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Carbon budgets, CCS and the energy transition


Staff member
Mar 22, 2024
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One of the key talking points of the recent Climate Summit in New York was the carbon budget available for a transition that could limit surface temperature warming to 1.5°C. In her speech to the UN, Greta Thunberg made note of the numbers;

“To have a 67% chance of staying below a 1.5 degrees global temperature rise – the best odds given by the [Intergovernmental Panel on Climate Change] – the world had 420 gigatons of CO2 left to emit back on Jan. 1st, 2018. Today that figure is already down to less than 350 gigatons . . . . . . . . . . With today’s emissions levels, that remaining CO2 budget will be entirely gone within less than 8 1/2 years.”

The physics and chemistry of the atmosphere tell us that the currently observed surface temperature warming can only be brought to a halt when society stops adding carbon dioxide to the atmosphere from long sequestered sources (fossil fuels, limestone for cement, global forests). Further, we also know that there are only two pathways forward for doing this – one is to stop the current practice of deforestation and using oil, coal, gas and limestone and the other is to at least remove an equivalent amount of carbon dioxide from the atmosphere for as long as these practices continue.

With this in mind, society has set out on a journey of energy transition, which involves reducing its use of fossil fuels as quickly as possible and reversing forest loss. The goals of the journey are based on our understanding of how much more warming will take place for a given amount of cumulative ongoing emissions; the data was published in the IPCC Special Report on 1.5°C released just over a year ago. It is also clear that the so-called ‘carbon budget’ for 1.5°C of warming (about 0.4°C above current levels) is very small and vanishing rapidly as emissions continue.​

Carbon Budget IPCC SR15

For a 2°C goal at 50%, the notional carbon budget of 1500 Gt CO2 looks achievable. While it represents only 35 years of emissions at current levels, on a 0.54 Gt per year linear declining emissions basis to net-zero it could extend to the 2090s, but that means emissions need to start falling from 2020. If there would be a ten year period of flat emissions prior to a fall, then the rate shifts to about 0.75 Gt per year. A fall of 0.54 Gt in the coming year would be about 1.4%, below the rises of the last two years.

Most published strategies that address the carbon budget problem make use of a set of technologies that are well understood and available at scale today, namely various applications of carbon dioxide capture and geological storage. Carbon capture and storage (CCS) can be used today to prevent emissions of carbon dioxide in the first instance when fossil fuels are used, but also offers the potential for removal of carbon dioxide from the atmosphere. But progress in actual scaling and deployment of CCS is essentially moribund, while other energy related technologies are moving ahead. In a world of growing climate anxiety, why is this?

In some cases, there is the belief that CCS is experimental and untested, yet this couldn’t be further from the truth. For starters, the technologies involved have been used in the oil and gas industry for decades, just not in the precise configuration that CCS requires. For example, separating carbon dioxide from other gases is a common practice in the natural gas sector where the gas coming from the well typically contains a low concentration of carbon dioxide, but it must be removed before the product is sent through pipelines and sold to customers. Furthermore, nineteen large scale CCS facilities are in operation around the world, including a Shell operated facility in Canada capturing and storing one million tonnes of carbon dioxide per annum. CCS technology may well improve, but it certainly isn’t experimental or untested, nor does it require pilot plant testing or demonstration – that phase is well and truly over. New CCS technologies will undoubtedly emerge and they will be subject to demonstration, but that is true for any technology pathway.

For others there is a belief that alternative technologies will emerge, be deployed very rapidly and effectively do away with the need for CCS. This is based on a view that the world can quickly move on from using fossil fuels, but is very unlikely to be the case. While there are clearly a set of technologies now available to generate electricity without fossil fuels, we are still very distant from a society based entirely on electricity using solar PV, wind turbines and nuclear reactors. Electricity makes up just 20% of the energy we use to provide services and historically that has shifted at a rate of two percentage points per decade. But even doubling or quadrupling the rate of change would still mean a century or more of transition and likely exceeding the desired carbon budget along the way. Further to this, there are many applications for combustion based energy provision where an electricity pathway doesn’t exist (e.g. cement manufacture, aviation). Some ideas are out there, but moving from concept to full scale commercial deployment is a multi-decade programme in itself.

We shouldn’t underestimate the time it takes to move from one system to another or build whole new systems. Even the internet has taken 25 years to deploy at scale, but that was on the back of an existing telephony system and was based on technologies that were first tested 25 years prior, in the late 1960s. In the field of energy transition, even longer time-frames are likely. The first Liquefied Natural Gas (LNG) carrier commenced operation 60 years ago, with the current market now reaching around 350 million tons per year; that’s about 17 EJ, or less than 5% of global energy demand. In the Shell Sky scenario we imagine a global hydrogen industry in 2100 that is four times this size and yet still only provides about 10% of final energy. Building a significant hydrogen and electricity based energy system (or any other system) to replace the current fossil fuel system is quite possible, but the time-span to do so will be measured in decades.

Finally, there are those who just claim that CCS costs too much, but usually without a reference to compare it with. Mitigating carbon dioxide won’t come at no cost, so the costs we do incur are all relative. Depending on the application, CCS projects can cover a range from as little as $30 per ton of carbon dioxide (e.g. in ethanol plants in the USA) to over $100 per ton in power stations. But as infrastructure develops costs will come down, as has been the case for many other technologies. Building a new electricity system based on renewable energy or deploying electric vehicles (EV) will also come at a cost, in some cases in excess of the cost of utilizing CCS, but the option to use CCS instead may not be available due to policy choices. This happens in instances where governments have given preference to certain energy technologies, rather than looking more broadly at the full range of opportunities for managing emissions. A challenge often faced by CCS is that its cost in CO2 terms is very transparent, against other technologies where costs on a CO2 basis are often not published or even used.

So we are left with the dilemma of a vanishing carbon budget and the eventual deployment of an alternative fossil fuel free energy system that will likely mean breaching that budget. Technologies and approaches that seek to remove carbon dioxide from the atmosphere or prevent emissions in the first instance can bridge this gap. This includes the full range of application of CCS technologies, but also the use of nature based solutions such as large scale afforestation. In the Shell Sky scenario the use of CCS in industry ramps up rapidly from the 2030s as this is an immediately available technology. It peaks in the 2070s and then starts to decline, as new technologies begin to deploy at scale, for example hydrogen based smelting of iron ore. This forty year gap is successfully bridged with CCS. We could imagine that by the middle of the 22nd century there is no further need for CCS in industry as a complete transition away from fossil fuels has taken place. But for 50 to 100 years, CCS has offered the possibility of no net addition of carbon dioxide to the atmosphere, even as fossil fuel use continues in legacy industrial processes.​

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