This essay was submitted to Open Philanthropy's Cause Exploration Prizes contest.

Currently we continue to add  35-40 billion tons of c02 to the atmosphere annually.   To stabilise our climate before tipping points create catastrophic damage to the biosphere we must rapidly reduce fossil fuel consumption globally and remove hundreds of billions of tons of co2 already emitted. 

While one critical step involves reducing fossil fuel consumption which will come mostly from policies such as switching to revenue neutral carbon taxation in place of income tax, stabilising the climate will also include removing hundreds of billions of tons of previously released carbo.  To achieve this we must develop an ultra cheap Direct Air Capture (DAC) device that can be scaled rapidly and improved with ease.  With innovations of this scale, a cellular approach (that is a series of small, interlinked machines) is preferable to large scale high investment cost designs because we expect that like solar panels, the technology will improve rapidly. 

Several DAC technologies are being developed. Most commercial techniques use liquid solvents to absorb CO2.   For example, sodium hydroxide reacts with CO2 and precipitates a stable sodium carbonate. This carbonate can be heated to produce a highly pure gaseous CO2 stream. Other chemical processes that are being explored include causticization with alkali and alkali-earth hydroxides, carbonation, and organic−inorganic hybrid sorbents consisting of amines supported in porous adsorbents.  All of these electro-chemical  techniques are experimental and where commercialised currently cost in the region of $1000 per ton captured, primarily due to high energy costs. If the energy used to capture the c02 is not renewable, it may be that the processing of capturing the c02 creates more co2 than it captures. 

Recently a new form of co2 capture has been discovered. Originally proposed by Dr. Klaus Lackner (the father of several DAC technologies) Moisture-Swing capture is based not on an electro-chemical process but  a wet/dry cycle.  This uses an Anionic Exchange Polymer which absorbs c02 when dry and releases it when wet. This moisture swing effect means that the total amount of energy needed to capture c02 is dramatically reduced, and may even be negligible if passive solar techniques are incorporated into the machine.  If Anionic Exchange Polymers can be developed which can go through many moisture swing cycles, the cost of capturing one ton of c02 could be as low as $20-30, making the whole process affordable.

There are a small number of companies exploring the technology, such as Carbon Collect https://mechanicaltrees.com/.  The Open-Air Collective (of which I am a member) is also active in this field. Because of moisture-swings promise as an ultra-low cost carbon capture technology, and because the basic science behind it has been replicated, Moisture-Swing Direct Air Capture (MSDAC) represents the most promising form of DAC being developed today and should be considered as a technology for a moonshot program. 

Gauging the technology readiness level (TRL) of moisture swing DAC is complex; the basic technology itself in the form of proof of concept is advanced, but there is much R&D work remaining to be done. We need more proof of concept research on the anionic exchange polymers and how to produce them economically, as well as iterating the design of the housing to reduce cost and incorporate passive solar energy.