During COP21, the international community set a long-term target : “Achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century” (Article 4). The formulation of this target comes in response to an apparently insurmountable challenge : we must rapidly reduce our emissions, but we will not be able to completely eliminate them. In addition, as there will soon be 10 billion people on the planet, even a very low level of emission per capita (for example, comparable to the current average level for Sub-Saharan African countries) would be enough to cause global warming above 2°C. The solution suggested in the Paris Agreement is simple : we need to reduce our greenhouse gas emissions to a minimum, then learn to remove from the atmosphere those which we have not managed to eliminate – consequently, in a manner of speaking, to produce “negative emissions”.
Ecology working for the climate
We would need to remove several billion tons of CO2 from the atmosphere every year in order to make a significant contribution to stabilizing the climate. Unfortunately, if we are beginning to know how to isolate the carbon contained in the smoke of emitting facilities, we are still a long way from being able to capture it in large quantities in ambient air where it is approximately 500 times less concentrated. Consequently, we currently do not know how to produce negative emissions, but a wide variety of methods are proposed.
A significant number of these methods are based on photosynthesis : plants are able to transform the carbon dioxide in the atmosphere into biomass, then we just have to ensure that the latter is withdrawn from the natural carbon cycle. These “ecological” methods include, for example :
- Reforestation or afforestation projects, possibly combined with sustainable timber use, in construction, for example ;
- Ecosystem restoration, in particular for coastal ecosystems (salt marshes, mangroves, seagrasses…), which are major carbon sinks ;
- The widespread implementation of agricultural practices which sequester carbon and generally increase yields (also known as “carbon farming”). This is, for example, the case with the use of biochar, e. coal produced from plant residues to improve soil quality. We can also mention the French “4 per 1000” initiative which aims to develop and promote these practices.
Technology to the rescue ?
Another family of methods uses industrial processes to replace or complete photosynthesis. In this case, we refer to technological methods which include :
- Direct air capture which exploits a chemical reaction able to isolate carbon dioxide in the atmosphere, with quicklime or caustic potash, for example ;
- Biomass + carbon capture : plants which have absorbed carbon dioxide during their growth are burnt to produce energy and the carbon released is captured in the effluents ;
- The production of synthetic fuels, here again combined with carbon capture. Methanation is a variant of this method which involves producing synthetic natural gas using carbon dioxide. It is an important element in a number of energy transition scenarios (for example, the Negawatt scenario), in addition to the large-scale deployment of intermittent renewable energies for which it would absorb the production peaks.
There is an additional problem with these methods : What can be done with the captured carbon dioxide ? An idea frequently put forward would be to sequester it in stable and impermeable geological formations. There are sometimes other proposals with a questionable climate balance (enhanced hydrocarbon recovery, hydrocarbon or plastic production, use as a solvent…).
Finally, certain methods are in a separate category, because they would involve the large-scale alteration of the planet’s ecological or chemical balances and are consequently associated with geo-engineering. This includes ocean seeding or cloud basification.
Issue of valuation
Most of these proposals are still at the development stage and we can assume that they will be the subject of intense research in the coming decades. At the same time, it will be up to the States which are parties to the Paris Agreement to determine which methods could be used and how the negative emissions will be accounted for and valued. These questions will probably be major issues during the next climate conferences.
In any event, it is likely that the ecological methods will be the first to reach maturity and be used commercially. This is especially because the activities they employ are not new : they already find their place in a number of development, environmental protection and food security projects. These projects could consequently benefit from new opportunities with the Paris Agreement.
This article is based on an analysis of the economic and technological implications of the Paris Agreement.
The opinions expressed on this blog are those of the authors and do not necessarily reflect the official position of their institutions or of AFD.