with its very large population, would be
underwater if a major hurricane passed over its western end.” Simply knowing
that there was a greater risk of hurricane-induced flooding during the LIA is
not really useful for those cities. They already know they are naturally in
danger. Those cities require early
warning systems to allow safe evacuation. Accurate early warning requires
useful science that can predict the effects of atmospheric circulation and
determine storm tracks, storm duration and storm intensity. Oddly Trenberth
2015 argued we should separate analyses of those most useful dynamics and focus
on thermodynamics (temperature) because CO2 forced circulation models do a very
poor job of simulating those critical dynamic changes. Trenberth 2015 wants to
focus on the effect of temperature anomalies in isolation to provide “a better
basis for communication of climate change to the public.” But examining
temperature anomalies separate from atmospheric circulation changes is dubious
science at best and blaming global warming does nothing to improve early storm
warnings or accurately assess the frequency of extreme events.
Centennial and Millennial
Hurricane Storm Surge
Trenberth 2015 suggested that for Hurricane Sandy, “the subways and tunnels may not have
flooded without warming-induced increase in
sea level and storm intensity
and size, putting a potential price
tag of human climate change in this storm in the tens of billions of dollars.”
[approaching 50% of the damage].But changes in sea level had little, if any, impact
on Sandy’s flooding. His statement may be useful for politicking climate change
but does nothing to improve early warning systems. The more useful question to
have asked is why was Sandy’s storm surge double that of recent hurricanes,
hurricanes that were far more intense but with similar sea levels?
Examining the graphic
on storm surge (below) posted by one of Trenberth’s colleagues at the
National Center for Atmospheric Research, we clearly see how extreme high water
events since 1900 are broken down into contributions from storm surge, high
tides and a century of sea level rise. The diagonal orange & white areas
represent sea level rise since 1900.
Sea level at Battery Park, NY has risen 11.2
inches over the past 100 years. Half of that rise happened naturally by
1950 - before CO2 had reached significant concentrations - and that natural sea
level rise has most likely continued into the present to some degree.
Furthermore due to glacial isostatic adjustments, 3 to 4 inches of that
relative sea level rise is due to land subsidence on the eastern seaboard. Thus
any theoretical contribution from human warming to current sea level is most
likely less than 3 inches, and less than 3% of Sandy’s high-water levels. Even if
we incorrectly assumed that CO2 caused the entire 1-foot rise in sea level, if
we remove that sea level increase Sandy would have still flooded New York’s
subways. By blaming global warming, Trenberth 2015 provided nothing useful that
would have predicted Sandy’s flooding.
In contrast to Trenberth’s global warming crusade, paleo-climate
studies of storm-washed sediments in New York City’s back-barrier marshes
show high storm surge was more common when the climate was cooler and sea level
was lower. As seen below in Figure 5, coastal flooding similar to Sandy’s
happened in 1788, 1821 and 1893. The conclusions from sediment analyses are further
supported by historical documentation. The 1893 storm surge was reported to
have destroyed Hog Island while driving large boats 100s of feet inland.
Because hurricane caused flooding was more prevalent during
the Little Ice Age when Atlantic temperatures averaged 1 to 2 degrees F colder
than today researchers concluded, “The frequent occurrence of major hurricanes
in the western Long Island record suggests that other climate phenomena, such as atmospheric circulation, may have
been favorable for intense hurricane development despite lower sea surface
temperatures.” In contrast Trenberth 2015 incorrectly argued analyzing the
causes of atmospheric circulation anomalies is not as “fruitful” as analyzing
temperatures.
Similarly Liu and Fearn
2000 investigated storm-washed sediments in northern Florida, concluding
the region was afflicted with millennial periods of hyperactivity for extreme hurricanes that alternated with a
thousand years of quiescent activity.
They reported that “no catastrophic hurricane of category 4 or 5 intensity has
made landfall in the Western Lake [northern Florida] area during the last 130
year documentary record” but “If future climatic changes, whether or not related to the
anticipated greenhouse warming, lead to a return of a “hyperactive”
hurricane regime characteristic of the first millennium A.D., then the
northeastern Gulf Coast is expected to experience a dramatic increase in the
frequency of strikes by catastrophic hurricanes.” Globally other
paleo-climate studies found the period of greatest hurricane activity for
Australia and the eastern USA both occurred during Little Ice Age times between
1400 and 1800 AD. And in Southeast Asia researchers
determined “the two periods of most frequent typhoon strikes in Guangdong (AD
1660–1680, 1850–1880) coincided with two of the coldest and driest periods in northern and central China during the
Little Ice Age.”
Trenberth 2015 wants to re-direct research questions and
ask, “Given an extreme storm, how was it influenced by anomalous SSTs?” or “ Was
the storm surge worse because of high sea levels?” Based on long-term studies
the answer is extreme storms and high storm surge happened more frequently with
cooler sea surface temperatures and long before rising CO2. Given that NYC
experienced 3 extreme high water levels associated with hurricanes between 1788
and 1893, but only one (Sandy) since then, we can reasonably argue that climate
change, whether human-induced or natural, has reduced the threat of high storm
surge.
Storm Tracks and Storm Surge
Early warnings and evacuation plans critically hinge on
projected storm surge, which primarily depend on the projected storm track. Perusing hurricane storm tracks since
1850 (illustration below) reveals it was Sandy’s unusual perpendicular approach
to the coast that enhanced storm surge. Consider the more intense Hurricane of 1938,
which made landfall on Long Island slightly north of New York City as a more
intense category 3 hurricane, implying sustained wind speeds between 111 and
130 miles per hour. In contrast Hurricane Sandy made landfall in New Jersey
slightly south of New York City as an extra-tropical storm implying winds
speeds less than 74 mph. Yet the more powerful hurricane of 1938 only generated
maximum water levels at Battery Park, NY of 8.8 feet, and does not make New
York City’s top ten high water levels over the past 100 years. It was the
difference in storm tracks that determined Sandy’s higher storm surge and
higher costs.
Hurricanes produce the highest winds to the right of the
hurricane’s direction of travel. Storms travelling parallel to the coastline
don’t aim the strongest winds at the coast. When Sandy took a 90-degree turn
and travelled perpendicular to the coast, she aimed her most powerful winds at
New York City for a more extended period of time as she approached. Due to
Sandy’s more eastward position when she started her approach, the fetch was
also greater and generated much bigger swells.
The degree of storm surge also depends upon how quickly a
storm moves up the coast. Sandy was a hybrid storm that had merged with a
cold-core extra-tropical storm typical of winter Nor’easters.
Unlike hurricanes that are powered by latent heat from warm sea surfaces,
extra-tropical winter storms along the eastern seaboard are primarily powered
by the pressure gradient produced by the contrast between the cold continent
and warm Gulf Stream. While Trenberth only draws your attention to anomalously
warm sea surface temperatures, the east coast was experiencing record
cold temperatures that increased the pressure gradient. Forecasters were
issuing both blizzard and hurricane watches. Furthermore extra-tropical storms
are 3 to 4 times wider than hurricanes, and merging with Sandy produced the
hybrid hurricane’s immense size. Extra-tropical storms and their hybrids move
much more slowly up the coast than a hurricane, thus the duration of Sandy’s
winds generated a much greater storm surge. As seen in Figure 5 above,
extra-tropical winter storms (light gray bars) have produced the greatest
abundance extreme storm surge. Apportioning partial causation of Sandy’s
destruction on global warming and ignoring all else only obscures the critical
dynamics required to make early warning predictions based on storm intensity.
Atmospheric Blocking
Due to the frequency of failed forecasts, the public often dismisses
media hype about the dangers of an approaching storm, preferring to stay
and take their chances rather than needlessly evacuate. Carelessly blaming
global warming only adds to the dubious hype and mistrust of useful science.
Fortunately the European ECMWF weather models accurately forecasted Sandy’s
storm track 8 to 9 days in advance due to a better understanding of atmospheric
blocking (in this case the high pressure south of Greenland) and the effects of
the jet stream. In contrast, the American National Weather Service’s GFS models
initially forecast Sandy to harmlessly head out to the mid Atlantic. Due to
such poor forecasting skills, Congress appropriated funds so the NWS
could adopt a more accurate weather model. Why did models differ so greatly
in forecasting Sandy’s storm track? All the models had access to the same sea
surface temperature data, so Trenberth’s temperature anomalies were never a
critical factor that could explain model differences.
In fact Trenberth 2015 cited Magnusson
2014 (a paper Trenberth helped craft) in which a ECMWF modeling experiment
compared the most recent 20-year average sea surface temperatures with a swath
of the Atlantic’s anomalously high temperatures during Sandy’s northward trek.
Although that experiment suggested anomalous temperatures could have possibly increased
storm intensity slightly, forecasting
intensity is still fraught with problems due to the complex contributions from
many other variables. More importantly the ECMWF experiment found changes in
sea surface temperature had little effect on Sandy’s storm track. Model runs
with failed forecasts underestimated the strength of the subtropical
high-pressure systems east of the storm track that had kept Sandy from
harmlessly veering into the Atlantic. In contrast to Trenberth’s 2015 lament
that atmospheric circulation patterns are not robustly simulated by CO2-driven
climate models, predicting storm tracks and blocking are the most critical factors for providing early warnings.
If researchers are interested in a link between Sandy’s
storm track and climate change, then a better question to ask would be ‘have Greenland blocking events been
affected by rising CO2 and climate change?” As illustrated in Figure 1 above
from the 2011 paper Atmospheric
Blocking and Atlantic Multidecadal Ocean Variability, the answer
would be there has been no trend in Greenland blocking days (estimates in black
and dark blue). Thus a CO2 global warming effect is again unlikely. In
contrast, Greenland blocking and hurricane activity are both significantly
associated with natural oscillations like the Atlantic Mulitdecadal Oscillation
(AMO). The dashed red line represents the AMO and the solid red line represents
the detrended AMO.
Klotzbach
2015 has shown that hurricane activity in the Atlantic is highly correlated
with the AMO and seemingly independent of climate change. Three peaks of the
AMO coincide with 3 peaks of hurricane activity centered on the 1880s, 1950s,
and 2005. The oscillation of Atlantic hurricane activity is also illustrated in
the Accumulated Cyclone Energy (ACE) index shown below. The AMO appears to be
transitioning towards its cool phase now coinciding with a period of below
average hurricane activity in the Atlantic since 2013.
Storm Intensity
According to Trenberth’s 2007 article Warmer
Ocean’s, Stronger Hurricanes, a one-degree increase in sea surface
temperature can increase the winds of a hurricane by one category and he argued
global warming will produce more intense category 4 and 5 hurricanes. But Sandy
only briefly reached category 3 status as she approached Cuba. Sandy quickly
lost intensity after passing over Cuba, devolving from a category 3 hurricane
to a mere extra-tropical storm before strengthening again to a weak category 1
hurricane. Clearly the ocean was not warm enough to produce a higher intensity
storm that Trenberth and global warming predicted. Or perhaps the dynamic
factors that Trenberth downplays had a more powerful part in limiting Sandy’s
intensity.
Days before making landfall, due to Sandy’s more westerly
storm track, Sandy interacted with an atmospheric trough and its cold Arctic
air mass that had dipped down over the eastern USA. The warm-core hurricane named
Sandy, eventually merged with a cold-core extra-tropical storm generated by the
jet stream. While tropical hurricane intensity is primarily driven by latent
heat from warm sea surface temperatures, an extra-tropical storm is primarily
driven by baroclinic processes (differences in the pressure gradient) such as
the gradient due to the contrast between the warm Gulf Stream and cold
continental air mass. As Magnussen 2014 noted, when tropical cyclones and
mid-latitude troughs interact to form a “hybrid storm”, it has been found that cyclones are more likely to intensify
than weaken. Thus it can be reasonably argued that it was abnormally cold
continental temperatures that intensified Sandy.
During a hurricane’s typical cold-induced extra-tropical
transition, a hurricane’s size greatly increases as observed in the extremely
large radius of Sandy. As reported by Galarneaux
2013, during the transition winds increased by 20% and Sandy’s central
pressure dropped to its lowest point of 940 hPa despite travelling over cooler
waters. During her “second trough interaction on 29 October, Sandy turned
northwestward and intensified as cold
continental air encircled the warm core vortex.”
Everyone agrees that hurricanes require warm waters to form
and indeed warmer temperatures can intensify a hurricane. As seen in Figure 10
below, Sandy’s storm track crossed the Gulf Stream (the reddish bands) before
making landfall. As she crossed the Gulf Stream, she briefly intensified to a
Category 2 hurricane before
devolving again to an extra-tropical storm when she crossed cooler coastal
waters (in blue and purple). But here again Trenberth’s attempt to separate the
dynamics of atmospheric circulation from a thermodynamic impact of higher
temperatures would be misleading. It was blocking that forced Sandy to cross
over the naturally warm waters of the Gulf Stream. Sandy’s brief increase in
intensity was ultimately the result of atmospheric circulation not global
warming. Otherwise she would have passed harmlessly out to sea.
The attempts by Trenberth 2015 to suggest global warming has
worsened disasters like Hurricane Sandy or the Colorado flooding (discussed in
part 2) simply fails to provide any useful science. Trenberth 2015 did not
accurately assesses risks or improve early warning systems. It simply reduced
climate science to “ambulance chasing” in order to scare up support for his
climate change politicking. Previously Trenberth has argued that extreme events
such as recent droughts and heat waves worsened due to CO2 warming and despite
the fact that climate experts found those events to be within the bounds of
natural variability (discussed
here). To communicate his brand of climate change, Trenberth attacked those
scientists on blogs as irresponsible. And here again Trenberth has hyped global
warming links to hurricane destruction in contrast to the opinions of many
hurricane experts.
In keeping with the long-term framework required by climate
science, hurricane experts like Chris Landsea, the late Bill Gray and Jim
O'Brien have consistently reported there are no links between global warming
and hurricanes. All the evidence such as the recent lull in Atlantic
hurricane activity supports their claims. But despite not being a hurricane
expert himself, Trenberth has been grandstanding for a decade to push a climate
of fear. After the devastation of Hurricane Katrina, Trenberth convened a press
conference to leverage human suffering and blame global warming. Trenberth’s
ill-informed bias resulted in hurricane expert Dr. Landsea’s resignation from
the IPCC. As Trenberth
acknowledged he purposefully convened the press conference to counter publicized reports by hurricane experts that
there was no link to global warming. Trenberth defended his conference as
necessary to “correct many very misleading and erroneous reports that global
warming had nothing to do with the hurricanes in recent times.” Yet evidence of Trenberth’s links to
global warming still remain elusive.
In contrast Landsea’s
IPCC resignation stated, “It is beyond me why my colleagues would utilize
the media to push an unsupported agenda that recent hurricane activity has been
due to global warming. Given Dr. Trenberth’s role as the IPCC’s Lead Author
responsible for preparing the text on hurricanes, his public statements so far outside of current scientific
understanding led me to concern that it would be very difficult for the
IPCC process to proceed objectively with regards to the assessment on hurricane
activity.” Yet here again
Trenberth 2015 continues to mislead the public suggesting a storm like Sandy
put “a potential price tag of human
climate change in this storm in the tens of billions of dollars.”