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Climate change (CC) and sea level rise (SLR) projection as well as developing respective scenarios are daunting tasks and possesses high uncertainty. However, if we want to proactively respond and adapt to climate change, such scenarios are indispensable.

The right image is the prediction that by 2050 the Mekong River Delta will be eliminated under the impacts of high tide


It is important that we know the nature of information presented in such scenarios so that they can be appropriately used. Highly reliable scenarios can be used as a scientific basis for long-term planning. However, scenarios with low-reliability can still happen. Therefore, scenarios with low reliability still can develop into disasters and they should be also taken into account and response measures should be prepared. This is called climate change risk management.

Collecting information is important, yet collating and analyzing information is more important. It is necessary to distinguish which information is correct or incorrect so that prudent and accurate decisions can be made. This is right step to make as far as the socio-economic development and environment strategy of the country is concerned.

“In 2050, Ho Chi Minh City and the Mekong River Delta will be eliminated" is not correct information with sound scientific basis. However, this can be considered as an alerting message during the formulation of planning options where land subsidence, sea level rise and flooding due to high tide will be taken in to serious consideration.

Public opinion is being stirred up before the information quoted from the New York Times and "Nature Communications" magazine which are dominating both social network and official news channels of Vietnam, including Vietnamese television as follows: “Based on standard sea level rise impacts, 150 million people who are currently living in these lands will be finally settled under the high tide by 2050, i.e. Ho Chi Minh City and the Mekong River Delta which will be eliminated due to high tide”.

According to studies of the Climate Central, a science organization based in New Jersey, sea level rise due to climate change can affect a number of people three times as many as what have been previously forecasted and can threaten everyone or even eliminate some of the world's great coastal cities. Authors of an article of the Nature Communications Journal published on October 30, 2019 have issued a forecast basing on a reliable impact assessment of sea level rise for vast areas that as many as 150 million people living on the mainland will be engulfed by extraordinarily high tides.


Proper assessment of issues and conclusion of relevant study results and forecasts/predictions must be considered as the focus of any socio-economic development strategy, especially in the medium and long run.

As far as I understand and as a matter of fact, when our deltas and developed urban areas are mostly located along coasts, on lowlands and on soft soil of the lower delta, the possibility of being submerged under the sea level due to land subsidence and high tide of these areas is something quite obvious.

The warning that by 2050 many coastal cities around the world will disappear has such positive implications that those who are responsible for national governance and scientists will be more concerned about proposing action plans to proactively cope with natural disasters. However, in order to avoid confusion among the public, such scientific arguments and premises as well as reliable data should be relatively clarified and scientifically proved so that a comprehensive, scientific and practical perspective can merge as the basis for the formulation of disaster response programs;

The same as a previous report by World Bank, the authors warned that by 2050 many cities would be eliminated due to high tide, just simply by comparing the ground elevation with sea level and then zoning the inundation area to see how many people will be affected. However, this article also mentioned: "Maps do not factor in potential coastal defenses, such as seawalls or levees, and are based on elevation, rather than flood models ”, which means they don’t take into account river banks, embankments, coastal structures, roads or using flow models to see if water can flow in these structures. Therefore, results of this method are not realistic due to following reasons:

- Sea water shall not suddenly flood the delta for about 5-10 m like in a Tsunami, yet it will rises slowly and flows in canals. As a results, many locations in the Mekong River Delta below the sea level will not be flooded as the tidal amplitude is quite small. The current maximum tidal amplitude in Phnom Penh in the dry season is 50cm. When the dry season flow decreases and the sea level increases, the tidal amplitude will be larger and the maximum tidal amplitude can be as much as 1m.

In contrast, all the points above the sea level to the height of 1m or more will be flooded during tidal peak. Therefore, it is necessary to take into account the tidal flow, not only the sea level. If only sea level is counted, the majority of the Dutch population will be all flooded.

- The sea level normally rises slowly from year to year and the ecosystem itself and humans also gradually adapt. It is not reasonable to base on sea level forecast data for tens-of-year periods to predict situation of existing ecosystem, structures and production activities. Adaptation measures of nature and humans should be taken into account. For example: if water level increases by 5 mm annually, sedimentation and coastal ecosystems will make gradual changes to adapt and people in flooded areas will carry out adaptive measures as well. Therefore, affected area and people will not be as forecasted in above threatening reports. At present, sediment flow to the delta is being strongly reduced and big floods in the delta are almost unseen recently. And it is not rational to say that the sedimentation rate can reduce climate change impacts.

However, it is noted that “tide” in Climate Central's research does not necessarily mean the astronomical tide but can also be meteorological tide, which is the storm surge. Given the current height of the Mekong River Delta dike (3m + 0.3m of breakwater), the sea level rise of 1m + high tide of 1.5m will not overflow the delta. However, the problem will be serious if there are big storms which are intensified by climate change trends.   

Vietnam has had an adaptive solution as the dyke system in the Mekong River Delta and in Ho Chi Minh City. In the future, the most important issue is just how to change people's livelihoods to adapt to the solutions accordingly.

The World and Vietnam’s understanding of climate change process as well as its impacts on socio-economic activities is still limited. The forecast showed even more complicated and unpredictable trends. Difficulties and challenges may be even more severe. Scientists have affirmed that climate change is a kind of force majeure, at least in the 21st century and adaptation measures are inevitable. Adaptation not only helps reduce consequences of climate change impacts but also contributes to mitigated climate change.

The Ministry of Natural Resources and Environment has developed climate change and sea level rise scenarios (2009-2012 and 2016) with quantitative and most reliable data yet still present such uncertainties as:

Uncertainty of climate change scenarios.

If we look back on the two climate change scenarios for 2009 and 2012, they are similar in the way that they are all based on 3 greenhouse gas emission scenarios (according to IPCC, 2007): B1 (low), B2, A1B (medium) and A2, A1FI (high); all use the same scenario formulation method of statistical downscaling and take the 1980-1999 period as the baseline period and consider B2 and A1B as the medium scenario, A2 and A1FI as the high scenario. However, they do not specify the scenario where the final results are based on whereas B2 and A1B vary significantly over time and A2 and A1FI also vary greatly. The two pairs only approximate the amount of greenhouse gas emissions by the end of the 21st Century (2090). This should be clarified. Both the 2009 and 2012 versions have not yet provided the necessary reliability of these scenarios (due to the lack of large enough sampling scale for evaluation).

Upon comparing to the 2012 version, the 2009 version is more detailed in term of time-based changes (for each decade of the 21st century), yet rawer in terms of spatial distribution (for 7 climatic regions only). The 2012 version shows a more detailed spatial distribution through map system yet not providing any specific information about changes over individual period in the 21st century, except for the B2 scenario in which the average annual temperature and annual rainfall are detailed for each decade whereas other factors are based on 2 milestones of mid and late 21st century.

The 2012 version has additional scenarios of average minimum and maximum temperatures and the number of days with highest temperature above 350C. The 2012 version (as presented in the report) referred further the results of PRECIS and MRI models (of Japan) and of SIMCLIM and SDSM statistical models.

However, in the 2012 version, the Ministry of Natural Resources and Environment has tried to come up with scenarios detailing spatial distribution. However, it is not guaranteed that these scenarios are reliable as data sources of maps presented in the PB2012 are not clear.

If these maps are developed based on monitoring network data, it is likely that the interpolation results from data of sparsely populated stations (high mountain areas, remote areas) will contain big errors. If only results of digital modelling are used (as MRI products are only available for 2 periods of 2015-2039 and 2075-2099), it is not sufficient and PRECIS's products have not been verified. If "mixing" the two above types, mixing methods should be worked out.

Despite using all existing products resulting from such methods as statistics, PRECIS and MRI based numerical models), the sampling set is still far too limited  for evaluation and estimation of uncertainties.

And therefore, the PB2012 scenario can neither be used as a scientific basis for the evaluation of climate change impacts nor a basis for the implementation of so-called climate change adaptation action plan. If we are not ensured about how climate change will affect, then how we can cope with such impacts and the outcome would be “spend money and only get worse".

In order to formulate a reliable scenario, it is necessary to mobilize efforts of many facilities and individuals who will work independently and produce different sets of climate change projections. Once the samples are large enough, the statistical sampling will take place to produce the best scenarios as possible.   At the same time, a large number of samples is needed to assess the reliability of scenarios. For example, 100 samples from individuals/collective groups showing changes of average temperature will determine the range of changes, the temperature amplitude will be defined and it is the range where most of (big probability) samples will fall in.

While waiting for reliable results of new scenarios, PB2012 scenario can be used as a warning message about climate change in Vietnam, thereby raising public awareness on the issue, especially awareness of the management and leadership.

Currently, SRES (Special Report on Emission Scenarios) has been replaced by RCP (Representative Concentration Pathway) with emission scenarios such as RCP 4.5 (low emissions), RCP 6.0 (medium emissions), RCP 8.5 (high emissions). Based on these GHG emission scenarios, global climate models (GCMs) will be run to predict climate for the future. Previously developed scenarios based on SRES such as A1B, A2, B1, B2, etc. in the 4th report of the IPCC (AR4) are now only for reference.

The benefit is that the PB2016 have been developed based on the results of many dynamic models and on RCP based emission scenarios. However, due to the limited number of component model products, reliability of the scenarios is still unknown. It is hoped that the development of climate change scenarios for Vietnam in the future will be improved.

Scenario “By 2050 the Mekong River Delta and Ho Chi Minh City will disappear” still have many "gaps".

The Mekong River Delta and Ho Chi Minh City are low lying and young lands which are now at risk of climate change and sea level rise impacts as well as soil subsidence. Proper listening and attention to the risk of land loss due to sea level rise are strongly advised. However, following considerations should be taken into account before any decision can be made:

The uncertainty in climate change scenarios is quite high, especially in the context of sea level rise scenarios. In theory, if all the snow and ice around the world, including those in both Earth's poles and glaciers, might melt into water,  that amount of water would cover the global surface (including mainland and ocean) for a thickness of 60 m. That means some places can be inundated under the depth of more than 60m.

In fact, this process would never happen in the history of the Earth. The estimation of sea level rise is very difficult and inaccurate because it is impossible to calculate dynamics of snow and ice and heat related expansion due to sea level. That is also the reason why in AR3 (the 3rd Report of the IPCC, 2001) it is estimated that the sea level will rise by 1m by the end of the 21st century. Given this scenario, the Mekong River Delta will lose nearly 40% of its land due to sea level rise (according to PB2009 scenario of Ministry of Natural Resources and Environment).

Therefore, during 2006-2007 UNDP and the World Bank warned that Vietnam is among the top 10 countries most affected by climate change. For this purpose, a new National Target Program to respond to climate change has been approved by the GoV in December 2008. But in the 4th Report (AR4, 2007) the IPCC only presented the estimated sea level rise of 0.6m. And in the 5th Report (AR5, 2013) under RCP8.5 scenario, the sea level in 2100 will be approx. 0.52-0.98m at a rising rate of 8-16mm/year during 2081-2100 (For RCP8.5), then the sea level by 2100 is 0.52 to 0.98 m with a rate of 8 to 16 mm/yr during 2081–2100).

Ho Chi Minh City and the Mekong River Delta are not completely flat areas. Some locations are elevated and others are depressed. The calculation of flooded areas requires a very detailed topographic map (large scale) to ensure necessary accuracy. So what is the basis of the statement that Ho Chi Minh City can be eliminated?

According to calculations of some international experts, the land subsidence rate of Ho Chi Minh City is about 3-4cm/year and 2.5-2.8 cm/year for the entire Mekong River Delta which means 10 times as much as the sea level rise. If the assumption is true, by 2050 (in 30 years), Ho Chi Minh City will be "sunk" for about 150cm at the most. So how many percentage of Ho Chi Minh City area having ground elevation less than 1.5m? Nha Be area which is considered as a "low-lying area" of Ho Chi Minh City is already over 1.87m above the current mean sea level, according to the national landmark elevation. Can we calculate the proportion of Ho Chi Minh City that is at lower level compared to Nha Be area?

High tide phenomenon resulting in inundation is not only caused by the nature (soil subsidence + sea level rise + combined effects of high tides and monsoon wind waves +...) but also by humans. This have been repeatedly mentioned (spatial planning issue). It is not easy to solve the consequences of wrong planning. However, for the moment, we can make prompt adjustment unless the problem will become worse.

According to the worst scenario (RCP8.5), up to 2025 the East Sea level will increase by 36cm compared to the beginning of the century. It is now 2019 and nearly 2/5 of the period has been through. From now on to 2025, another rise of 22cm will be experienced.

Currently, the high tide in Phu An have reached the record level of 1.77m and 1.80 m in Nha Be Area. By 2050, unless some unexpected leveling may take place, the high tide in Phu An will  reach the level of 1.90m. Except for Can Gio where nobody is living, the majority of ​​Ho Chi Minh City area will be 2.0m higher than the sea level. If the highest sea level is not higher than the existing terrain, how the tide can submerge the entire city? Moreover, if only sea level and topography are taken into account, why the Netherlands is not totally flooded or even eliminated? So it can be concluded that "Ho Chi Minh City will be eliminated by 2050" is only an overstated and exaggerated way of saying.

For conclusion

"In 2050 Ho Chi Minh City and the Mekong Delta will be eliminated" is not scientifically based information, nonetheless it is a message to be take into account during the development of planning options as far as land subsidence, sea level rise and high tide related inundation are concerned.

Regarding sea level rise, we have a system of hydro-meteorological stations and agencies in charge of monitoring, measurement, analysis and calculation of sea and river levels etc. We have annual and updated data of sea level rise in Mekong River Delta through which sea level rise trends are understood and the GoV will have a sound basis to propose reasonable solutions to sea level rise issues for the Mekong River Delta and Ho Chi Minh City.

This article presents author's scientific viewpoints, not VnDMA’s official opinions.

To Van Truong