Debates on Carbon Dioxide Removal (CDR) approaches

Debates related to CDR

Many scholars project that although it may not show rapid consequences compared to other approaches like solar geoengineering, with intensive efforts of CDR implementation over the long-term (e.g. several decades) it is expected to significantly reduce the level of atmospheric CO2 concentrations (Caldeira et al., 2013). Also, the scale of carbon removal capacity is huge - if implemented successfully - as shown in the geological reservoirs and oceans; geological reservoirs have the capacity to store several thousand Pg C (Metz et al., 2005) and oceans also expected to store few thousand Pg C in dissolved inorganic carbon forms for centuries (Caldeira et al., 2005 in Metz et al., 2005). 

Concerns on CDR 
Despite impacts of carbon removal from atmosphere in extensive scale is expected from CDR implementation, there are serious concerns raised and is ongoing debate in the academic discourses: 

     1. Cost efficiency of CDR is still questionable - it will be very costly if CDR is implemented in grand scale (i.e. national level) while the effects on the Earth's system are expected to come relatively slow in time. It is highly likely that there will be no or little appreciable influence on global climate for decades after the implementation, so the impacts such as rapid reduction of global surface temperature will not be shown directly. Thus, it is still on debate whether CDR implementation will be 'cost-effective'.

     2. Concerns over physical leakage of carbon from the storage (e.g. geological sinks) are raised as well. Among various CDR methods, carbon capture and storage considered to uptake most of the atmospheric carbon (in expected quantity) and issues on  'permanence' of such carbon storages remain questionable. Some studies suggest that the temporary carbon storage may be equivalent to a delayed release of carbon back in the earth's system (Herzog et al., 2013). Particularly, carbon storage in land (e.g. terrestrial biosphere) has a possibility of leakage since future land-use change or other unexpected factors like fire may lead to release of stored carbon back to atmosphere (Caldeira et al., 2013).

     3. Some scientists postulates that the impact of anthropogenic CO2 emissions won't be solved even though CDR is implemented in scale, since the anthropogenic CO2 stored in the ocean and land are in great amount and they also need to be removed as well when it outgasses back to the atmosphere in near future (Cao and Caldeira, 2010). 
     For instance, Cao and Caldeira (2010) modelled a climate-carbon system response to a 'instantaneous removal of all anthropogenic CO2 from the atmosphere', using an Earth system model. The results revealed that the one-time removal of 100% excess atmospheric CO2 offsets less than 50% of the warming (in temperature) experiencing at the removal time (Cao and Caldeira, 2010). This implies that both anthropogenic CO2 in the atmosphere and carbon stored in ocean and land should be removed as well to achieve appreciable decrease in temperature and CO2 concentrations. 
     Additionally, a study by Kirschbaum (2006) illustrates that despite the short-term reduction in atmospheric CO2 concentrations may be attained by carbon capture and storage, this may lead to other effects that could lower the concentration gradient between the atmosphere and carbon reservoirs including the oceans; the rate of CO2 removal from the atmosphere can be reduced as a result of this. Hence, if carbons are released in the future from such temporary carbon reservoirs/storages, the subsequent atmospheric CO2 concentrations can be higher than 'without temporary carbon storage' (Kirschbaum, 2006). 


Apart from the points illustrated above, there are also arguments on environmental and health problems related to specific CDR approaches, such as enhancing weathering, as shown the diagram below:

A diagram briefly explaining about the advantages and concerns on enhanced weathering as one of the CDR method (Source: Mooney, 2016 in The Washington Post). 

Among various arguments worrying about the CDR implementation, the issue of 'permanence' in carbon storage appealed very crucial to me. Assume that the international community implement a grand-scale carbon capture and storage project and pushed into the deep geological reservoir in near future. What would happen if such storage starts to leak back to the atmosphere? how can we be sure whether the leakage will happen or not in the future? do we have the appropriate technology to stop the leakage or turn back to the state before the carbon storage implementation? Lots of questions to be asked and remain unresolved..