Why We Need CCS, Part 4: Carbon Negative Solutions
- Posted on July 1, 2014
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- The economic growth ambitions of the developing world combined with the very tight carbon budget prescribed for the 2 ᵒC scenario could potentially demand a very large deployment of carbon negative solutions from the middle of the 21st century.
- CCS is the best candidate for achieving negative CO2 emissions – both in the form of bio-energy with CCS and direct air capture.
- Extensive modelling studies performed by the IPCC show that removal of this option makes the achievement of the 450 ppm scenario much more expensive or even impossible.
As discussed in a previous post, CCS is likely to play a very important role if climate science is eventually proven correct and long-term atmospheric CO2 concentration levels of ~450 ppm are confirmed as a top global priority. The possible role of CCS retrofits to the very young fleet of fossil-fueled industry currently being built in the developing world was discussed as a medium-term possibility in the case where CO2 prices rise very rapidly in the next decade. Given the massive coal-fired push towards economic growth in the developing world and the very tight CO2 budget related to the 450 ppm scenario, however, this is unlikely to be sufficient.
Indeed, with every passing year of much climate talk and little climate action, various agencies around the world adjust their 2 ᵒC scenarios towards something that looks even more unrealistic than the previous iteration. The most recent of these scenario analyses comes from the IPCC where it was flatly stated that many models could not achieve the 450 ppm scenario if CCS is eliminated as an option. And a substantial portion of the projected CCS impact comes in the form of carbon negative solutions resulting in net-negative emissions from the electricity sector starting mid-century (see below).
Carbon negative solutions
Taking CO2 from the air is quite a lot more difficult than putting it there, but, as shown in the figure above, almost all 450 ppm scenarios require large net-negative emissions from the electricity sector towards the end of the century. The IPCC assigns this responsibility to bio-energy with CCS (BECCS), but also mentions direct air capture as an option.
Naturally, there is great uncertainty tied to BECCS, primarily related to the feasibility and impact of building out bio-energy on such an enormous scale. However, if it eventually turns out that the 450 ppm scenario is indeed a vitally important target for the future of the planet, we will have little choice other than making this work.
The amount of emissions mitigated by CCS in various scenarios producing long-term CO2 concentrations in the range of 430-530 ppm is shown below. A rapid upscaling is clearly visible with the storage rate in the year 2100 being close to the entire energy-related emission rate of today – a rather incredible expectation.
The ~3 GtCO2/yr storage rate in 2030 might not look very impressive in comparison, but the enormity of this upscaling effort becomes evident when compared to the large subsidy-driven renewable energy rollout in recent years. For example, wind energy has expanded at a very impressive 30% CAGR over the past 15 years, but, as shown below, currently avoids only about 0.235 GtCO2/yr – more than an order of magnitude less than is expected from CCS within a similar timeframe under the 430-530 ppm scenario. (CO2 mitigation from wind is calculated under the assumption that wind displaces natural gas at 0.45 tCO2/MWh and that additional emissions associated with embodied energy, fossil fuel balancing and the rebound effect are negligible.)
The IPCC details several mitigation scenarios which deviate from the “all of the above” baseline case. The difference in total mitigation costs relative to the baseline scenario for these different scenarios is given on the left in the figure below.
It is clear that the “No CCS” scenario results in the biggest cost increase among 450 ppm scenarios, followed by the “Limited Bioenergy” scenario. It should also be noted that many of the “No CCS” model runs could not achieve the 450 ppm scenario.
One of the reasons given for the substantial cost increase caused by these two mitigation scenarios is the absence of BECCS (negative emissions) in the second half of the century. In comparison, the “Nuclear Phase Out” and “Limited Solar/Wind” scenarios show very small cost increases relative to the baseline because they are limited to the electricity sector and cannot achieve negative emissions.
It should also be noted that the above-left figure is generated under highly idealistic assumptions of a global least-cost climate change mitigation effort starting immediately. Under more realistic assumptions regarding delays in climate action and regional differences in ambition, mitigation costs increase further (as shown on the right in the figure above) and model predictions become increasingly dependent on large negative emissions from CCS in the second half of the century.
The role of CCS is increasingly becoming more clearly defined as an insurance policy against the scenario where climate science actually turns out to be correct. This climate insurance can potentially be claimed through retrofits to the enormous new fleet of fossil-fueled infrastructure still being rapidly deployed in the developing world and through the carbon negative solutions of bio-energy with CCS and direct air capture.
Even though the existence of this insurance policy can make the potentially catastrophic longer-term effects of climate change appear less daunting, it should not be relied upon too heavily. An enormous push towards carbon negative solutions towards the middle of the century is likely to be unnecessarily expensive and potentially environmentally destructive due to the enormous amounts of bio-energy that will be required. We can make things a lot easier for ourselves by establishing a true technology-neutral CO2 mitigation policy framework sooner rather than later.