Desert
Flooding

About

About 10% of the world's surface is desert, which is cheap, uninhabited, unproductive land that is drenched in some of the most powerful solar radiation on the planet. This approach seeks to leverage these attributes by flooding the desert with water to create millions of 1 km2 oases (shallow reservoirs with water) that can absorb and retain carbon with the potential benefit of creating newly habitable areas.

This system of oases would be used to grow phytoplankton. With additional desalinated water, it could irrigate the surrounding area to propagate vegetation as well as provide fresh water to nearby communities. These oases would operate similarly to the ocean phytoplankton cultivation concept but executed in a relatively controlled and, thereby, safer environment than in the ocean. Unlike BECCS, the oases would absorb CO2 via phytoplankton growth - phytoplankton produce biomass faster than agriculture, reducing the necessary surface area by almost 4x - which would be periodically harvested to extend the length of sequestration and set up downstream use as fertilizer or other higher value products. This would be the largest infrastructure project undertaken, making its scale the main challenge.

How much CO2 it could absorb if everything worked perfectly?

Creating 4.5 million oases that are 1 km2 would allow us to sequester more than current global emissions (41+ giga tonnes of CO2 per year), while only requiring the equivalent of half the landmass of the Sahara desert.
41Gt CO2
per year
Here's the maths
  1. Algal beds can fix 2.5 kg of C/m2/yr and 2.5 kg of C works out to 9.2 kg of CO2.
  2. The total Sahara desert surface area is 9.2 million km2, with 4.5 million km2 being less than half.
  3. The maximum CO2 assimilation rate of algal beds 9.2kg/m2/yr * 4.5 trillion m2 = 41.4 giga tonnes of CO2 assimilation annually, using the equivalent of half of the Sahara for this project

In-depth description & science

To build such a system, we would need to fill our reservoirs with water, which requires 9 trillion m3 at a 2m reservoir depth. This water would have to be transported from its source with industrial pumps. In addition, we would need to design the system to replenish 100% of the water per year based on observations that at tropical latitudes evaporation rates can be up to 2,000 mm per year. In order to fill and continuously support this system, we would need to have renewable energy sources or develop a net negative carbon system using non-renewable energy while extending the oases total size.

Connecting the entire system would require the construction of a piping system to transport the water from the coastline to the reservoirs. This piping infrastructure may look something like our own vasculature, decreasing in size moving away from the source with numerous intermediate staging points to amass water and reduce the distance needed to pump water into our reservoirs.

Next, we would have to build the reservoirs. Instead of digging millions of holes, we could sinter a base and put a 3m barrier up for each reservoir - in essence, a large glass pool. Other methods of constructing these reservoirs with various materials is possible as well. There has been research into using desert sand as construction material to avoid the use of environmentally destructive inputs to create concrete, but this would require more materials that are not readily available in the desert environment. With the advances in construction automation we could imagine using drones to automate the whole construction process.

While building the infrastructure of the oases reservoirs would be the first part of the project, cultivating phytoplankton is the second challenge that would allow us to sequester carbon at the massive scale required. Therefore, we would need to design systems that monitor culture growth, potential contaminants, and distribute nutrients on demand. This would be a coordinated feedback loop between aquatic drones that patrol the reservoirs and their flying counterparts that image reservoirs from the air while shipping nutrients quickly across our vast operations of oases. We are pretty good at making things smaller, faster, cheaper so we would need to focus our attention on details like improving practical biological sensing in the wild as well as optimizing load-bearing capacity for efficient distribution of nutrients.

Assuming we can refine the technical implementation of the distributed infrastructure and engineer the phytoplankton to further increase carbon sequestration and/or produce commercial byproducts, the next step would be to stabilize the ecosystem around the reservoirs. We would attempt to reforest using the phytoplankton biomass as the nutrient source - making sure it isn't consumed by bacteria and the carbon respired back into the atmosphere - while adding seeding duties to our flying drones.

What makes it a frontier technology?

This would be the largest infrastructure project ever undertaken. This notion in itself warrants the invention of new design and construction technologies to save even a fraction of the total cost of the project.

There would need to be significant breakthroughs to the desalination process to make it cheaper and more energy efficient - currently at 3.5kwh per m3, but a theoretical minimum of 1kwh per m3 - in order to facilitate the development of a sustainable ecosystem in between the reservoirs of phytoplankton.

Creating the appropriate physical structures, whether for human habitation or for infrastructure such as man-made reservoirs or piping, using local materials - e.g. desert sand - has been an ongoing area of research. While the chemistry challenge hasn't changed, the use of various forms of drones to build, maintain, and observe - in essence, terraform - has yet to move past the prototype stage. The beauty, in theory, is that these systems would be able to respond in real-time to issues causing degradation of infrastructure which would give more flexibility in the choices of structural materials.

Our biological engineering tools have vastly improved to the point of being able to transfer external, desired biochemical pathways into our model organisms of choice. However, lab-scale production is incredibly difficult to move to industrial-scale, so the leap to controlled cultivation at reservoir or ocean scale comes with a series of unknown unknowns. The hope we have is that future automation and use of distributed software/hardware systems will allow us to have unprecedented control over the ecosystem.

Energy requirements

Moving vast quantities of water - 9 trillion m3 per year - is the main energy requirement of the project. Desalination, at this scale and its current rate of energy efficiency, would itself require 10.8 tw of additional solar capacity, compared to 6.4 tw of total global power capacity in 2016. While this energy requirement dwarfs the energy of constructing and maintaining the system of reservoirs, the millions of drones necessary to do this will add significant additional energy requirements.


Connection to Other Ideas

The transferability of these biological, software & hardware technologies across a wider array of problems has the potential to add to the commercial value beyond keeping us alive and our ecosystem in habitable form. For instance, the secondary effects of a portion of the desert land being newly habitable for humans could alter the global real estate landscape. Figuring out how to autonomously terraform could bolster our abilities to do so on the Moon as well as Mars. The gigatonnes of cell cultures we would be producing could themselves be at least a commercial fertilizer, if not higher value products.


Estimated Costs

The key components that drive capital costs are: pumping seawater, desalination plants, installed electricity generation capacity and the piping system. Altogether, they put the cost of the project well above $50tn. Economies of scale as well as breakthroughs in material science and construction technology will all be necessary for success.

The desalination plants need an annual 9 trillion m3 capacity, which translates to a rate of 24.7 trillion liters of water per day. Currently installed plants vary in capital costs from $1 to $10 per liter per day and less than $1 per 1000 liters (m3) in operating costs, but we can expect $1 per liter per day going forward based on low cost plants. This would result in a roughly $25tn capital cost for building desalinating capacity and $25 billion in daily operating costs. Even if we scrap the desalinated water for irrigation, the desalination energy requirements are a decent proxy for the energy needed to just pump water into our phytoplankton reservoirs, given the multitude of variables ranging from elevation variance as well as the distance traveled.

For installed electricity generation capacity, the likely option assuming current technologies would be large-scale solar, which we could assume at that scale to fall under $1 per watt of installed cost. The effectiveness of desalination is assumed to be 3.5 wh per liter, which means 31.5 twh needed per year or a 10.8 tw installed capacity. This gives us an installed cost of $10.8 trillion dollars to supply the desalination plants with enough electricity to operate continuously.

The cost of installing the piping system with current technologies is also significant, even when stretching them. Assuming each set of 2 reservoirs can be supplied with 1 km of additional piping, 2.25 million km of piping are needed. At a 4m/s flow velocity, which results in average pressures, 2.4m diameter pipe can support about 430k l / hour, which means it can supply 2 reservoirs constantly. At a $48,000/cm-km installed cost for onshore pipeline, the total cost equals $25 tn. We may be able to cut costs another 5-20x by using polymer-based piping, resulting in a still-massive $5tn bill. If we are able to greatly reduce the distance between sea water and our reservoirs by - really reaching here - carving a river from the ocean through the desert, we may be able to further reduce the cost of piping.


Risks

While any large scale infrastructure project has the potential to fail or have unintended consequences, the benefit of using currently inhospitable land such as deserts is that we constrain the magnitude of risk. In relation to the potential ecological impact of culturing phytoplankton in the oceans, doing so in desert reservoirs reduces systemic risk and exposure of the marine ecosystem to our widespread meddling. On the scale we would need to get to emissions-neutral, there is simply no way around impacting global weather patterns and, consequently, still exposing the entire ecosystem to potential collapse.

What is the 2040 vision for the idea? How would it work 20 years from now?

Making once inhospitable land productive and livable while putting in place a distributed climate control system will allow us to engineer the next phase beyond this climate crisis. We, also, have the means to provide power-water-nutrition around the globe. For humanity to continue to accelerate and thrive we need to optimize our supply chains for sustainability as well as global accessibility.

The lessons from developing this idea would have far reaching consequences, from enabling cheaper food production to giving us insight on how to terraform other planets, while deploying the newest technologies to support its operations.

2018 2040

References

Lucas J. Stal 1981 . BMR Journal of Australian Geology & Geophysics, 6, 1981, 307-317 Geobiological role of cyanobacterial mats in sedimentary environments: production and preservation of organic matter John Bauld _____Cyanobacterial Mats and Stromatolites.