The discussion about deliberate intervention in the climate system has produced a wealth of new terms over the years. This makes the subject hard to approach. For about a decade, the term climate engineering has been used to denote methods of deliberately intervening in the climate system on a large scale to reduce the consequences of human-caused climate change. Terms used synonymously with climate engineering include geoengineering and occasionally climate intervention or climate remediation. All of these terms traditionally cover both carbon dioxide removal (CDR) and radiation management (RM) methods.
RM itself is subject to similar terminological variety. Frequent alternatives include solar radiation management (SRM) and albedo modification. As these do not cover the idea of intervening with the long-wave part of the radiation budget by reducing cirrus clouds, we have opted for RM.
Methods discussed under RM include altering the radiation budget by spreading aerosols in the stratosphere. Examples of methods proposed under CDR are increasing the CO2 uptake of the oceans or afforesting entire regions. ‘Large scale’ means that the methods significantly affect the planetary radiation balance or the CO2 content of the atmosphere. Painting a few houses or roofs white or planting a few trees therefore does not count as climate engineering because the global impact is negligible. Reforestation of large areas of land in order to have a noticeable effect on the atmospheric CO2 concentration and reduce the impacts of climate change would, on the other hand, be a deliberate large-scale intervention in the climate system. This is because turning entire regions into forest plantations not only affects ecosystems and biodiversity but also, for example, the water cycle and the reflectivity of the Earth – and hence the climate system.
RM and CDR basically work in fundamentally different ways. RM acts on the Earth’s radiation budget to reduce warming without removing CO2 from the atmosphere. CDR acts instead on the Earth’s carbon cycle to take CO2 out of the atmosphere, meaning that it tackles the main cause of human-induced global warming.
A further term has come into common use for CDR methods in connection with the 2015 Paris Climate Agreement: negative emission technologies (NETs), where negative emissions mean the removal of CO2 from the Earth’s atmosphere. Occasionally, the term greenhouse gas removal (GGR) is used instead of CDR or NETs in order to include other greenhouse gases besides CO2. RM methods are not negative emission technologies in principle, even where they have an impact on greenhouse gas concentrations as a side effect.
In the current debate, there are increasing voices that CDR and NETs should no longer be generally counted as climate engineering. This is because CDR helps reduce the CO2 concentration and thus addresses the cause in the same way as the afforestation already established at a small scale as a mitigation measure. What matters here is the definition of mitigation. The IPCC defines it as “a human intervention to reduce emissions or enhance the sinks of greenhouse gases.” Removing CO2 from the atmosphere by increasing terrestrial or marine CO2 uptake comes under sink enhancement. Because of this, based on the IPCC definition, such CDR methods are frequently considered part of mitigation. As climate engineering is highly controversial and viewed critically in science and society, taking most CDR methods out from under the climate engineering umbrella and reclassifying them under mitigation could aid their social acceptance and political implementation.
In this publication, the term climate engineering, which combines CDR and RM, is nevertheless retained as a generic term in instances where attributes (such as large scale and deliberate intervention) and principles are referred to that apply to both categories. This is in line with the terminology in common use. When it comes to actually assessing the opportunities and risks of individual CE options, the less meaningful generic term is frequently irrelevant and unhelpful. In that context, we therefore uphold the distinction between radiation management and carbon dioxide removal. Since there are very different interpretations of the term mitigation, we do not use that term at all and speak instead of emission avoidance when we mean preventing greenhouse gas emissions and of carbon dioxide removal when we mean creating sinks. ◆