Seeing The Big Picture

CE methods have to be applied on a huge scale if they are to change the climate. This means they could, inadvertently, conflict with other human development goals. That applies equally to carbon dioxide removal (CDR) and radiation management (RM) methods. Scientists are already able to outline the potential areas of conflict: Every hectare that is reforested or turned into biomass plantation for climate purposes may be a hectare too few in the long term when it comes to feeding the steadily growing world population. Futuristic visions such as afforestation of the Sahara fail at obstacles such as the lack of water. Enhanced weathering of large quantities of minerals could harm biodiversity in rivers and coastal waters.

As with climate change itself, the side effects of CDR and RM methods are unevenly distributed. They create winners and losers by favouring or disadvantaging specific population groups or countries. To prevent political tensions, cross-border side effects must therefore be taken into account from the outset and proposals made for solutions or compensatory measures. Also, from the moment people start deliberately influencing the climate, every hurricane, every drought and every flood could potentially lead to political upheaval because it will be hard to say exactly who or what caused the weather phenomenon

One thing that is certain is that RM methods cannot restore the climate to its former state as we know it today. This is because CO2 and reflective particles added to the atmosphere act very differently. CO2 influences heat radiation from the Earth. As heat radiates quite evenly from around the planet, this effect is relatively similar globally. Reflecting particles added to the atmosphere, on the other hand, influence incident solar radiation, and this varies considerably both from place to place and over time. It is at its strongest in the tropics during the daytime. In regions subject to polar winter, on the other hand, the particles have no effect on the radiation balance. It would also have a very diverse effect on rain and the distribution of precipitation. All in all, the advantages and disadvantages of RM are likely to be spread unevenly across the world’s regions, leading to major political tensions and demands for financial compensation. Radiation management methods also harbour the risk of what is called termination shock. This refers to the fact that while global warming could be limited for a time by means such as spreading sulphur particles in the upper atmosphere, the cooling effect would be lost again if particle spreading ceased and the particles fell back to the surface. The temperature would then rise at a far faster pace than it would have without any intervention in the climate. In the event of such abrupt warming, many animal and plant species would presumably have great difficulty adapting to the new conditions.

Generally speaking, it is currently nearly impossible to predict the consequences of RM methods, and also of many CDR methods if applied on a large scale. However, the limits of climate engineering are not only of a natural or technological, but also of a political character. The governance and control of climate engineering is an unresolved challenge.

Plenty of methods, but which to choose?

As the list of the various CDR and RM methods shows, there is a wealth of ideas that could potentially be used in the future alongside efforts to reduce emissions. Some, such as afforestation or the production of biochar, are already technologically mature, while others are no more than ideas. One thing that is certain is that rather than relying on one single method, there has to be a mix of methods selected according to location and the state of the climate. It is also important to remember that many CDR methods also directly affect the radiation budget by modifying the albedo and the water cycle. Similarly, many RM methods also cause changes in the carbon cycle, notably via temperature effects acting on biological processes. Thus, it would not always be possible to tease apart the effects, and side effects, of CDR and RM methods used at the same time.

Deploying a portfolio of CE methods could also lead to interactions between their respective effects. This would basically resemble the situation of a patient fighting an illness with a cocktail of drugs where it is not known precisely how they will affect the individual patient or how they interact with each other. Not enough research has yet been done into the scale of interactions and how to make the most of them. It may be difficult to split out the effectiveness and side effects of individual methods and regulate their use accordingly. International agreement on their deployment and a coordinated approach would therefore be essential requirements.

Whether CDR or RM methods are deployed alongside emission reduction efforts will ultimately depend on policy decisions and social values. This is because the use of climate engineering is not only a question of technical feasibility. On the contrary, it also touches on fundamental social questions such as intergenerational equity. ◆


  • CDR methods help to combat the cause of global warming by removing CO2 from the Earth’s atmosphere. RM methods instead merely combat the symptom of global warming.
  • CDR and RM methods are only climatically significant if used on large scales. At such scales, however, all these methods harbour certain risks.
  • None of the CDR or RM methods is mature enough to be deployed on a mass scale. Many methods are so far no more than theoretical proposals that at best have been tested in small field experiments.