Urgently sought – ways out of the climate crisis
WOR 8 The Ocean – A Climate Champion? How to Boost Marine Carbon Dioxide Uptake | 2024

Solutions to the greenhouse gas problem?

Lösungen für das Treibhausgas-Problem? fig. 1.18: Perfect Lazybones/Shutterstock

Solutions to the greenhouse gas problem?

> Climate change is man-made and undeniably a consequence of the unchecked emission of greenhouse gases. Stopping emissions is thus the only way out of the climate crisis. There is presently an abundance of suggestions for how human societies can avoid a large portion of their emissions. However, it will certainly not be possible to eliminate all emissions by the year 2050, even if a great effort is made towards that end. Residual amounts will thus have to be compensated for by the deliberate removal of carbon dioxide from the atmosphere.

Humankind alone is responsible for climate change and its consequences

Halting climate change and preventing its drastic consequences is the duty of humans because they alone are responsible for the global warming that has occurred up to now. There is no longer any doubt that climate change is man-made. According to the Intergovernmental Panel on Climate Change, global warming over the past 120 to 170 years can be clearly attributed to human-induced greenhouse gas emissions. The primary contributors include carbon dioxide, methane, nitrous oxide (laughing gas) and chlorofluorocarbons (CFCs), as well as 16 additional chemicals.
The enrichment of these greenhouse gases in the atmosphere is steadily reducing our planet’s ability to radiate heat energy into space. The surplus heat in the Earth’s atmosphere first warms its air masses, then subsequently also the ocean. This process is based on the same physical principle that warms a garden greenhouse. The consequences of increasing atmospheric greenhouse gas concentrations are therefore known as the greenhouse effect. It is important to understand that a significant portion of the total warming triggered by greenhouse gases is not yet being observed by humans or in nature because it is masked by the cooling effects of aerosols like soot particles and sulphur dioxides, as well as by changes in the reflectivity of the Earth’s surface. Without these cooling components the level of global warming would already be at 1.5 degrees Celsius today.
1.14 > Humans are causing climate change. This is clearly evidenced because the measured warming of the Earth (black line) can only be realistically represented in climate models when they combine the natural with all human-influence factors (grey dotted line and shading).
fig. 1.14: after IPCC, 2021, Climate Change 2021: The Physical Science Basis. doi:10.1017/9781009157896, FAQ 3.1 Figure 1
The concentrations of greenhouse gases in the Earth’s atmosphere are being monitored around the world by research institutions such as the US American National Oceanic and Atmospheric Administration (NOAA). Each year NOAA publishes its Annual Greenhouse Gas Index (AGGI). This is a numerical expression of how much additional heat energy has remained in the atmosphere as a result of man-made greenhouse gas emissions compared to the reference year 1990, and is continuing to drive global warming. In 2022 the NOAA Greenhouse Gas Index rose to a value of 1.49. This means that the greenhouse gases released by human activities trapped an astonishing 49 per cent more heat energy in the Earth’s atmosphere in 2022 than they did in the reference year.
The greatest proportion by far of this increasing heat accumulation, around 80 per cent, has been contributed by carbon dioxide (chemical formula: CO2). This greenhouse gas is especially long-lived. It does not break down chemically in the atmosphere, and thus can only be removed through a variety of processes (such as CO2 uptake by plants). For this reason, carbon dioxide can remain in the Earth’s atmosphere for as long as 1000 years and thus has a long-term effect on the climate.
1.15 > Global warming is a result of anthropogenic greenhouse gas emissions. Aerosols released by human activities, mainly sulphur and nitrous oxides, have so far had a cooling effect by reflecting incoming sunlight back into space.
fig. 1.15: after IPCC, 2021, Climate Change 2021: The Physical Science Basis. doi:10.1017/9781009157896, Figure SPM.2

Direct and indirect

Direct emissions are closely related to activities within a clearly defined area, region, sector or company (for example, CO2 emissions by the burning of oil in the heater of a building). Indirect emissions, on the other hand, are produced outside the defined area (heating a building by district heat: Indirect emissions result from combustion in the geographically removed gas or coal power plant).

Carbon dioxide is emitted as a product of almost all human activities. It is primarily produced by:
  • the burning of fossil fuels such as coal, oil and natural gas: According to the Intergovernmental Panel on Climate Change, around 34 per cent of the global carbon dioxide emissions in the year 2019 came from the energy sector, while the traffic and transport sector accounted for 15 per cent and the industrial sector for 24 per cent;
  • the decomposition of organic materials (animal and plant remains) due to land-use changes: Agricultural, forestry and other land-use changes accounted for around 22 per cent of the global carbon dioxide emissions in 2019;
  • industrial processes such as the production of cement: Cement is made of limestone, which is burned at temperatures of 1450 degrees Celsius to achieve the required material properties. During the burning process, carbon dioxide escapes from the primary material in large quantities. The process-related emissions from cement production alone accounted for around 2.6 per cent of the total global carbon dioxide emissions in the year 2019. This amount does not include indirect emissions, which include the energy used in the process and for transport. In Germany, the production of one tonne of cement is responsible for around 600 kilograms of carbon dioxide emissions. Approximately two-thirds of this amount are due to raw-material processing emissions, and one-third to fuel emissions.
1.16 > Overview of the direct and indirect greenhouse gas emissions for the individual sectors in the year 2019. The total emissions are recalculated to carbon dioxide equivalents. The percentage values shown in the sums do not always add up to 100 per cent due to rounding.
fig. 1.16: after IPCC, 2022, Climate Change 2022: Mitigation of Climate Change. doi:10.1017/9781009157926, Figure 2.12.
1.17 > The amount of all relevant anthropogenic greenhouse gases has steadily increased during the period from 1990 to 2019.
fig. 1.17: after IPCC, 2022, Climate Change 2022: Mitigation of Climate Change. doi:10.1017/9781009157926, Figure SPM 1
The annual worldwide carbon dioxide emissions resulting from cement production and the burning of fossil raw materials now add up to around 36 billion tonnes of CO2. Added to this are the emissions from agriculture and forestry as well as changes in land use, at levels of around four billion tonnes of carbon dioxide. On a global scale, these emissions have been increasing for the past 270 years, although their growth has slowed down for the present.

fig. 1.18: Perfect Lazybones/Shutterstock


1.18 > Gravel is produced at a Chinese quarry. Industrial companies like this one are responsible for more than one-third of the world’s greenhouse gas emissions.

Record values – every year

Consistently high emission levels are resulting in a steady rise in carbon dioxide concentrations in the Earth’s atmosphere. For the month of May 2023, the carbon dioxide monitoring station at the Mauna Loa Observatory on the Island of Hawaii observed a record-high monthly value of 424 parts per million (ppm), an increase of 3.0 ppm compared to the value of may 2022, and the highest atmospheric CO2 concentration in the past two million years. Carbon dioxide is definitely the strongest driver of climate change, but it is not the only one. In addition to the long-lasting gas, human societies are also increasingly releasing more short-term climate-impacting pollutants such as methane (CH4), laughing gas (NO2) and fluorinated greenhouse gases. Unlike carbon dioxide, these compounds break down chemically in the atmosphere. As a rule, their effect on climate becomes negligible in less than 20 years. But for as long as they exist in the atmosphere, the short-lived greenhouse gases do contribute significantly to climate change. Methane, for example, over a period of 20 years, retains 80 times more heat in the Earth’s atmosphere than the same amount of carbon dioxide.
In its current report, the Intergovernmental Panel on Climate Change concludes that the increased emissions of methane from 1850 to 2019 were responsible for around 0.5 degrees Celsius of the global warming observed during that time. Converting methane concentrations and their climate impacts to carbon dioxide equivalents reveals that anthropogenic methane emissions accounted for around 18 per cent of total emissions in 2019.
Methane concentrations in the Earth’s atmosphere have been directly measured since 1983. According to NOAA, the average methane concentration in the year 2022 was exactly 1911.8 parts per billion (ppb). In the year 1750, based on climate archives, it was only 729 ppb. This means that the Earth’s atmosphere now contains 162 per cent more methane than it did at that time. Methane concentrations have not been this high in the past 800,000 years.
1.19 and 1.20 > Various regions of the world contribute to greenhouse gas emissions to greatly different degrees – both at present and in retrospect, whereby all emissions are ­added cumulatively.
fig. 1.19 and 1.20: after IPCC, 2022, Climate Change 2022: Mitigation of Climate Change. doi:10.1017/9781009157926, Figure SPM 2


fig. 1.19 and 1.20: after IPCC, 2022, Climate Change 2022: Mitigation of Climate Change. doi:10.1017/9781009157926, Figure SPM 2
Methane is released, on the one hand, through natural sources such as swamps, mangrove forests, salt marshes and seagrass meadows. But it is also released by human activities, particularly:
  • in agriculture: digestive processes of ruminants, rice cultivation, and manure, slurry and digestate management;
  • in the energy sector: coal production, oil and natural gas production and transport, burning of biomass and biofuels; as well as
  • in solid waste and wastewater management: releases from landfills, wastewaters and sewage sludges.
These anthropogenic methane emissions can be reduced with relatively little effort. Furthermore, because atmospheric methane breaks down chemically within a time frame of about nine to twelve years, thus losing its impact on climate, strategies to reduce the release of methane are seen as especially promising measures in the struggle against climate change. Recent research indicates, for example, that by the year 2050 around 0.25 degrees Celsius of additional warming could be prevented through the immediate implementation of all the presently known options for curbing man-made methane emissions.

Carbon dioxide equivalent
In order to compare the impacts of the different greenhouse gases, researchers calculate how much carbon dioxide would be required, within a certain time frame, to produce the same effect on a particular climate parameter with a given amount of methane, laughing gas, or a mix of other greenhouse gases. This calculated amount of CO2 is referred to as the carbon dioxide equivalent.

When will global warming exceed the 1.5-degree mark?

Every additional tonne of released greenhouse gases continues to advance the progress of global warming. This near-linear relationship has been well documented by ­science, at least for carbon dioxide. It is now known that 1000 billion tonnes (one thousand gigatonnes) of carbon dioxide emissions cause the global surface temperature to rise an additional 0.27 to 0.63 degrees Celsius – and this occurs every time that the atmosphere is newly enriched by this amount of carbon dioxide.
But a much more common question in the climate change debate is when a particular warming level will be reached. The 2015 Paris Climate Agreement, for example, sets a target of limiting global warming to well below two degrees Celsius, and if possible to 1.5 degrees Celsius compared to preindustrial levels. A difficulty with this, however, is that the Agreement explains neither how the specific warming levels are defined, nor exactly what time period is meant by the term “preindustrial”.
Climate researchers, therefore, have agreed on a common baseline. The warming level is defined with respect to the time period from 1850 to 1900 – although with full awareness that industrialization had actually begun 100 years earlier and that carbon dioxide emissions had already risen rapidly, especially in Great Britain. Data of acceptable quality on the global surface temperatures of the Earth, however, only extend back to the year 1850. Researchers have therefore selected the period of 1850 to 1900 for comparison purposes.
1.21 > Farmers in the province of Sindh, Pakistan, herd their goats over flooded terrain. Heavy rains and flash floods in July and August 2022 inundated large areas of Pakistan, causing severe damage in half of its provinces.
fig. 1.21: picture alliance/Xinhua News Agency/Stringer
1.22 > Climate change could be mitigated effectively if human societies were able to reduce their methane emissions. The steps needed to do this are well known. But the solutions would have to be implemented comprehensively.
tab. 1.22: after Ilissa B. Ocko et al., 2021. doi:10.1088/1748-9326/abf9c8
fig. 1.23: Central Press/Getty Images


1.23 > A photograph from the year 1971: Dark plumes of smoke rise from the four chimneys of the coal-fired Battersea Power Station in London. The power station on the Thames has been shut down since 1983. Where coal was once burned, luxury apartments and offices have been built since 2013.

The answer to the question of when global warming exceeds a certain temperature limit is constrained by calculating warming as an average value over a 20-year period. For climate researchers, this means that the 1.5-degree limit is reached when the average surface temperature over a 20-year period lies 1.5 degrees Celsius above the average value between 1850 and 1900. But what exactly would that value be?
Precisely predicting the trend of temperature change is still difficult because the amount of future warming depends on four factors: the amount of future greenhouse gas emissions, the internal variability of the climate system (i.e., the natural fluctuations), climate sensitivity, and the uncertainties in determining the temperature levels for the reference time period of 1850 to 1900. Researchers use the term “climate sensitivity” to refer to the amount of long-term climate warming that would be triggered by an abrupt doubling of the carbon dioxide concentration in the Earth’s atmosphere. According to current figures from the Intergovernmental Panel on Climate Change, there is a 90 per cent probability that this value would be between two and five degrees Celsius, whereby it would take several decades to centuries for the warming to occur and for the climate system to return to a state of equilibrium after the disturbance (i.e., after the doubling of the carbon dioxide concentration).
1.24 > Because carbon dioxide accumulates in the atmosphere, scientists can calculate the amounts that can still be released before a certain level of warming is reached. In the year 2020 this amount was an additional 400 gigatonnes of carbon dioxide for the world to reach the 1.5-degree target with a probability of 67 per cent.
fig. 1.24: after IPCC, 2021, Climate Change 2021: The Physical Science Basis. doi:10.1017/9781009157896, FAQ 5.4 Figure 1

Paris Climate Agreement
The Paris Climate Agreement was adopted on 12 December 2015 at the 21st Climate Conference in Paris and entered into effect on 4 November 2016. By September 2022, 194 countries and the European Union had signed and ratified the agreement.

Because the range of this value has a span of three degrees Celsius, climate models can produce significantly different results. If scientists use an intermediate sensitivity value in their climate models, calculations based on the five Shared Socioeconomic Pathways indicate that the 20-year average temperature in the time frame from 2020 to 2039 will reach the 1.5-degree limit, totally regardless of what amounts of greenhouse gases humans release in the coming years. If emissions remain at the present high levels or continue to increase, global warming will exceed the limit of two degrees by the year 2050.
The amount of time remaining to curb climate change can be assessed by what scientists call the carbon budget, which is an expression of how much carbon dioxide can still be emitted by human activities before a given level of warming is reached. The calculations for this are based on the assumption that the global surface temperature rises by around 0.45 degrees Celsius (0.23 to 0.65 degrees) when humankind releases an additional 1000 billion tonnes of carbon dioxide into the atmosphere. Other factors that are considered include past warming, the contribution of greenhouse gases other than carbon dioxide to future warming, and the question of how long the warming will continue to progress even if humans manage someday to reduce their carbon dioxide emissions to zero.
During the period from 1750 to 2019, human societies emitted around 2560 billion tonnes of carbon dioxide. Taking all methodological uncertainties into account, the Intergovernmental Panel on Climate Change finds that this amount of greenhouse gas is probably already enough to reach the 1.5-degree mark. According to the experts, this means that the remaining carbon budget would be zero, although the probability of this would be low. However, if the “best estimates” are used for the most important parameters, the calculated carbon budget is greater than zero.
Nonetheless, the results indicate that it is still small:
If humans want to limit global warming to 1.5 degrees Celsius with a probability of 67 per cent, they can only release a total of 400 billion tonnes of carbon dioxide, calculated for the period beginning on 1 January 2020. This corresponds roughly to the amount of carbon dioxide emitted by the international community over the past decade (2010 to 2019). The budget for the two-degree target is 1150 billion tonnes. Based on constant continued emissions at the current level of around 40 billion tonnes per year, the two budgets would be exhausted by the years 2030 or 2050, respectively.
The following statistic also shows how little margin we have remaining: If humankind were to allow all fossil infrastructures already in operation in 2018 – i.e., coal and natural-gas power plants, oil refineries, blast furnaces, combustion engines, etc. – to continue running at the same capacity as they have in the past until the end of their respective lifetimes, an additional 660 billion tonnes of carbon dioxide would be released in the coming decades. If this calculation is expanded to include all of the installations that were planned or under construction in the year 2018, another 187 billion tonnes of carbon dioxide would have to be added to that sum. Limiting global warming to less than two degrees Celsius under these conditions would be in serious jeopardy. A ban on new coal or natural-gas power plants would thus be an important step towards preventing future emissions.

Surplus scenario
A development in which the global surface temperature rises above a defined climate target (for example, 1.5-degree target) for an initial time period of one or more decades, but subsequently falls again below the temperature threshold, is called a surplus scenario. However, the temperature decline can only occur if the greenhouse gas concentration in the atmosphere is really decreased through a process of carbon dioxide removal.

The ultimate goal: greenhouse gas neutrality

Limiting global warming to 1.5 degrees in the coming decades will now hardly be possible – at least not without overshooting the temperature target for a few decades (surplus scenario). Through a huge effort, however, it may be possible to limit global warming to less than two degrees Celsius. To realize this goal would require immediate and wide-ranging reductions of global greenhouse gas emissions, as well as achieving net zero carbon dioxide emissions by the year 2050.
There are ideas for far-reaching emission reductions in every sector. According to the Intergovernmental Panel on Climate Change, it is possible to cut global greenhouse gas emissions in half by 2030 based on known options. More than half of the potential reduction can be realized through measures that would cost less than 20 US dollars per tonne of carbon dioxide that is eliminated, a fact that is especially important for poorer countries. Examples of these include the worldwide expansion of wind-power and photovoltaic systems for generating electricity from renewable sources, an end to deforestation and the draining of wetlands, improved carbon storage capacities in many fields through sustainable and soil-conserving agriculture, a substantial reduction in meat consumption, construction of energy-efficient buildings, the use of alternative fuels in industry, and measures to curb methane emissions.
This may appear to be a perfectly feasible programme. It requires, however, the successful implementation of comprehensive structural and societal changes, as well as restructuring and rethinking at all levels, including new ideas about what people need (and must consume) and do not need to live. Furthermore, cutting the emissions by half would only be the first step.
This would have to be followed by a reduction of greenhouse gas emissions to such an extent that greenhouse gas neutrality is achieved as soon as possible. The term “greenhouse gas neutrality” and the synonymously used term “net-zero greenhouse gas emissions” both describe a world in which humans or individual entities such as states and companies only release as much greenhouse gas as they can remove again from the Earth’s atmosphere. Experts distinguish the terms “carbon neutrality” (net zero carbon dioxide emission) and “greenhouse gas neutrality” (net zero of all greenhouse gas emissions, including carbon dioxide). The reason, in terms of climate physics, is that the global surface temperature could be stabilized if humans would release only as much carbon dioxide as they can remove, while at the same time reducing the release of short-lived air pollutants such as methane and laughing gas by a certain amount. If all greenhouse gas emissions could be reduced to net zero, on the other hand, the global temperature would even begin to fall over the long term. A net zero of carbon dioxide emissions is thus a major, indeed fundamental prerequisite to halting global warming. But with the added help of a net zero of all greenhouse gas emissions it would even be possible to roll back global warming by a small amount.
1.25 > Approaches are now available that would effectively reduce greenhouse gas emissions in all areas of life by the year 2030. This figure from the Intergovernmental Panel on Climate Change lists the most effective measures and shows the costs at which the reductions would be possible. It is important to note that investing in such reductions would cost much less than remedying the consequences of continued climate change.
fig. 1.25: after IPCC, 2022, Climate Change 2022: Mitigation of Climate Change. doi:10.1017/9781009157926, Figure SPM 7

Methods for carbon dioxide removal

The term “carbon dioxide removal” (CDR) is used to discuss and research the methods that can be applied for removing carbon dioxide from the atmosphere. Although ideas for the removal of methane are also beginning to be suggested, scientific assessment of their feasibility is not yet possible due to insufficient research at present.
CDR covers a wide range of processes that can be used to remove carbon dioxide from the atmosphere and then store it permanently. Possible storage sites include the deep geological subsurface, the oceans and sites on land, especially soils and vegetation. A fourth option would be to use the extracted carbon dioxide to make various products from carbon.

Carbon dioxide removal – offsetting residual emissions that are difficult to avoid

Climate researchers assume that the international community, despite its highly ambitious climate policies, will still be emitting several billion tonnes of residual greenhouse gases (carbon dioxide, methane, laughing gas) by the middle of the 21st century. These hard-to-avoid residual emissions will be generated, for example, in the production of cement and steel, in aviation and heavy-duty transport, and in agriculture and waste incineration.
To achieve greenhouse gas neutrality, these residual emissions will have to be compensated for using carbon dioxide removal methods. There are various proposals for solutions that involve either the expansion of natural carbon sinks or technological approaches. Experts assign the numerous CDR methods to four categories:
  • enhancement of the biological carbon dioxide sinks on land, e.g. through reforestation,
  • enhancement of the biological carbon dioxide sinks in the ocean, e.g. through the restoration of damaged or dead mangrove forests and seagrass meadows,
  • geochemical approaches, and
  • chemical methods.

The IPCC’s definition of CDR
The term “carbon dioxide removal methods” applies exclusively to such practices in which the carbon dioxide removed comes from the atmosphere, its subsequent storage is durable, and its removal is an outcome of human action and is thus additional to the natural carbon dioxide removal processes of the Earth system.

It is important to note that only those methods can be counted that result from human efforts to enhance the removal of carbon dioxide from the atmosphere. Trees that naturally establish themselves somewhere, photosynthesize, absorb and sequester carbon dioxide should not be included in the CDR balance. The official CDR definition of the Intergovernmental Panel on Climate Change is so narrow that even approaches in which carbon dioxide from fossil sources is captured at the emission site and subsequently stored underground (Carbon Capture and Storage, CCS) or processed into products (Carbon Capture and Utilization, CCU) may not be considered as CDR. In this case carbon dioxide is not actually removed from the atmosphere, rather its escape into the atmosphere is simply prevented.
Some CDR methods have been carried out for centuries, although not with the explicit purpose of removing carbon dioxide from the atmosphere. These include the reforestation of deforested areas, the sustainable management of existing forests, and the conservation of peat- and wetlands. They also include regenerative types of agriculture that lead to increased humus or carbon content in the soil by removing carbon dioxide and other carbon compounds from the atmosphere and storing them in the soil, mostly in the form of organic material (plant remains, manure, etc.). The best-known practices for enriching soils with carbon include the cultivation of perennial grasses and legumes, improved crop rotation including catch cropping, the application of compost and manure, and reduced soil tillage.

Afforestation and reforestation
The Intergovernmental Panel on Climate Change defines the term “afforestation” as the planting of trees in an area that was not forested in the past. One could therefore also refer to this as “forestation”. “Reforestation”, on the other hand, means the planting of young trees in an area whose former forest cover has been destroyed by clearing, fire or other human activities.

There are other comparatively new CDR methods, however, whose specific purpose is to decrease greenhouse gas concentrations in the atmosphere. These include methods such as the capture of carbon dioxide from the air and its subsequent storage (Direct Air Carbon Capture and Storage, DACCS) or the generation of bioenergy with subsequent carbon dioxide capture and storage (Bioenergy with Carbon Capture and Storage, BECCS). Experience and knowledge of these approaches are growing, but they are still being applied on a comparatively small scale.
Furthermore, CDR methods differ with respect to the length of time that the carbon dioxide is removed from the atmosphere. The possible time frames range from a few decades to millions of years, and depend specifically on the storage site. Carbon dioxide that is absorbed by the ocean or is stored in deep-lying rock layers usually remains there for a longer time than carbon dioxide that is sequestered by forests on the land. Natural storage sites on land are also more susceptible to disruption. Wetlands, for example, can dry out, and forests can burn down. In both cases the carbon dioxide will escape into the atmosphere again. The risk of escape is somewhat lower when trees are felled and used for durable timber elements (e.g. roof beams) or when long-lived products are made from captured carbon dioxide.
Last of all, the various CDR methods differ from one another in the extent to which they can be applied, how much carbon dioxide can be removed from the atmosphere with their help, what possible risks and advantages a method poses, the costs associated with their large-scale application, and whether the necessary technology has even been developed and is ready for implementation. Science is presently searching for the answers to these and many other questions in various research projects.
1.26 > Iceland has achieved some small-scale success in removing carbon dioxide from the atmosphere and sequestering it underground. The process involves dissolving the extracted gas in fresh water and injecting it into the warm volcanic basalt rocks. The components of the rocks react chemically with the carbon dioxide, resulting in its mineralization and conversion to rock material itself.
fig. 1.26: Arnaldur Halldorsson/Bloomberg via Getty Images

Extra Info Climate goals –
progress at a snail’s pace open Extra Info

No substitute for comprehensive emission reductions

Considering the enormous speed at which the Earth’s climate is changing, there is no longer any question as to whether mankind must remove carbon dioxide from the atmosphere in order to limit global warming to a tolerable minimum level for humans and nature. Without a doubt the answer is yes! But the unresolved questions now are how, to what extent, with what goals, and under what basic conditions such removal should and can happen.
It is certain that if humankind is to achieve the Paris climate goal, removing carbon dioxide can never be accepted as a substitute for comprehensively reducing emissions. The Intergovernmental Panel on Climate Change says that the level of greenhouse gas emissions is far too high for that. The use of CDR methods is conceivably a way to compensate for residual emissions that are difficult to eliminate. They can help to reduce the net anthropogenic emissions more quickly in the near future. In the long term, CDR would help humanity to compensate for unavoidable carbon dioxide residual emissions as well as the emissions of other greenhouse gases. In the best case, it would one day be possible to achieve net-negative emissions. This would mean that the amount of carbon dioxide being removed from the atmosphere would exceed the amount of CO2 equivalents being released. As a consequence, the greenhouse gas concentrations in the atmosphere would decrease, which would be followed by a decline in the global surface temperature.
But the first milestone along this path would be to achieve net-zero carbon dioxide emissions. The goal of comprehensive greenhouse gas neutrality would then follow after about ten to 40 years, or maybe even much later depending on the amount of residual greenhouse gas emissions (methane, laughing gas, etc.) that would have to be compensated for by carbon dioxide removal.
For a global net zero of carbon dioxide emissions, not all countries would have to offset their residual emissions. If some countries are able to remove more carbon dioxide than they release into the atmosphere by emissions, there would be a condition of net-negative emissions, or an emission credit. Other countries could then redeem this credit. They would then have more time to reduce their own greenhouse gas emissions without an increase in the overall carbon dioxide concentrations in the Earth’s atmosphere and the accompanying rise in average global temperatures.
1.28 > Processes of carbon dioxide removal from the atmosphere could be employed both on land and in the ocean. This chart shows the different approaches, sorted by type of removal and by subsequent storage medium.
fig. 1.28: after IPCC, 2022, Climate Change 2022: Mitigation of Climate Change. doi:10.1017/9781009157926, Chapter 12, Chapter Box 8, Figure 1

Major concerns and many unanswered questions

So far, only a few countries have adopted CDR methods beyond afforestation and reforestation in their long-term climate strategies. Nevertheless, according to the Inter-governmental Panel on Climate Change, there is concern in many circles that simply the theoretical possibility and feasibility of increased carbon dioxide removal could lull governments and other societal stakeholders into half-hearted attitudes in implementing ambitious greenhouse gas reduction plans, or lure them into placing their trust in technologies that have not yet been sufficiently developed and researched in the fight against climate change.
A further misgiving is that the hope for effective CDR measures could induce decision-makers to fail to rigorously address the challenges associated with drastic greenhouse gas reductions, and instead defer action to the future. This would mean that the next generation would have to deal with the steadily growing problem.
It is also unclear how the costs, risks and burdens of large-scale CDR efforts can be evenly distributed, and how negative effects can be avoided, particularly in the areas of food production, biodiversity and the availability of land.
Furthermore, reliable and globally standardized methods would be needed to measure, verify and assess the carbon dioxide removal and storage achieved through CDR measures. A transparent and functioning market, in which emission credits could be traded and financial resources generated for the implementation of CDR measures, can only be realized when these conditions are met. In the view of the Intergovernmental Panel on Climate Change, there are still many challenges that need to be overcome before CDR methods more sophisticated than reforestation can be implemented on a large scale. These include the many unanswered research questions, the immature state of technological development, high costs, and the fact that the possible implementation of new kinds of CDR methods in the future needs to conform with the overarching development and sustainability goals of the international community. This calls for matching laws and regulations, along with the corresponding decision-making processes, before novel CDR methods can be implemented.
1.29 > The active removal of carbon dioxide from the atmosphere will be necessary to reduce net anthropogenic emissions in the short term, to achieve the goals of carbon-dioxide and greenhouse gas neutrality in the intermediate term, and in the long term to reduce the carbon dioxide concentration in the atmosphere by negative emissions.
fig. 1.29: after IPCC, 2022, Climate Change 2022: Mitigation of Climate Change. doi:10.1017/9781009157926, Chapter 12, Chapter Box 8, Figure 2

How much CDR is needed in the future?

Science is investigating approaches and ideas for the struggle against climate change with the help of Integrated Assessment Models (IAMs). These are being developed in order to understand how particular societal or economic developments affect nature and the climate. To this end, the models are fed with information about the Earth system as well as about society. The models thus consider natural laws as well as the behavioural changes of humans, and they also assess the undesirable side effects or desired advantages of particular measures and decisions. Although the model predictions are always subject to some degree of uncertainty, IAMs do provide valuable insights. They can demonstrate, for example, how our economy, society and energy supply would have to change in order to achieve a given climate goal, or show us what impacts certain emission reductions would have for humans and nature.
Researchers in IPCC Working Group III evaluated thousands of such integrated assessment models for the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. This work has clearly illustrated that all models that project a limit on global warming of two degrees Celsius or less include a robust implementation of methods for carbon dioxide removal at significantly higher levels than any that are being carried out at present.
1.30 > Here, near the Brazilian city of Porto Velho, slash-and-burn clearing of the Amazon rainforest has provided arable land for the cultivation of soya beans. Along with the forests, enormous areas of carbon storage are lost because the trees store carbon in their wood and leaf mass as well as in the forest soils.
fig. 1.30: Victor Moriyama/The New York Times/Redux/laif
The amount of carbon dioxide that will have to be removed from the atmosphere in the future in order to stabilize the climate has not yet been clearly determined. The model results only allow rough estimates. But for land-based biological methods such as afforestation and reforestation, the estimates fall within the order of 900 million tonnes of net carbon dioxide in the year 2030. In this case, net means that the carbon dioxide removal through afforestation and reforestation must be 900 million tonnes greater than the sum of global land-use emissions produced at the same time (such as deforestation in certain regions). Two decades later the net removed amount would have to be almost three billion tonnes of carbon dioxide if global warming is to be held to less than two degrees Celsius over the long term. In addition, similarly large amounts of carbon dioxide would have to be removed through energy generation from biomass and through direct air capture. For both of these methods the captured carbon dioxide would subsequently have to be safely and permanently stored somewhere.
In light of these high estimates, the IPCC has concluded that existing programmes of land-based carbon dioxide removal need to be expanded massively and very rapidly. It is questionable, however, whether this can be achieved at the necessary scale.
The assessment models being studied by the IPCC have not yet been able to integrate ocean-based methods of carbon dioxide removal. The Sixth Assessment Report therefore does not provide any information on how much they could contribute to achieving the Paris Climate Agreement goals. The first research teams, however, including scientists from Germany, have begun to take on the task of developing IAMs with components of marine-based carbon dioxide removal. Their motivation for this work is fuelled by the knowledge that the ocean has already absorbed and stored one quarter of the carbon dioxide emissions caused by human activities in the past, with wide-ranging consequences for humanity and nature. Textende