CCS and climate change mitigation

Carbon capture and storage (CCS) is a method used to reduce the amount of anthropogenic emissions of carbon dioxide (CO2) in the atmosphere in an attempt to limit the effects of global climate change. CCS can be employed to achieve a number of goals regarding climate change mitigation, such as preventing average global temperature from reaching certain levels above the pre-industrial average. In December 2015, the Paris Agreement articulated a census to not exceed pre-industrial global temperatures by more than 2 °C and recognized that different countries would have different contributions to help realize this goal.[1] Under the Paris Agreement, different scenarios and climate models were analyzed for different temperature goals considering a wide range of mitigation methods from a temperature goal of less than 2 °C to an upper limit of exactly 2 °C increase above the pre-industrial average.

The terms CCS and CCUS (Carbon Capture, Utilization, and Storage) are often used interchangeably. The difference between the two is the specified 'utilization' of the captured carbon and refers to its use for other applications, such as enhanced oil recovery (EOR), potentially making liquid fuel, or the manufacturing of useful consumer goods, such as plastics. Since both approaches capture emitted CO2 and effectively store it, whether that be under-ground in geological formations or long-term trapping in material products, the two terms are often treated the same.

CCS is considered as a basis of one climate stabilization wedge, which is a proposed climate mitigation action to reduce approximately 1 billion tonnes of carbon emissions over 50 years.[2]

CCS and different climate models
Carbon dioxide emissions and atmospheric concentrations over the 21st century

Large scale CCS plays a crucial role in reaching climate change stabilization. According to the IPCC, the carbon emission patterns can greatly vary based on the uncertainty of human power consumption. A file regarding the fluctuations of greenhouse gas emissions is shown to the right. However, CCS' primarily role is to delay the shift from fossil fuels and thereby reducing transition costs. The implementation of default technology assumptions would cost 29-297% more over the century than efforts without CCS for a 430-480 ppm CO2/yr scenario.[3][4] The Paris agreement upholds a goal to reach no more than a 2.0 °C increase above pre-industrial temperatures. If the 2.0 °C goal is to be reached in time, CCS must be utilized to achieve net zero emissions by 2060-2070. After 2060-2070, negative emissions will need to be achieved to remain below the 2.0 °C target. The variations in methods depend heavily on the climate change model being used and the anticipated energy consumption patterns. It is widely agreed upon, however, that CCS would need to be utilized if there is to be any negative climate change mitigation.[5]

CCS and 2.0°C target

The concept of a 2.0 °C came to light in the European Union of 1996 where the goal was to reduce the global temperature range relative pre-industrial levels. The decision of the 2 °C range was decided mostly on the evidence that many ecosystems are at risk if average global temperatures exceeded this limit. In order to limit the anthropogenic emissions such that there is no more than a 2 °C change relative to the periods between 1861 and 1880, carbon emissions would need to be limited to about 1000 GtC by 2100 since that period. However, by the end of 2011 about of half of the budget was already released (445 GtC) indicating that a lower budget is necessary.[6]

A distinctive path that aims for a 2.0 °C limit might have complications. The first complication involves the lack of positive feedback loops in IPCC climate models. These loops include reduction of ice sheet size, which would mean less sunlight is reflected and more is absorbed by the darker colored ground or water, and the potential release of greenhouse gases by thawing tundra. Since the lifetime of CO2 in the climate atmosphere is so long, these feedback loops have to be taken into consideration. Another important factor to consider is that a 2.0 °C scenario necessitates tapping into alternative fossil fuels sources that are harder to obtain. Some examples of these methods are the exploitation of tar sands, tar shales, hydrofracking for oil and gas, coal mining, drilling in the Arctic, Amazon, and deep ocean. Therefore, 2.0 °C scenarios result in more CO2 produced per unit of usable energy. Further, the danger of extra released CH4 via mining processes must be taken into account.[7]

Global greenhouse gas emissions by gas type

Different models are based on when the peak of carbon emissions happen on a global scale. In one article regarding the 2.0 °C scenario with respect to pre-industrial levels, possible approaches are short term and long term emission resolutions as well as the considering the cost effectiveness of different solutions to reduce carbon emissions. Short term goals are set to quantify progress towards the temperature goal. In a short term goal, looking ahead to the year 2020, the allowable carbon emissions must be between 41 and 55 GtCO2 per year. The short term 2 °C scenario is not feasible without CCS.[8]

Currently, greenhouse gas emissions would need to be reduced by 7 Gt of carbon equivalent each year by 2050 to achieve 2 °C stabilization. This requires power generation with CCS at 800 coal-fired power plants of 1 GW energy generation capacity, 180 coal-synfuel plants, or natural gas plants worth 1,600 GW.[9] In this scenario, one of the wedges, or 1 Gt of carbon is accounted for by CCUS.[10] The cost of capturing CO2 is estimated to be $500/tC. If the goal with the 2.0 °C is to store a total of 7 Gt carbon per year, then the collective amount needed to achieve this is around 3.5 trillion U.S. dollars per year. The economic demand needed to achieve this goal is high. This amount of money is equivalent to gross national incomes for countries such as Russia or United Kingdom, and it represents 18% of United States' 2017 gross national income (19.61 trillion dollars).[3]

CCS and below 2.0°C target

Achieving below 2.0°C target

A change of temperature below 2 °C is, to certain extent, almost impossible to achieve due to the current carbon emission practices. The IPCC notes that it is difficult to assess a climate mitigation scenario that would limit average global temperature increase to only 1.5 °C above pre-industrial levels. This is mainly due to the fact that few reliable multi-model studies have been conducted to thoroughly explore this scenario. Nevertheless, what few studies that have been done agree that mitigation technologies must be implemented immediately and scaled up quickly and reflect energy demand decrease.[11] A change below 1 °C with respect to pre-industrial era is now inconceivable because by 2017 there was already an increase of 1 °C.[12]

Because of the immediate inability to control the temperature at the 1 °C target, the next realistic target is 1.5 °C. There is enough confidence that past emissions alone (pre-industrial time) will not be enough to go beyond the 1.5 °C target. In other words, if all anthropogenic emissions were stopped today (reduced to zero), any increase beyond the 1 °C change for more than half of a degree before 2100 is unlikely. If anthropogenic emission are considered, the probability for the planet increasing for more than 1.5 °C before 2100 are high. Then, scenarios where the degree change is maintain below 1.5 °C are very challenging to achieve but not impossible.[13]

For a below 2.0 °C target, Shared socioeconomic pathways (SSPs) had been developed adding a socio-economic dimension to the integrative work started by RCPs models. The advantage of using SSPs is that they incorporate social standards, fossil fuel use, geographical development, and high energy demand. SSPs also incorporate the use of six other models such as GCAM4, IMAGE, MESSAGE-GLOBIOM, and REMIND-MAgPIE. The combination of models and scenarios concluded that by 2050, annual CO2 emissions are in the range between 9 and 13 billion tons of CO2. All of the scenarios estimated that temperature will remain below 2.0 °C change with a 66% probability of success. To do so, a 1.9 W/m2 within the year 2100 is necessary. Net zero GHG emissions have to be achieved between 2055 and 2075, and CO2 emissions have to be in a range between 175 and 475 GtCO2 between the years 2016-2100. All SSPs scenarios show a shift away from unabated fossil fuels, that is process without CCS.[13]

Assumptions for below 2.0°C target

To achieve a 1.5 °C target before 2100, the following assumptions have to be considered; emissions have to peak by 2020 and decline after that, it will be necessary to reduce net CO2 emissions to zero and negative emissions have to be a reality by the second half of the 21st century. For this assumptions to take place, CCS has to be implemented in factories that accompany the use of fossil fuels. Because emissions reduction has to be implemented more rigorously for a 1.5 °C target, methods such as BEECS, and natural climate solutions such as afforestation can be used to aim in the reduction of global emissions.[14] BECCS is necessary to achieve a 1.5 °C. It is estimated by the models that with the help of BECCS, between 150 and 12000 GtCO2 still have to be removed from the atmosphere.[13]

Another negative emission strategy which includes CCS can also be approached through DACCS. Direct Air Carbon Capture and Sequestration (DACCS) is a carbon negative technology that utilizes solid amine based capture and it has proven to capture carbon dioxide from the air even though content of the air is much lower than of a flue gas from a coal plant.[15] However, it would require renewable energies to power since approximately 400kJ of work is needed per mole of CO2 capture. Furthermore, it is estimated that the total system cost is $1,000 per tonne of CO2, according to an economic and energetic analysis from 2011.[16]

Going forward in the utilization of models such as SSPss and RCP, feasibility of the model has to be to take into consideration. Feasibility includes concerns in various fields, such as geophysics, technology, economics, social acceptance, and politics, all of which can serve to facilitate or obstruct the carbon capture and sequestration of emissions needed in order to achieve the global temperature targets. Uncertainty in feasibility is especially a problem with more strict temperatures limits such as 1.5 °C. Real world feasibility of SSPs models, or any other models, in general are coarse approximations of reality.[13]

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