Future SLR Projections & Biggest Worries

By Dr. Nicole Khan 24 November, 2020

In this follow up interview, Dr Khan shares concerns on future sea level rise rising higher & faster than we think

Last month, we sat down with Dr Nicole S. Khan, an Assistant Professor from the University of Hong Kong to talk about the scientific truth of sea level rise (SLR). We couldn’t get enough and given the significant risks – which we extensively dive into in our newly released CWR Coastal Climate Threat Series – we did a follow-up interview with Dr Khan. This time Dr Khan shares her concerns on the faster & higher future projections of SLR and talk about what variables to use for building the best scenarios. 

If you missed our interview with Dr Khan last month, check it out here.

CWR: Thank you for chatting with us again Professor Khan. This time we want to really dig into how we can best prepare for sea level rise (SLR). This means building scenarios. It seems to be common practice to use the median value of the projected SLR to assess the risk at each climate scenario. Do you think it is too conservative, or should we be using the full range or the higher end of SLR projections, since the risk has always underestimated?

Dr. Nicole S Khan (NK): Approaches focused on median values omit information about high-end outcomes that are critical for risk management. Central ranges or probabilistic projections provide the likelihood of a range of possible future sea-level rise projections occurring. For example, in our study, experts projected it is likely (i.e. there is a greater than 66% probability) that global mean sea level will be between 0.6 and 1.3 meters in 2100 under a high-emissions scenario, whereas it is very likely (i.e., there is a greater than 90% probability) that global mean sea level will be between 0.5 and 1.7 meters for the same time range and emissions scenario.

Median value approaches omit info critical to risk mgmt…

e.g. 17% chance that global sea level will be >1.3m

Or put another way, there’s a 17% chance that global sea level will be higher than 1.3 meters and a 5% chance it will be higher than 1.7 meters. Using this approach for future sea-level rise is useful for decision frameworks such as cost-benefit analyses that employ probability distributions of future change as an input to derive metrics of the economic or social cost of greenhouse gas emissions.

CWR: There are still many uncertainties when projecting global mean sea level (GMSL), key ones being warming seas, accelerating melting glaciers in Greenland & Antarctica and thawing permafrost in Russia; even scientists say they are hard to assess. Can you help walk us through how these impact SLR and share what your biggest worry is?

NK: There’s a lot to unpack in this question. Let’s take it one process at a time.

When oceans heat up, they expand. The contribution of this process to GMSL given a certain warming scenario is fairly well known, and it contributes between 0.2-0.6 meters of rise per °C of warming. For example, if temperatures rise by 1°C, this effect will be small causing GMSL to rise 0.2-0.6 meters, but if temperatures rise by 4°C, this effect is much larger and contributes 0.8-2.4 meters to GMSL.

The thawing permafrost is worrying…

A more worrying related process here is the impact that permafrost thawing in Russia may have. Permafrost across the Arctic is beginning to irreversibly thaw and release carbon dioxide and methane, a greenhouse gas that is around 30 times more potent than carbon dioxide and has long-lasting effects.

If methane emissions were to cease, 75% of the methane-induced thermal expansion persists for 100 years, and approximately 40% persists for 500 years. With the Arctic warming at least twice as quickly as the global average, the boreal forest in the subarctic is increasingly vulnerable.

The response of ice sheets to warming is the largest uncertainty in how GMSL will rise with future warming. Part of this uncertainty is due to the non-linear behavior of ice sheets as they disintegrate and the complexity of interactions among ice sheet forcings (e.g., atmospheric temperature and precipitation, ocean temperatures, and even sea level itself). Put more simply, ice sheets can reach certain thresholds or tipping points that result in dramatic and irreversible changes.

…then there’s the ~14m of GMSL that may have already started irreversible melting

Certain sectors of the Antarctic ice sheet may have already passed a tipping point. The Amundsen Sea embayment of West Antarctica is retreating irreversibly, and modelling suggests that when this sector collapses, it could destabilize the rest of the West Antarctic ice sheet, leading to about 3 meters of GMSL rise over centuries to millennia. The Wilkes Basin, part of the East Antarctic ice sheet, might be similarly unstable, which could add another 3 to 4 meters to GMSL on timescales beyond a century. The Greenland ice sheet, which holds enough ice to raise GMSL by ~7 meters, could disappear within a millennium if we continue on a ‘business as usual’, high-emissions trajectory.

Already historical carbon emissions have locked-in 1.2-2.2m SLR…

…moving to zero emissions over ~90 years will lock-in another ~9m

Some studies have assessed the long-term sea-level response to a given emission future, which is sometimes called a “sea-level commitment.” One such study used physical models of ocean warming, glaciers, ice caps, and ice-sheet contributions to assess the sea-level change arising from two millennia of exposure to a constant temperature. Over two millennia, they project a commitment of 1.4–5.2 meters from 1°C of warming, 3.0–7.7 meters from 2°C of warming, and 6.0–12.1 meters from 4°C of warming. Another study estimates that historical carbon emissions have already locked-in 1.2–2.2 m of sea-level rise, and phasing emissions down to zero over the course of the next ∼90 years will lock-in another ∼9 m.

CWR: These numbers sound very scary and will have serious consequences for all of us. Currently many governments are still building resilience on the base case that we only reach 1.5ºC of global warming by 2100 and so use the lower emission scenario (RCP2.6) to plan their adaptation strategies. Given all the uncertainties you have just mentioned, clearly it is not enough. Should we be doing more so that we won’t trigger these “instabilities”?

NK: A recent study came out in August that indicates that current sea-level rise is on track with IPCC high-end projections. In the same month, another study was published that suggests that the Greenland ice sheet has reached a tipping point in coastal portions of the ice sheet that likely cannot be reversed.

Need holistic approach of mitigation & adaptation strategies

It is becoming increasingly likely that we will see impacts beyond projections for low-emissions scenarios (i.e., we’ve locked in or committed ourselves to a certain level of sea-level rise), but it’s important to consider the time span over which these impacts may occur and their severity centuries into the future, both of which can still be reduced if emissions are cut.

Therefore, we need a holistic approach that includes mitigation measures to limit emissions or sequester GHGs from the atmosphere to reduce the immediate impacts of climate change, and also adaptation to impacts we’ve already set in place. In view of the extreme flooding that has ravaged China, the uncontrollable wildfires that have devastated the US and Australia, and tropical cyclones that have devasted western Atlantic coastlines this year, it is clear that we are already beginning to feel the impacts of a warming climate.

More On CWR Coastal Capital Threat Series…

• The CWR Survival Guides to Avoiding Atlantis – Sea levels can be 3m by 2100, putting urban real estate equivalent to 22 Singapores underwater in just 20 APAC capitals & cities. With US$5.7trn of annual GDP at stake, get on top of the new risk landscape to survive
• Surviving Rising Seas – 20 APAC Cities: Who’s ahead & Who’s Behind? – The homes of 28mn to 100mn+ residents could be submerged in just 20 APAC cities. Which cities are more prepared? We walk you through the Top 5 Most Proactive & the Bottom 5 Laggards in our CWR APACCT 20 Index
• CWR APACCT 20 Index – Finance Sector Input On Methodology – Coastal threat assessment can be complex & daunting, so we did the heavy lifting by creating the index. But we did not do this alone – 100+ finance experts helped. See who they were & how feedback shaped the index
• Sovereigns At Risk: Lots Of Capital In Vulnerable Spots – Clustered nature of rising coastal threats plus lax govt action put APAC sovereigns at risk. CWR’s analysis of GDP, trade, markets & bank loans reveal intense concentration of risks. As no-sense strategies pervade, see who’s in CWR’s watchlist
• Existential Coastal Threats: 8 Things You Must Know – Rapid SLR will happen sooner than we think, yet we are still driving investments to vulnerable locations. CWR’s Debra Tan shares 8 things you need to know about the existential threat from SLR – from glaciers in the mountains to ice sheets in our poles, permafrost + more

Further Reading

• Sea Level Rise – What The Science Tells Us – What’s the latest on sea level rise projections? HKU’s Dr. Nicole Khan shares key findings from her survey of 100+ sea level experts, as well as talks risks to Hong Kong and what we should take away from COVID-19
• 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