What’s CDR?

Through the Paris Agreement, countries have together set quantifiable goals to reduce (or “mitigate”) climate change. The principal goal is defined in terms of temperature:

Holding the increase in the global average temperature to well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels.

In support of this long-term temperature goal, the Paris Agreement further sets out a goal for emissions and removals:

to achieve a balance between anthropogenic [i.e. human-caused] emissions by sources and removals by sinks of greenhouse gases in the second half of this century, on the basis of equity, and in the context of sustainable development and efforts to eradicate poverty.

The definition of CDR used by the Intergovernmental Panel on Climate Change (IPCC):

Human activities capturing CO2 from the atmosphere and storing it durably in geological, land or ocean reservoirs, or in products. This includes human enhancement of natural removal processes, but excludes natural uptake not caused directly by human activities.

Principle 1: The CO2 captured must come from the atmosphere, not from fossil sources. The removal activity may capture atmospheric CO2 directly or indirectly, for instance via biomass or seawater.

Principle 2: The subsequent storage must be durable, such that CO2 is not soon reintroduced to the atmosphere.

Principle 3: The removal must be a result of human intervention, additional to Earth’s natural processes.

Routes through the carbon cycle: CDR methods encompass a range of capture processes and storage pools. Processes which carry out the initial capture from the atmosphere are often referred to as sinks. Between capture and ultimate storage, carbon may be converted and transferred through a number of carbon pools. Some methods involve multiple steps, while others combine capture and storage in a single step.

Capture processes:  There are three capture process.

1) Biological capture. Through the process of photosynthesis, CO2 is taken up from the atmosphere by trees, crops and aquatic biomass such as kelp and seagrasses.

2) Geochemical capture. A range of minerals can bind atmospheric CO2, as can alkaline waste materials from construction and industry. The CO2 is bound in the form of solid carbonate (which can be used as a product, such as aggregates) or dissolved bicarbonate, both of which are durable carbon pools.

3) Chemical capture. CO2 can be captured directly from air using chemical solvents and sorbents designed to re-release it as a concentrated CO2 stream for use or storage.

Storage processes: There are three storage process.

1) Biological storage (on land and in oceans). While annual plants do not retain carbon durably, trees can retain their carbon for decades, centuries or more. Soils and wetlands are a further store of carbon, derived from compounds exuded by roots and dead plant matter. In the oceans, aquatic biomass may sink to the ocean floor and become marine sediment. Carbon can be retained durably in these ecosystems, especially if managed carefully to reduce disturbances.

2) Product storage. Many carbon-based products do not constitute durable storage. However, construction materials and biochar (a carbon-rich material produced by heating biomass in an oxygen-limited environment) can store carbon for decades or more. These carbon-based products can be made from conversion of harvested biomass (in the cases of biochar and wood in construction), from concentrated CO2 streams or even from CO2 from ambient air (in the case of aggregates).

3) Geochemical storage. Concentrated CO2 can be stored in geological formations, using depleted oil and gas fields or saline aquifers, or reactive minerals such as basalt. Geochemical capture leads directly to long-term storage of CO2 in the form of carbonate minerals or bicarbonate in the ocean.

Durability: Different carbon pools have very different characteristic timescales for storage and risks of reversal, and there is no clearly agreed definition of durability. Well chosen geological and mineral formations offer the longest and least reversible storage. Nevertheless, choosing to include only these methods excludes others widely regarded as valid CDR, such as those that store carbon in trees, biochar and soils.