>> Read the original text from "Aktuel Naturvidenskab" in Danish

Humanity finds itself at a critical point in history, and the effects of climate change are becoming more visible every year; increasing surface temperatures, melting polar ice, and extreme weather are a few examples of ongoing events that are threatening Earth´s natural balance. In the middle of this climate crisis, we find one of Earth´s most common elements, carbon. Carbon, as a chemical compound, can store massive amounts of energy that we can and have harvested and used all over the world, from the first bonfires to modern space travel. The burning of carbon-based energy sources is the primary driving force behind humanity´s development over thousands of years. But every time we use those energy sources, we release invisible gases into the atmosphere that now cover the Earth like a blanket and cause global warming and climate change. The molecule responsible for these changes is carbon dioxide, or CO₂.

CO₂ is essential to the Earth´s energy balance, and it has been present in the atmosphere throughout Earth´s 4.5-billion-year-old history, where it acts as a greenhouse gas. CO₂ acts as a greenhouse gas by absorbing heat radiation that is reflected off Earth´s surface. Some of this heat is reflected back to Earth creating more warming, and some of the heat is reflected to outer space beyond the atmosphere. Here is an interesting thought experiment: if we removed all CO₂ from the atmosphere, the result would be a frozen planet that would be unable to sustain complex lifeforms, therefore we have a lot to thank CO₂ and other greenhouse gases for. However, in our current scenario, increased levels of CO₂ and other greenhouse gases will continually trap more heat between the Earth and the atmosphere, thus increasing global temperatures. Causing global warming as is happening right now.

Where are we?

In 2015, 196 countries signed the Paris Agreement, pledging to take steps to reduce their CO₂ emissions to limit the global temperature rise to 1.5 degrees Celsius compared with pre-industrial temperatures. FN´s climate panel, the IPCC (Intergovernmental Panel on Climate Change), monitors progress regarding the agreement and provides periodic assessments of climate change, climate impact, and climate risks as well as providing advice on how to adapt and reduce these risks. An important question right now is: how much more CO₂ can the atmosphere absorb before we surpass the 1.5 degrees Celsius temperature limit? According to the IPCC we only have 260 gigatons of CO₂ left before we surpass 1.5 degrees Celsius, and around 1009 gigatons of CO₂ (figure 1) before we reach 2 degrees Celsius. But the more interesting question is: how much time will it take to use the 260 gigaton allotment of CO₂? That is the time we have left to solve the climate problem, and the answer to this question depends on how humanity manages its CO₂ emissions going forward.

_{Figure 1. The IPCC has observed an almost linear relationship between the total amount of CO2 in the atmosphere and the Earth's surface temperature. The more CO2 there is in the atmosphere, the more heat is reflected back, and less heat is released into space. The figure illustrates this relationship by showing the cumulative accumulation of CO2 emissions since 1876 and the corresponding increase in surface temperature.}

In 2021 we emitted around 42.2 gigatons of CO₂-equivalents of greenhouse gases, and our emissions are expected to be higher in 2022. If we use 2021 as a starting point, we have roughly six more years left before we surpass the 1.5 degrees Celsius temperature increase. Fortunately, significant changes can still happen! We saw, for example, a significant reduction in yearly emissions during the COVID-19 pandemic and saw that we can affect the emissions. However, it is doubtless that this will require significant effort.

However, it has become more unlikely that we can keep the temperature increase under 1.5 degrees Celsius. Many climate models are now predicting scenarios in which the temperature rises above 1.5 degrees for a period but falls again afterward. For example, you could temporarily exceed the 1.5-degree increase now to eventually achieve the climate goals of 2050 or 2100. That is a risky climate strategy, but it can give us more time to develop technological or systemic solutions to reduce CO₂ emissions before time runs out. But how can that be achieved?

Where should we be?

The IPCC is working on long-term solutions and trying to incorporate all collective knowledge in climate solutions and strategies. Overall, future model scenarios are being prepared in which we can achieve climate neutrality and comply with the Paris agreement continuously until the year 2100. Some of these scenarios incorporate “overshoots”, while others operate without “overshoots”. If you look at the various emission scenarios for the year 2100, there are a lot of large differences in their endpoints in terms of emissions, largely dependent on the level of reductions now, and if we allow the temperature to overshoot. However, each of these endpoints must fall below zero by year 2100, and falling below zero is called global negative emissions.

Negative emissions are a complicated and controversial subject, but overall, it means that we must remove more CO₂ from the atmosphere than we emit; it is no longer enough just to reduce emissions to zero. CO₂ can be removed from the atmosphere through a range of nature-based solutions (soils and forests), or man-made technological solutions designed to remove CO₂ from the atmosphere at a massive scale.

_{Figure 2. IPCC P1-P4 are scenarios for how we keep the temperature rise below 1.5 degrees. You can see that the graph goes below zero in all cases, which means that negative emissions are needed – in other words, we reach a point where more CO2 is removed from the atmosphere than is added. P4 (gray) allows for a larger overshoot and requires greater negative emissions in 2100.}

What scenario we choose to follow has major implications and greatly affects the amount of negative emissions required in the future. As a rule, we can say, the more CO₂ we emit now, the more we allow “overshoot” the more negative emissions will be required by the year 2100. Conversely, the more we can reduce CO₂ emissions now, the less we will need to remove them in the future.  And it matters a lot whether we need to have 5 gigatons or 20 gigatons of negative CO₂ emissions by the end of the century, as it is hugely expensive and difficult.

Besides the need for negative emissions, we should also find technical and systematic solutions to avoid and reduce emissions, which are inertly hard to reduce. For example, green energy can be used for air transport, heavy duty transport, and industry in cases where they cannot be electrified. The solution can be to make greener fuels from CO₂ caught in biomass through photosynthesis or directly from the atmosphere or points sources (an example is the process of making cement). When these fuels are burned to harvest energy, the CO₂ will go back into the atmosphere, but the climate impact is less than oil-based fuels in theory. It is called net-zero-fuels.

 In summary, we should focus on three things to comply with the Paris Agreement: reduction of current emissions, net-zero technology, and lastly, negative emissions. And Denmark’s emissions reduction goal is 110% by 2050, which means that Denmark will have to produce at least 10% negative emissions!

Reduction, Converting, And Negative Emissions

An effective way to reduce one’s own CO₂ emissions is to use a bicycle instead of a car. Cycling emits almost no CO₂, but your car emits much more. Emissions that your car would emit are simply taken out of the equation. The same can be said for electric vehicles- however, cars, electric vehicles, and bicycles all emit CO₂ in their production processes. One should also consider the entire “cradle to grave” cycle when calculating the CO₂ emissions of products. And electric vehicles only have low emissions if they run on renewable energy.

On the other hand, if one travels using energy derived from biomass, that travel would be considered net-zero emissions. Crops, which remove CO₂ from the atmosphere through photosynthesis, are later refined into biofuels and burned off in a vehicle’s combustion engine. It then ends up in the atmosphere as the same amount of CO₂, resulting in an unchanged net-amount of CO₂ in the atmosphere, meaning it is carbon neutral. However, this assumes that every step in the refinement process is carbon-neutral “from cradle to grave.”

If you drive the car on biofuel and capture the CO₂ you emit, and then store it permanently (for over 1000 years), it will be considered a negative emission since the origin of the CO₂ is the atmosphere, and the destination is permanent storage. In fact, this is one of the methods that is currently highly regarded and is called BECCS (which stands for Bioenergy Carbon Capture and Storage). BECCS takes place at combined heat and power plants that heat their boilers by burning biomass such as wood chips or straw. The CO₂ in the flue gas produced during combustion is captured and can then be stored. This way, you obtain energy from biomass and achieve negative emissions. However, the burning of biomass is highly criticized as it takes land for production of "energy crops". Land that could otherwise be used for purposes, such as nature conservation or food production.

So, the capture of CO₂ is a central element in achieving negative emissions. But therein lies the challenge, as unfortunately, capturing CO₂ is not so straightforward. This brings us directly back to the issue of energy.

The energy in everything

To follow the IPCC’s calculations, there is a need for technical solutions for reduction, negative emissions, and net zero solutions. Before 2030 most reductions should come from the use of sun and wind energy, which are each responsible for saving 4 gigatons of CO₂ per year. However, to follow the IPCC’s plan, we will need to remove and store 1 gigaton of CO₂ (CCS, Carbon Capture and Storage). On the surface, it can be a quick solution to solve climate change. However, the existing technology to perform these tasks is currently extremely energy-dependent, and their development is still in the initial stages. There are only a few plants in the world that are currently using these young technologies. Therefore, in the coming years, it is important that we build more of these plants and learn how to reduce the costs of CO₂ capture. Research plays a decisive role in this context, especially when there is a need for much more research to find the best methods and technologies for various situations. There is rarely a universal solution, therefore research is critical to discovering the right approaches.

One example that illustrates the huge energy needs is the green production of methanol (CH3OH)- a type of energy that can be used for heavy transport in the future. To produce a gigaton of methanol from CO₂, 0.14 gigatons of hydrogen are required. The hydrogen for the process is produced by splitting water in electrolysis, and to produce 0.14 Gt of hydrogen, 6.710 TWh of power must be supplied to the water. For comparison, the United States’ energy use per year is just under 4000 TWh, so it will require one-and-a-half times the USA’s energy consumption just to produce one gigaton of methanol, and that energy must be derived from renewable sources (only 12.5% of the USA’s energy consumption is from renewable sources). In Denmark, due to wind turbines and burning of biomass, we have a lot of renewable energy compared to many other countries, and this is also one of the reasons why we will see many of the so-called Power-to-X (PtX) projects in Denmark. Energy consumption is not the same throughout all hours and seasons of the day, and sometimes more is produced than we consume. It is therefore smart that we can produce fuels and other useful chemicals with the surplus green energy.

We must turn the tide!

Since the invention of fire, humans have gotten their energy from carbon-based sources. We have emitted CO₂ from biomass, and later fossil fuel, and emitted substantial amounts of CO₂ into the atmosphere. Now we face the urgent task of diverting the carbon flow so that it goes back from the atmosphere to biomass and the subsoil. It requires freeing ourselves from our dependence on fossil fuels and obtaining enormous amounts of energy to reverse the process.

Thankfully, researchers all over the world are engaged in innovating solutions to make technology scalable, cheaper, and more energy efficient. We can compare our situation with a budget and see how humanity should invest in negative emissions and smart solutions to buy more time- and in this case, our currency to buy more time is renewable energy.

However, technological solutions cannot save us alone. Firstly we should focus on areas where we can achieve signification reductions quickly! It is not only the most cost-effective approach but also the most viable.

>> Read the issue of Aktuel Naturvidenskab themed around CO₂

Sources

• _{Figure 1: IPCC Special Report on Global Warming of 1.5°C. Source: IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 3-24, doi:10.1017/9781009157940.001.}
• _{Figure 2: IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 3-24, doi:10.1017/9781009157940.001.}