CDR, or Carbon Dioxide Removal, the technology that removes carbon dioxide from the atmosphere once it has been produced and released into the air, is part of a larger group of interventions known as "climate engineering" (or geo-engineering). In order to discuss CDR and the logic of how it works, we must learn more about the earth's climate system and its determinant variables. The prime mover of the earth's climate is the sun. Every day, the earth receives from the sun an enormous amount of energy in the form of light (long wave radiation): part of this energy is directly reflected by the atmosphere (phenomenon called atmospheric albedo) or by the earth's surface (earth surface albedo) and returns into space. Another part, however, reaches and heats the soil, water and air and is then re-emitted in the form of heat (short wave radiation). Certain gases (called greenhouse gases) present in the atmosphere retain part of this reflected heat (greenhouse effect), warming the earth. Basically, only a part of all the heat present on earth (i.e. that received during the year and that accumulated in previous years) manages to reach space outside the atmosphere and is dispersed.
Earth’s energy budget
Over the period of one year, the balance between the energy that enters the earth's system (in the form of sunlight) and the energy that leaves it (in the form of reflected heat) is called the earth's annual energy budget: if it is positive, there is more incoming than outgoing energy and the earth warms up; if it is negative, the opposite occurs and the earth cools down; if it is zero (i.e. balanced), the temperature is stable. There are many natural causes that can alter this complex balance and the earth’s temperature over the millennia, with the recent addition of humans with their greenhouse gas emissions. In turn, the average temperature of the earth is fundamental for determining the earth's climate: in fact, variation in temperature alters the complex and continuous exchange of energy and matter between air, water and soil, in the end changing permanently (and not occasionally) the average value of the numerous climate indicators (rainfall, windiness, humidity, intensity and number of extreme events, heat waves, temperature itself and many others). Obviously, if in the long run the earth's energy budget is zero (i.e. balanced), the earth's temperature stabilizes and the climate too remains stable over time (always remember that we are talking about the climate, i.e. average values measured over many decades, and not the weather conditions, which instead can vary from day to day and from season to season).
The influence of the greenhouse effect
There are many variables that can cause the earth's energy budget to change. We have seen the three most important: atmospheric and earth surface albedo and the greenhouse effect (determined by the concentration of greenhouse gases in the atmosphere). Over recent decades, the concentration of greenhouse gases has progressively increased due to emissions generated by human activities. These emissions have intensified the greenhouse effect as compared to that of the pre-industrial era and, consequently, the earth’s energy balance has risen above zero and the temperature has increased. To re-balance the budget and counteract the global warming in progress, we can both take action on the main cause (reducing and eliminating human emissions) and also adopt measures to alter the characteristics of the various elements that determine this balance, causing the earth's energy budget to move in the opposite direction (i.e. negative) to that triggered by the anthropogenic emissions of greenhouse gases (positive). This type of intervention falls within the field of climate engineering and is divided into two main categories: the removal of carbon dioxide present in the atmosphere (CDR) and the futuristic (and somewhat controversial) Solar Radiation Management (SRM). As the name implies, the objective of CDR is to remove from the atmosphere - using natural or artificial techniques - part of the carbon dioxide contained and emitted, over the years, by human activities. This reduces the concentration of carbon dioxide in the air and, consequently, the intensity of the greenhouse effect, which helps limit global warming.
How do we remove carbon dioxide from the atmosphere
There are numerous techniques classified under the name CDR: these range from aﬀorestation and reforestation (which use the natural ability of plants to capture and store CO2), to interventions that exploit the ability of the oceans or soil to fix atmospheric carbon dioxide by transforming it into carbon compounds, and even sophisticated man-made “machines” that filter enormous quantities of air and are able to separate and capture the CO2 to then store or use it. The flow of carbon dioxide removed from the air thanks to these techniques is often described as “negative emissions” precisely because, instead of introducing new gas into the atmosphere, they remove it. The great advantage of interventions that generate negative emissions is that they remove carbon dioxide emitted in the past or offset existing emissions that are not easily eliminated. Therefore, CDR is an instrument that offers a greater degree of flexibility in the energy transition process and, aside from this, is an effective contribution to the decarbonization objective. This is why it is beginning to attract considerable attention, even though, as with all the other instruments, it has its advantages and disadvantages.
Read more about decarbonization
Selected content on this topic.