Climate engineering methods: Can global warmingbe slowed down by deliberately intervening in the climate system?
Various methods are proposed to prevent further global warming. They essentially follow two basic strategies: removing CO2 from the atmosphere or reducing solar radiation. None of the methods discussed has yet reached maturity. Neither their potential nor any risks involved can yet be accurately estimated. For some methods, however, it is already foreseeable that they could have serious side effects.
The term ‘climate engineering’ (CE) has been in use for some time to denote large-scale technical means of intervening in the climate in order to slow down human-made climate change. It covers two fundamentally different strategies:
1. The first category, referred to as carbon dioxide removal (CDR), changes the Earth system by influencing the carbon cycle. Methods discussed here aim to remove CO2 from the atmosphere and store it long term. The aim is to remove the main cause of global warming from the atmosphere: the increased CO2 concentration caused by emissions from fossil fuels.
2. The other group of methods discussed intervene in the global radiation budget. Their aim is for less radiation to reach the Earth’s surface or for more radiation to be released into space. This category of methods is referred to as radiation management (RM). They aim to reduce global warming even though the greenhouse gases remain in the atmosphere – including very long-lived CO2.
Scientists around the world have been working on various methods of carbon dioxide removal (CDR) and radiation management (RM) for over 20 years. A number of methods have already been tested in the laboratory or in small field experiments, while many approaches remain theoretical for the time being. Conclusions regarding their effectiveness are so far mostly based on findings from modelling. It is unclear to what extent their use would be feasible in practice and if they would suffice to limit global warming to 1.5 °C or below 2 °C, even in combination with a drastic reduction in emissions.
There are currently no CDR or RM methods capable of being deployed on a large scale. Not enough is known about their respective potential and side effects. Also, they are not yet technically mature, or there is a lack of strategies for their widespread application. In many cases, field experiments and deployment come up against scientific, legal, ethical or political reservations.
What timeframe and spatial scale are we talking about?
For CDR and RM methods to significantly affect the planetary radiation balance and the CO2 content of the atmosphere, they would have to be applied on a very large scale and in some cases for a very long time. However, CDR and RM methods differ here in a fundamental respect:
→ RM methods do not have a permanent effect in principle. Instead, they only go on working for as long as the deliberate intervention in the radiation budget continues. As they do not combat the cause of global warming by removing CO2, RM methods would have to be kept up until the long-lived CO2 is removed from the atmosphere by natural means or by accompanying CDR measures. The main natural sink is the ocean. This currently absorbs about 20 to 25 percent of the CO2 emitted today. Uptake of CO2 by the ocean is very slow, however, taking centuries to millennia. Accordingly, RM methods would have to be kept going, and keep being financed, for many generations and be paralleled by a successful transformation to a carbon-neutral society, possibly with the aid of CDR. This would presuppose a stable world order over many decades or centuries so the international community could pull together and cooperate in radiation management.
→ If CDR could be established on a large scale, it might be possible – in combination with massive emission reductions – to maintain the atmospheric CO2 concentration at today’s level or even to reduce it below that level. An important distinction is made in CDR, however, between what is referred to as permanent and temporary storage. An example of temporary storage is trees. As they grow, trees remove CO2 from the atmosphere and store it in their wood. Beech trees, for example, can live longer than 400 years and are able to remove the greenhouse gas from the atmosphere for a correspondingly long period. If their timber is subsequently used for construction, the CO2 remains captured for a long time in buildings. Biochar could possibly store CO2 for several thousand years. Another key store of CO2 is the ocean. CO2 captured in the ocean, for example by algae sinking to the sea floor, returns to the surface and re-enters the atmosphere after about 1,000 years. If the CO2 dissolved in seawater is neutralised with alkaline substances as proposed in one CDR method – enhanced weathering of minerals – then the greenhouse gas is permanently removed from the system. One permanent storage method consists of locking up CO2 in rock. Mixed with water, the greenhouse gas is pumped at high pressure into volcanic basalt deposits deep underground. In a natural process, the basalt rock then chemically reacts with the CO2 to form carbonate minerals similar to limestone, in which the gas is permanently captured
Should society decide to supplement emission reductions with climate engineering, carbon dioxide removal methods would make more sense in the long term with regard to the climate targets than would radiation management methods. This is because they directly combat the cause by removing human-emitted CO2 from the atmosphere. Carbon dioxide removal also counteracts ocean acidification. CDR methods are not suitable as a quick fix for the Earth’s climate processes, however. Due to the quantities of carbon they would have to remove, it is estimated that they would need five to 15 years to have any effect on the climate. So CDR measures, too, would have to be applied early and for relatively long periods to have an impact. Many methods also require monitoring to ensure that CO2 storage is permanent.
A further key factor for all methods is scale. Only if applied on a large scale would their impact on the planetary radiation balance or atmospheric CO2 content be relevant to the problem of global warming. One CDR method under discussion, for example, is afforestation. Capturing significant quantities of CO2, however, would involve afforestation of huge areas. Yet the potential global area of land suitable for afforestation is limited. The same goes for the large-scale cultivation of biomass plantations. The land taken up is then also lost to other uses, including food production. Biomass-based CDR methods thus have a direct bearing on food security issues.
One method that could in principle be used without restriction, on the other hand, is what is called direct air capture. This involves the use of machines to remove CO2 from the air. The CO2 captured from the air could then be reused by the chemical industry (carbon capture and usage, or CCU). To remove it from the atmosphere for a long time, however, the CO2 would have to undergo energy-intensive processes in order to convert it into long-lived products. The potential of CCU is not estimated to be very large relative to the quantities of CO2 needed to attain the climate targets. An alternative would be to store the captured CO2 in deep rock strata, such as in empty, exhausted natural gas or petroleum deposits. Experts refer to this as carbon capture and storage (CCS: read here). Geologists estimate that subterranean formations are available worldwide with sufficient volume to take up all anthropogenic CO2 emissions for the long term and thus remove them from the atmosphere.
The different methods
The various CDR and RM methods vary significantly with regard to their potential benefits and potential risks. For CDR methods, there is also the disputed and difficult question of whether they should properly be considered a form of mitigation or if they indeed count as climate engineering. It is therefore important to look at each method individually.
The following pages present carbon dioxide removal (CDR) methods – subdivided into terrestrial and oceanic -applications followed by a detailed description of radiation -management (RM) methods. Estimates are given for the potential and side effects of all methods, although it may not always be necessary to use the full potential in order to attain the Paris climate targets. According to the Intergovernmental Panel on Climate Change (IPCC) assessment report, if rapid progress in drastically cutting emissions were achieved, we then would require approximately 10 – 20 billion tonnes of CO2 to be removed from the atmosphere each year towards the end of the century.