USA – WMO Expert Team Statement on Hurricane Harvey

WMO (World Weather Research Programme) Expert Team on Climate Impacts on Tropical Cyclones statement on possible linkages between Hurricane Harvey and anthropogenic climate change.

Flood waters brought on by Hurricane Harvey surround a highway near Houston, Texas, Aug. 31, 2017. Hurricane Harvey formed in the Gulf of Mexico and made landfall in southeastern Texas, bringing record flooding and destruction to the region. U.S. military assets supported FEMA as well as state and local authorities in rescue and relief efforts. (U.S. Air Force photo by Master Sgt. Jason Robertson/Released).

Hurricane Harvey brought record multi-day rainfall to the Houston area, after it stalled in the region following landfall.  Harvey had intensified before landfall after travelling over unusually warm waters (~2°C) in the western Gulf of Mexico.

There is no clear evidence that climate change is making the occurrence of slowly moving land-falling hurricanes in the Houston region, such as Hurricane Harvey, more or less likely. However, some aspects or “ingredients” of the Harvey event may have linkages to climate change.

For example, the rainfall rates associated with Harvey were likely made more intense by anthropogenic climate change.  This is a consequence of the tropical atmosphere generally holding more water vapor due to climate warming (about 7% more water vapor per degree Celsius sea surface temperature increase). This higher water vapor content leads to higher rainfall rates in hurricanes that have been simulated using climate models.

A detectable anthropogenic influence on water vapor near the earth’s surface has been identified with medium confidence according to the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5).  A regional detection-attribution study indicates that human activities have caused a detectable warming of Gulf of Mexico sea surface temperatures over the past century, with one study reporting a linear increasing trend of about 0.5°C over the period 1873-2005. However, a detectable (i.e. outside the range of natural variability) anthropogenic influence on hurricane rainfall rates has not yet been convincingly documented in observed data.

The inferred impact of anthropogenic warming on Harvey’s rainfall rates is an indirect attribution based on the previously identified attributable human influence on water vapor together with model simulations and physical understanding of hurricane rainfall processes.

Model simulations also indicate that hurricanes in a warmer climate are likely to become more intense, and that it is more likely than not that the frequency of category 4 hurricanes like Harvey will increase over the 21st century, even if overall tropical cyclone numbers do not increase, or even decrease.  Such changes are not yet clearly detectable in observed data due in part to limitations of existing datasets.

Ongoing sea-level rise, attributable in part to anthropogenic climate change, also exacerbates storm surge for land-falling hurricanes such as Harvey. Damage resulting from the geophysical event itself will be influenced by the vulnerability of the affected region, which is increased by factors such as population and infrastructure growth, and potentially decreased by mitigation measures such as flood control systems. Extensive coastal development has generally led to large increases in hurricane damage in U.S. coastal communities over the past century.

For more information see expert team’s previous assessment on this topic: “Tropical cyclones and climate change”, Nature Geoscience (2010)

Expert Team on Climate Impacts on Tropical Cyclones

Tom Knutson (GFDL/NOAA), Chairperson, Expert Team on Climate Impacts on Tropical Cyclones

Emeritus Members: Kerry Emanuel (MIT), Jim Kossin (NOAA’s National Centers for Environmental Information”), Johnny Chan (CUHK)

Current Members: Suzana Camargo (Columbia University), Chang Hoi Ho (Seoul National University), Masaki Satoh (University of Tokyo), Kevin Walsh (University of Melbourne) and Liguang Wu (Nanjing University of Information, Science and Technology), M. Mohapatra (India Meteorological Department)

Website here

Source: WMO

Flood Summary

Last updated: September 6, 2017
Event
Hurricane Harvey, USA, August 2017
Date
August 26, 2017
Type
Urban flood, River flood, Storm surge
Cause
Storm surge, Extreme rainfall

Locations

A - Cypress
B - Spring
C - Katy
D - Barker Dam
E - Addicks Dam
F - Alvin
G - Atascocita
H - South Houston
I - Houston
J - Mont Belvieu
K - Clear Creek Village
L - Baytown
M - Rockport
N - Port Arthur
O - Beaumont
P - Lake Charles, Louisiana

Magnitude

Rainfall level
680 mm in 24 hours
Pearland, Brazoria County - August 26 to August 27, 2017
Rainfall level
584 mm in 24 hours
Alvin, Brazoria County - August 26 to August 27, 2017
Rainfall level
697.23 mm in 24 hours
Dayton, Liberty County - August 26 to August 27, 2017
Rainfall level
558 mm in 24 hours
South Houston - August 26 to August 27, 2017
Rainfall level
622.3 mm in 24 hours
Santa Fe, Galveston County - August 26 to August 27, 2017
Rainfall level
511 mm in 24 hours
Mount Houston - August 26 to August 27, 2017
Rainfall level
604 mm in 24 hours
Bacliff, Galveston County - August 26 to August 27, 2017
River level
12.04 metres
Greens Bayou at Houston - August 28 to August 28, 2017
level as of 28 August: 39.51 feet / Previous record: 39.4 feet
River level
39.05 metres
Cypress Creek near Cypress - August 28 to August 28, 2017
level as of 28 August: 128.11 feet / Previous record: 127.6 feet
River level
29.27 metres
Cypress Creek near Westfield - August 28 to August 28, 2017
level as of 28 August: 96.06 feet / Previous record: 94.3 feet
River level
11.04 metres
Navidad River at Sublime - August 28 to August 28, 2017
level as of 28 August: 36.23 feet / Previous record: 34 feet
River level
8.97 metres
Navidad River near Speaks - August 28 to August 28, 2017
level as of 28 August: 29.44 feet / Previous record: 28.2 feet
River level
20.56 metres
Buffalo Bayou at Piney Point Village - August 28 to August 28, 2017
level as of 28 August: 67.46 feet / Previous record: 61.23 feet

Damages

Evacuated
35,000
August 26 to August 31, 2017
35,000 people were evacuated and staying in over 200 shelters. According to media reports, over 1 million people were displaced overall: in Texas: 779,000 mandatory evacuations; 980,000 voluntary evacuations. In Louisiana: 7,000 mandatory evacuations; 133,000 voluntary evacuations (source: Reuters)
Fatalities
44 people
August 26 to August 31, 2017
According to media reports
Buildings destroyed
7,000
August 26 to August 31, 2017