Sequestering seawater on land: a water-based solution to global issues

Introduction

Although the impacts of climate change on the oceans are ‘harder to see than receding glaciers’¹, the rise in sea-level and its economic and social consequences are already visible for people inhabiting low lying oceanic islands². Seawater thermal expansion and the melting of glaciers and polar icecaps^3,4 have led to an average sea-level rise of 3.2 [2.8 to 3.6] mm per year between 1993 and 2010⁵. Rising oceans cause coastal land to be lost or become inhabitable⁶ and will likely generate millions of ‘climate change migrants’⁷ as well as major economic and environmental damage in the near future^1,8. However, this ‘surplus’ of water offers an unprecedented opportunity to tackle a number of global issues through a very pragmatic process: shifting the excess water from the oceans onto the land.

Storing desalinized water

Here we propose that sea-level rise could be mitigated through the desalination of very large amounts of seawater in an international network of massive desalination plants.

The resulting economic, environmental and health benefits would be considerable. Desalinized seawater could be used to grow crops in desertified and drought-prone areas⁹, which could directly contribute to increased food security in countries where water resources for agriculture are limited, by the reliable production of local food. Water is also needed to refill lakes and river systems dried up from human consumption and rising temperatures, as is already done for the Jordan River in Israel¹⁰. The second largest reserve of freshwater after polar icecaps, is groundwater, but its depletion in recent years due to increased water demand (mainly for agriculture) and to a very long recycling time¹¹, is also contributing to sea-level rise¹². Desalinized water could be used to counterbalance groundwater depletion and maintain current levels. To ensure long-term ’storage’ on land, desalinized water could also be ‘captured’ in the form of restored wetland vegetation and novel forested areas. Wetlands provide essential ecosystem services, particularly relevant in a changing climate¹³, but 87% of wetland areas have been lost since 1700 AD¹⁴. Novel forests will not only capture and store water, they would also act as important carbon sinks (Figure 1), thereby supplementing existing forests, which may have reached saturation¹⁵, and mitigating climate change¹⁶. These restored or newly created habitats will also contribute to the conservation of particularly vulnerable and declining biodiversity^14,17, thus tackling yet another major global issue.

Figure 1. Schematic illustration of how excess atmospheric carbon and excess seawater could be captured in the trunk or stem of different woody species; clockwise: baobabs, Douglas fir, vines and screwbean mesquite (data from 42–45).

Desalinizing the excess seawater

Today’s desalination plants are designed mainly to produce potable water for human consumption, but also to support agricultural activities. As freshwater resources are becoming more unreliable in many parts of the world, the number and the size of these facilities are rapidly increasing. The world’s largest desalination plant, Ras al Khair has recently been built in Saudi Arabia. Once fully operational, it will be capable of producing 1,025,000 m³ of desalinated water per day. Another example of megaplant is the Sorek desalination plant in Israel, which reached full capacity in 2015 and is now producing up to 624,000 m³ of desalinized seawater per day, thereby providing potable water for 20% of Israeli households¹⁸.

Although most of the ca. 18,000 existing desalinization plants are smaller than the Saudi Arabian or the Israeli mega plants, globally 85 million m3 of desalinised water were produced per day in 201519 and this figure is predicted to further increase in the future. Yet this is still a drop out of oceans that expand every year by 9–12 trillion m3. To counteract the current increase, 46,000 mega plants like the one in Israel would be required. With an individual price tag of US$ 500 million, the Israeli model (Sorek) is much cheaper than the Saudi Arabian Megaplant ($7.2 bn). But building 46,000 of them would still cost US$ 23 trillion. Such amount is difficult to comprehend as it represents more than the GDP of Europe or that of the US ($18 tn each) and more than twice the GDP of China ($11 tn). However, it needs to be put into perspective with the potential cost of inaction. For example, in absence of sea level rise mitigation action, coastal flood damage in the world’s 136 largest coastal cities could cost US$ 1 trillion per year by 2050²⁰ and the relocating the 13.1 million people living in coastal areas threatened by sea-level rise in the US only is projected to cost more than $ 14 trillions²¹.

Feasibility, on-going costs and environmental impacts

In addition to the costs of building desalination megaplants, the on-going processing costs of sea water also needs to be taken into account. The methodology used in Sorek and most modern desalination facilities is reverse osmosis²² (RO), a process which has high energy efficiency and is currently used in 65% of desalination facilities around the world²³. During the RO process, seawater is forced through semi-permeable membranes at 27 times atmospheric pressure to overcome the osmotic pressure of seawater²⁴. To achieve this, RO still requires large amounts of energy, which on average accounts for 30–60% of the production cost of desalinised water^25–27. The cost of producing 1 m³ of potable water in Sorek is currently $0.58¹⁸. This type of plant may therefore require $102 millions of operational costs per year and is predicted to use 525 GWh megawatts in 2020²⁶. Based on the above rate, operating 46,000 clones of Sorek could cost around $4 tn and require the equivalent of 15% of the global primary energy supply of the year 2014²⁸. When Frieler et al.²⁹ investigated the possibility of storing the excess sea water as ice in the Antarctic, they estimated that the energy costs of pumping sea water onto the Antarctic ice shelf, could require 12% of the global primary energy supply of the year 2012 under optimistic assumptions. The authors concluded that producing such amount of energy in the Antarctic was a major technical limitation to their solution. Similarly, operating the proposed network of desalination megaplants represents a major energetic challenge, not only because of the enormous amount of energy required, but also because the production of this energy should not rely on fossil fuel and result in massive emissions of carbon dioxyde. Therefore, an important area of research and innovation lies in the production of renewable energy such as solar, wind and tide-generated electricity to power desalination plants. Facilities that are being built today are already turning towards renewable sources of energy^30,31 and it is likely that the energetic impediments described above will reduce with time and as technology advances³². The recent launch of the Global Clean Water Desalination Alliance, at the Paris COP21 meeting also demonstrates a political ambition to reduce carbon dioxide emissions from desalination processes.

Rapid decline in the total cost of desalinised water with increasing plant capacity³³ has led to cost of desalinated water to approach that of conventional water supply²⁰. However, transportation of desalinised sea water remains a major hurdle, which has the potential to significantly affect the cost and therefore feasibility of the proposed concept. If most desalinised water was used within 1000km from the cost and 500m elevation, the production and transportation costs will likely remain under $2 per cubic meter³⁴. If such costs are likely to be acceptable when relating to potable water for human consumption, they may be less justifiable if the water is used to grow forests, or replenish wetlands, but, could be defendable for agriculture in areas where food security is poor. Another important consideration is the fact that desalinised water used in agriculture (and by extension for the restoration of natural habitats) does not require the same quality as drinking water³⁵. An acceptable salinity for irrigation water is < 1600 ppm of total dissolved solid as opposed to less than 400 ppm for drinking water³³. As a consequence, desalination for agriculture purposes or storage on land is technically less challenging and can be significantly cheaper^33,36.

One recurrent issue with sea water desalination is the potential impact on the environment. Reverse osmosis produces significant waste in the form of hyper-salinized brine³⁷, which is usually released in the ocean³⁸, and used membranes, which are usually disposed of in landfill³¹. These by-products could cause significant environmental issues in a scenario involving the construction of thousands of large desalination plants. A review by Roberts et al. 2010³⁹ revealed that the main environmental effects occur within tens of meters of the discharge site. The careful selection of these sites is therefore of prime importance to limit the potential adverse impacts of desalination plants on marine ecosystems. The development of new, more efficient and environmentally-friendly desalination technology is another important area of research.

Incentives to engage on the proposed path

The projected acceleration of sea-level rise⁴⁰ means that the desalinating capacity required for the proposed response is always increasing. A critical tipping point is the melting of Antarctica’s ice shelves, which is projected to become irreversible if atmospheric warming exceeds 1.5 to 2 degrees Celsius above current temperatures⁴. This point could be reached in less than 50 years under the current emission scenario⁵. Given the timeframe required to deploy a worldwide array of massive desalination facilities, as well as the means and infrastructures to redistribute desalinized water where needed, it may not be achievable in the next 50 years. However, benefits from engaging on the proposed path will be perceptible from the onset. The consequences of initiating the construction of massive desalination facilities will directly contribute to 9 of the United Nations’ 17 Sustainable Development Goals⁴¹ (Figure 2). The progression to these positive outcomes is stepwise. At a local scale, these include the creation of jobs, the development of infrastructure, the driving of innovation in water treatment methods, and the possibility of increasing food production in famine-prone areas through the reliable and sustainable provision of water for agriculture. These outcomes should be seen as incentives to engage in the development of large-scale seawater desalination facilities, particularly in areas where freshwater availability is unreliable and food security is poor.

Figure 2. Milestones of the proposed sea-level rise mitigation strategy, direct benefits and how they relate to the United Nation Sustainable Development Goals (SDGs).

Conclusion

Although a radical option, the proposed strategy remains simple in principle, relies on existing and continuously improving technology and is scalable to mitigate sea-level rise and contribute to addressing a number of global issues including food security, climate change and biodiversity loss. Because these issues are intimately intertwined, solutions that only address one of them have a limited chance of success. There is a pressing need for proposing and testing more proactive and ambitious ways to address multiple global issues under one sole umbrella. The massive financial investment required to mitigate sea level rise through seawater desalination is likely to be balanced by significant socio-economic and environmental benefits. This perspective raises a number of critical questions relating to the financing and ownership of thousands of mega desalination plants around the world; the mechanism for distributing desalinised water to agriculture and other means; and the governance of such a global cooperation. The other major hurdle, is a global political plan to engage in a worldwide coordinated effort for mitigating sea-level rise.