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 massive desalination plants.

The resulting economic, environmental and health benefits would be considerable. Desalinized seawater can be used to grow crops in desertified and drought-prone areas⁹. This can 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 26–29).

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 Sorek desalination plant built in Israel in 2013 has 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 17,000 existing desalinization plants are smaller than the Israeli mega plant, globally 80 million m³ of seawater were processed per day in 2013¹⁹ and this figure was predicted to reach 97.5 million m³ in 2015²⁰. Yet this is still a drop out of oceans that expand every year by 9–12 trillion m³. To counteract such increase, 46,000 mega plants like the one in Israel would be required. With an individual price tag of US$ 500 million, they would cost US$ 23 trillion to build. However, technological advances and even bigger plants could significantly reduce this cost. The methodology used in Sorek and most modern desalination facilities is reverse osmosis²⁰, where seawater is forced through semi-permeable membranes at 27 times atmospheric pressure to overcome the osmotic pressure of seawater²¹. To achieve this, reverse osmosis requires large amounts of energy. Therefore, an important area of research and innovation is the production of renewable energy such as solar, wind and tide-generated electricity to power desalination plants. Another important consideration is the fact that water used in agriculture does not require the same quality as drinking water²². As a consequence, desalination for agriculture purposes is technically less challenging and significantly cheaper²³.

Incentives to engage on the proposed path

The projected acceleration of sea-level rise²⁴ 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 Goals²⁵ (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 largely balanced by 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.