Sea Level Rise – What The Science Tells Us

By Dr. Nicole Khan 20 October, 2020

What's the latest on sea level rise projections? HKU's Dr Khan shares her survey findings from 100+ sea level experts

The consensus of a 100+ sea level experts is with a 4.5ºC rise: SLR will be 0.6–1.3 m by 2100 & 1.7–5.6 m by 2300; the latter range for 2300 is much higher than in the original survey 5 years ago
Increase in projections is related to modelling developments that incorporate dynamic processes related to ice cliffs & sheets; IPCC projections comparatively are conservative to many experts
As for HK, it is vulnerable to extreme sea levels yet, HK’s relative sea level’s mechanism is not well-known; COVID-19 is a warning that we need to prepare for the high-end risks of climate change
Dr. Nicole Khan
Author: Dr. Nicole Khan
Dr. Nicole Khan is an Assistant Professor in the Department of Earth Sciences at the University of Hong Kong. She received her B.A. in Earth Science and Mathematics from Boston University and Ph.D. from the University of Pennsylvania, where she studied past changes in sea level. Her research concerns deciphering the internal and external drivers of past sea-level change to better understand how these mechanisms will affect future changes. Dr. Nicole Khan has published in eminent peer reviewed journals, including Science, Nature Geoscience and Geology. She is project leader of the HOLSEA (Geographic variability of HOLocene SEA level) and on the steering committee of PALSEA (PALeo-constraints on SEA-level rise) international working groups and is a member of the American Geophysical Union, the Geological Society of America, International Geoscience Programme (IGCP) Project 639. She is also a contributing author to the upcoming IPCC AR6 report.
Read more from Dr. Nicole Khan →

Many sea level rise experts think that the latest IPCC projections are conservative due to multiple uncertainties across all elements of climate change. But as the global warming is accelerating faster than we thought, these uncertainties are certainly becoming more realistic.

We sat down with Assistant Professor Nicole S. Khan, a sea level change expert who just relocated to Hong Kong, to ask her more about this. One of her co-authored papers “Estimating global mean sea-level rise and its uncertainties by 2100 and 2300 from an expert survey” was published in npj Climate and Atmospheric Science this year in May. Check out our interview with Dr Khan below; we talk get her throughs on the progress of SLR projections to understanding uncertainties and what it means for Hong Kong.

CWR: Congratulations on your publication in npj Climate and Atmospheric Science. Can you walk us through the key findings in your study?

Dr. Nicole S. Khan (NK): The study repeated a survey conducted 5 years ago to solicit projections of global mean sea-level change under two climate scenarios from more than 100 sea-level experts, who were chosen objectively by identifying a pool of the most active publishers of scientific sea-level studies. Because projecting sea-level rise comes with many uncertainties, such as glacier and ice cap melting, use of water for irrigation, etc, different modelling approaches can yield significantly different answers as to how much sea levels may change in the future.

Survey of 100+ sea level experts predicts a 4.5ºC rise = SLR of 0.6 – 1.3m by 2100

By surveying leaders in the field, the study offers more broad-based assurance on the range of future sea-level rise projections. In a scenario where warming is limited to 2ºC above pre-industrial levels, experts predicted ~0.5 m of rise by 2100 and 0.5–2 m by 2300. In a high-emissions scenario with 4.5ºC of warming, experts predicted a correspondingly larger rise of 0.6–1.3 m by 2100 and 1.7–5.6 m by 2300.

CWR: How do these results compare to those of the IPCC?

NK: Expert projections from our survey exceed those from the IPCC. For example, the 2014 IPCC report and its 2019 Special Report on the Oceans and Cryosphere (SROCC) projected a likely upper limit of Global Mean Sea Level (GMSL) rise of 0.98 and 1.10 meters, respectively, for RCP 8.5 by 2100. The likely upper limit of expert projections from our survey was 1.32 meters.

IPCC projections seem conservative to many experts

The latest IPCC projections appear to be conservative in the eyes of many sea-level experts. The IPCC increased its sea-level projections by ~60% between its 2007 and 2014 assessment reports, and again in SROCC, given new data on the instability of ice sheets and the fact that sea levels are already rising faster than expected.

CWR: In your recent study, how does the scientists’ consensus on SLR projections differ from the conclusions drawn in your original survey conducted 5 years ago? If different, what could be the reason?

NK: Expert projections for 2100 are similar to those from the original survey, although the projection for 2300 has extended tails and is higher than in the original survey. The upper likely estimate increased from 1.5 to 2.2 meters for the low-emissions scenario and from 3.6 to 5.6 meters for the high-emissions scenario.

Key change is the 2300 projection at 3.6-5.6m; much higher than in the original survey at 1.5-2.2m

We interpret this increase in long-range projections to be related to recent modelling developments that incorporate dynamic processes known as marine ice cliff instability (MICI) and marine ice sheet instability (MISI) that influence ice sheets grounded below sea level. The former process describes the potential for marine ice cliffs to experience runaway collapse when they exceed a certain elevation above sea level due to physical characteristics of ice; the latter process relates to a series of responses of marine ice sheets grounded on inland-sloping beds to increased atmospheric/ocean temperatures or ocean circulation changes that lead to self-reinforcing feedbacks that accelerate ice sheet retreat.

The West Antarctic ice sheet and several sectors of the East Antarctic ice sheet, which contain enough ice to raise sea levels ~3 and ~19 meters, respectively, are particularly vulnerable to these processes.

CWR: You have recently relocated to Hong Kong and we are glad to have an SLR expert in town. Do you think Hong Kong is vulnerable to SLR? What will you your research focus on, now that you are here?

NK: Hong Kong is particularly vulnerable to extreme sea levels. The combination of potentially more frequent or more powerful storm surges from typhoons on top of a baseline of rising sea levels has the potential to inundate and flood villages in Lantau and the New Territories as well as some of Hong Kong’s highest valued infrastructure and financial and commercial centers in Central and Tsim Sha Tsui.

HK is particularly vulnerable to extreme sea levels…

The npj Climate and Atmospheric Sciences study described global mean sea level, which is the volume of water in the oceans divided by their area. What is more important for coastal planning and risk management are projections of relative sea level. Relative sea level is the change in sea level experienced at coastlines due to changes in the height of the sea surface and the land surface.

Relative sea level varies from place to place and can be greater or less than the global mean. For example, relative sea levels in the Canadian Arctic and Scandinavia are actually falling currently because there is rebounding or rising up in response to the disappearance of ice sheets that used to be present in these locations ~20,000 years ago. In comparison, in some locations such as Jakarta, relative sea level is rising at a rate in some areas 40 times faster than the global average due to excess extraction of groundwater from aquifers that has resulted in the ground sinking.

…yet, HK’s relative sea level’s mechanism is not well-known

The mechanisms that cause relative sea-level rise in Hong Kong and elsewhere on the China coast to deviate from the global average are not well-known. I am working to better understand and quantify these mechanisms.

I am also interested in improving our knowledge of how natural systems, such as mangroves, can be used to mitigate and adapt to climate change. Mangroves have a high capacity to draw down and store carbon from the atmosphere, and also play a critical role in buffering coastal communities from sea-level rise and storm surges. These ecosystems are receiving increasing consideration in carbon accounting of climate offsets and as nature-based solutions to protect against coastal flooding, but further knowledge of how they will respond to future warming and rising sea levels is needed.

CWR: Are there any other things you would like to share with us or words of advice?

NK: Recent comparisons have been made between the COVID-19 global pandemic and climate change – they are both high impact scenarios that require globally coordinated responses to prepare and build resilience. Disease experts for years had warned that a pandemic was possible. Institutions across the world did not do enough to prepare, and now are trying to make up for lost time.

COVID-19 provides a warning for what could happen if we neglect to prepare for the high-end risks of climate change

The COVID-19 pandemic is a cautionary tale and provides a warning for what could happen if we neglect to prepare for the high-end risks of climate change. However, it also suggests that if we start now with the same urgency we’ve responded to the COVID-19 pandemic, we can avoid making the same mistake with climate change.

Finding solutions to the increasingly urgent issue of climate change will require vast amounts of capital resources, which the public sector alone cannot bear. The resources and expertise of the private sector must be part of the solution. In turn, solutions for mitigation or adaptation to climate change will create investment risks and opportunities, and investors will need to integrate climate science into the equation. Improved coordination between climate scientists and financial professionals is an important step to tackling and responding to the many uncertainties and risks that future changes in climate will bring.

More On Sea Level Rise

  • It’s Time To Prioritise Sea Level Rise – CWR’s Debra Tan says it’s time to be FOMO about our rising seas. From emission accelerants to accelerated impacts she runs through three reasons to rethink our attitudes towards sea level rise – it’s a big deal, sea level rise is worse than you think. This time, even she’s depressed
  • The Present & Future Flood Risks In Bangkok – The nature of flood risks in Bangkok are complex and there is yet to be any holistic plan or authoritative projections on SLR for Bangkok. Chulalongkorn University’s Dr. Chanita Duangyiwa breaks it down & looks ahead
  • Awareness Of Typhoon Risks In Mekong – Vietnam is already vulnerable to typhoons and climate change will increase the number and intensity of typhoons. Yet, awareness and preparedness among locals are inadequate. Dr. Le Tuan Anh & Dr. Hiroshi Tagaki unpack why
  • Typhoons And Storm Tides Risks In Guangdong – Already vulnerable Guangdong Province, accounts for 34.5% of typhoons landing on China, has to now also deal with increasingly unpredictable storm tides and escalating damage. SYSU’s Dr. Xiaohong Chen & Huiwen Bai expand
  • Rethinking Of Typhoon’s Randomness And The Impact On Economy – See why Dr. Hiroshi Takagi from Tokyo Tech believes typhoons have an unpredictable random nature and why we need to look and other factors and re-evaluate risks

Further Reading

  • Thirsty And Underwater: Rising Risks In Greater Bay Area – How will water & climate risks, including rising sea levels & droughts, threaten the already water-stressed Greater Bay Area (GBA)? CWR’s Tan & Mirando explain in their latest CLSA report and highlight companies’ failure in climate risk disclosures
  • No-Sense Climate Strategies: From DSD To HSBC – Hong Kong’s shortsighted & unrealistic climate plans will leave key assets & infrastructure exposed that mean the government, companies, investors and the public are even more exposed. China Water Risk’s Dharisha Mirando & Debra Tan expand
  • It Happened – Central Banks And Water Risks – Half a dozen new reports by the NGFS means that CWR has achieved a key milestone in embedding water risks in finance. Debra Tan and Dharisha Mirando expand on these game-changing moves by the central banks. The credit evolution has started
  • Capital Threats Remain Post COVID – There is no vaccine for climate & water risks, yet some in the financial sector are still burying their heads. CWR’s Dharisho Mirando reminds us how our capital is at risk & steps we can take to reduce them while going green
  • HK Submerged? Is This Map For Real? – Rising sea level is a catastrophe waiting to happen but we have to avoid alarmism & choose the right map to visualise the risks. Getting the right scenarios also matter. Find out more in our review