HK Submerged? Is This Map For Real?
By Debra Tan, Chien Tat Low, Dharisha Mirando 18 November, 2019
With so much press on the new sea level rise numbers - CWR's Tan, Low & Mirando fact check to see if we need to be worried
There has been much noise about sea level rise (SLR) recently thanks to a research paper from Climate Central that uses a new map called CoastalDEM to gauge the impact on coastal areas. The paper revealed catastrophic results – even with moderate levels of global warming, SLR will permanently flood areas that are home to around 150 million people by 2050; and by 2100, the number displaced will rise to 200 million.
In the case of Antarctic instability, annual coastal floods will reach further inland, affecting a total of 300 million people by 2050 and 480 million by 2100. The majority of people living on affected land are across eight Asian countries: China, Bangladesh, India, Vietnam, Indonesia, Thailand, the Philippines, and Japan – seven out of eight are developing countries.
While it is great that there is increased attention, the impact on HK is overstated– but that is not to say we are not at risk
These findings have certainly generated concern worldwide, with coverage by major media including The New York Times and BBC. In Hong Kong, a map of the affected areas in Hong Kong was published in Mingpao and we have received several queries as to whether this is real. While it is great that there is increased attention to the issue, the impact on Hong Kong is overstated– but that is not to say we are not at risk. Here’s why …
Real or just alarmist? The problem with the Hong Kong map in Mingpao…
The above is the map in the Mingpao article. It looks bad. Mingpao created the map using data from CoastalDEM, assuming moderate global warming (RCP4.5).
The article implies that the above map is caused solely by SLR by 2050, but actually storm surge was included
The result, though shocking, is misleading. Two reasons – firstly, the article implies that the above is caused solely by SLR by 2050, but to reach the flood levels depicted in the above map, maximum high tide levels and maximum coastal flooding from typhoon storm surge were included.
There is a difference here – the impact of SLR due to global warming is permanent and likely irreversible, while the storm-surge-driven coastal flooding is event-driven. Below is the same map only with SLR by 2050; it looks slightly better but it’s still not great. If you want to play around with the CoastalDEM map with different scenarios, click here.
Secondly, but more importantly, there are issues with the new CoastalDEM map for Hong Kong…
The CoastalDEM map overestimates the impact on Hong Kong…
So how bad is the flooding?
Taking the high end of the local range of 0.41m, we added the local high tide of 2.5m bringing maximum SLR to 2.91m by 2050
We tested the calculations… There are 2 factors – (1) the height of SLR and (2) the underlying map. As we are heading to 3°C-4°C of global warming, to determine SLR we took the worst-case local scenario provided by the Hong Kong Observatory (HKO), which predicts SLR of 0.25m-0.41m under RCP 8.5 by 2050. Taking the high end of the local range of 0.41m, we added the local high tide of 2.5m bringing maximum SLR to 2.91m by 2050.
It is important to pick the right map when estimating the impact
And now for the maps… it is important to pick the right map when estimating the impact on Hong Kong as the results can be significantly different as seen from the graphic below – from hardly any impact on the left to moderate impact on the right.
- HK DTM map (5m grid) – hardly any impact (LH Side) – a 5m-grid Hong Kong Digital Terrain Model (DTM) released by the Lands Department of Hong Kong. It is clear that nearly the whole coastline of Hong Kong Island and Kowloon remain unaffected at 2.91m; and
- NASA SRTM map (30m grid) – moderate impact (RH Side) – produced by NASA’s Shuttle Radar Topography Mission (SRTM). This map shows parts of Kowloon, like Kai Tak and Tsing Yi, starting to show signs of coastal flooding at 2.91m.
So which is real?
Although CoastalDEM aims to improve on SRTM at global and national scales, results need to be interpreted with caution when it comes to individual cities like Hong Kong which has densely clustered tall buildings in coastal areas.
For Hong Kong we believe that CoastalDEM materially overestimates the impact of SLR by 2050, assuming they’re using the local projection for SLR. How do we know this? Because of Mangkhut. In September 2018 Super Typhoon Mangkhut brought storm tides that affected a few low-lying coastal areas. Storm tides measuring 3.88m at Victoria Harbour only affected Lei Yu Mun, Hang Fa Tsuen, and Tseung Kwan O but not the large areas highlighted in the CoastalDEM map. Since the projected SLR by 2050 in the worst case is only 2.91m, we know that the CoastalDEM map is an overestimation.
The Hong Kong Observatory (HKO) has also questioned the output
The Hong Kong Observatory (HKO) has also questioned the output, as it looks different from HKO’s own research and past observations. And it’s not just Hong Kong; even the Malaysian authorities think this new research is an “overestimation”.
Therefore, for Hong Kong use the HK DTM map which is freely available and is the most accurate.
CoastalDEM may be wrong for Hong Kong, but Hong Kong needs to be prepared for SLR PLUS storm surges
Hong Kong may appear to be safe from SLR by 2050 on the HK DTM map but it may not be so immune to extreme storm surges coupled with SLR.
According to CWR, HK is at risk of storm tides at 5.87m brought about by typhoons by as early as 2030…
…impacting areas like IFC & Exchange Square & has implications for almost 3/4 of HK’s GDP
According to CWR’s New Base Case Scenario in the recent co-authored CLSA report, Hong Kong is at risk of storm tides at 5.87m brought about by typhoons by as early as 2030. This would impact large swathes of Kowloon and Hong Kong Island. Central, Hong Kong’s financial district – including IFC, Exchange Square, and the headquarters of HSBC and Standard Chartered – would also be inundated as can be seen below. This has implications for almost three quarters of Hong Kong’s GDP due to the vulnerabilities to the real estate, trade and financial sectors. Read more here.
As we are on the track for warming of 2.9°C-3.4°C instead of 1.5°C-2°C, Hong Kong is heading for a “Hot, Thirsty, Sweaty & Wet” future. Scenarios must be adjusted to fit the new base case for future planning.
HK must seriously step up its adaptation plans alongside aggressively decarbonising its economy
To remain a global financial hub, Hong Kong must seriously step up its adaptation plans alongside aggressively decarbonising its economy. Unfortunately, Hong Kong’s adaptation plans still fall short and the current no-sense climate strategies leave key assets and infrastructure exposed. So get on top of Hong Kong’s water and climate risks now!
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