As a person who has spent years on carbon dioxide (CO2)
capture technologies innovation and development, I am often asked why is it that
CO2, this tiny molecule in the air, can cause global warming? And
this is a fair question.
And my response always starts with: “it is not only CO2, but a group of gases called greenhouse gases (GHGs)”, and then follows with this simple explanation: “sunlight reaches the earth, some energy is reflected back to the space, some is absorbed and re-radiated as heat, CO2 and other GHGs have the ability to absorb the heat emitted from the earth surface and keep it from escaping out to space, and then reflect it in all directions, warming the earth”.
CO2 is the most important driver of global warming, but
it is not the only one. Certain group of gases in atmosphere that are
long-lived and remain semi-permanently there and do not respond physically or
chemically to changes in temperature are considered as “forcing” climate change,
e.g. Carbon Dioxide (CO2), methane (CH4), Nitrous oxide
(N2O), Chlorofluorocarbons CFCs. Moreover, there are other gases,
e.g. water vapour, which respond physically and chemically to changes in
temperature, these gases are defined as “feedback”.
CO2 is the most long-lived and abundant forcing gas in
atmosphere. It releases through natural processes and through human activities
such as deforestation, land use changes, and burning fossil fuels. Human
activities caused increase in atmospheric CO2 concentration by more
than a third since the Industrial Revolution. Before the Industrial Revolution
in the 19th century, global average CO2 concentration in atmosphere
was about 280 ppm (part per million), and in 2019 this value reached to nearly 415
ppm with an incremental growth of 2.3 ppm per year (average of the last decade),
considering 1 ppm of atmospheric CO2 is equivalent of 2.13bn tonnes
of CO2, each year nearly 5bn tonnes of CO2 is added to
atmosphere. Most credible climate scientists believe that the world needs to
reduce its atmospheric CO2 concentration to the “safe” level at 350
ppm at the most. This means we need to reduce the CO2 concentration of
around 65 ppm to hold the line against irreversible climate change. The awful
truth is if all decarbonisation methods are effectively used from today and the
whole world achieves Net-Zero emissions today (e.g. full electrification, 100%
renewables, all electric/hydrogen transportation, etc.), we are only eliminating
the growing rate (i.e. 2.3 ppm per year). And the 65 ppm which is the
cumulative atmospheric CO2 concentration since industrial revolution
needs solutions other than those mentioned above for removal.
This means we need to counterbalance the legacy CO2 by
taking it back out of the atmosphere, which is also necessary to achieve
Net-Zero Emissions by 2060 according to the IEA Technology Perspective Scenario
to keep global warming below 2°C. This makes Negative Emissions solutions as
part of the “Plan A” to combat climate change contrary to previous notion that
negative emissions solutions are down the road far off solutions.
Negative emissions solutions refer to any means or technology that capture atmospheric CO2 and store it permanently in a safe place. There have been several solutions proposed that could contribute to reduce atmospheric CO2. Here I am reviewing and evaluating the top three promises:
- Reforestation and afforestation
Reforestation and afforestation are great natural strategies to combat climate change via negative emissions as trees are fundamentally designed to absorb CO2 from atmosphere. Reforestation and afforestation are universally desirable solutions to tackle climate change, and they can represent up to 30% of share for climate change by 2050. Many countries are already practicing reforestation, such as Brazil, which has a target to restore 12m hectares of forest by 2030. Estimations suggest a capture rate of nearly 3.7 tonnes of CO2 per hectare per year is achievable with associated costs of $20 to $100 per tonne.
Reforestation means restoring areas where the trees have been cut
down or degraded. Afforestation means planting trees where there were
previously none. In recent decades there has been a movement toward people
moving into cities which have freed up some lands and plus better farming methods
could help to turn those lands to forest.
There are some considerations for this solution: e.g. land availability
and suitability, plus forests cannot be planted indefinitely. Planting new
forests could conflict with other requirements for land such as growing food. Also,
planting vast areas of forests could cause complex changes in cloud cover,
reflectivity, and the soil-water balance. Moreover, rising competition from
bioenergy (will be discussed in the following section) is creating a conflict.
- Bioenergy power generation with Carbon Capture
& Storage (BECCS)
BECCS is another negative emission technology which is the most-popular
and the most-talked about technology. BECCS achieves net negative emissions
through capturing and storing CO2 emitted from burning the biomass
in a power station, where all the CO2 absorbed by the plant (energy
crops) during its lifetime is not re-released to the atmosphere but removed
permanently from the atmosphere resulting in net negative emissions. BECCS has been
considered in almost all climate management scenarios. For instance, BECCS
accounts for 20% share in the IEA Energy Technology Perspective for limiting
the global warming below 2°C.
What makes BECCS attractive is its negative emissions being able to
compensate for the remaining emissions from other sectors such as transport or
industry that are technically and economically very difficult to abate (e.g.
the case for aviation or cement industry). Deployment of BECCS at scale is
crucial to ensure the energy industry achieves net-zero emissions by mid-century
with current carbon budgets.
Despite BECCS being considered to remove an average of 3.3bn tonnes
of CO2 per year by 2100 (according to the IPCC scenarios to limit
global warming below 2°C), it faces one significant limiting factor, which is
not the technology, but the supply of biomass. One study in 2018 estimated to
achieve negative emissions using BECCS, every billion tonnes of CO2
stored per year requires approximately 30 to 40 million hectares of BECCS
feedstock. This is equivalent of 300 to 700 million hectares of land devoted to
bioenergy crops depending on the efficiency of production. The cost of BECCS
deployments varies widely depending on the source of biomass too. A cost range
of $20-400 per tonnes of CO2 captured are estimated depending on biomass
sources and the sector being used.
Deployment of BECCS would also be limited by availability of land.
Growing biomass (energy crops) enough for negative emissions of 10bn tonnes of
CO2 would take up more than one-fifth of the area currently used for
growing food. BECCS benefits could be reversed (contributing to positive
emissions) if existing forests should be removed to make land for energy crops.
Direct air capture, sometimes referred to as DAC, is a relatively
new and innovative technology that collects and removes CO2 directly
from the atmosphere, and when combined with permanent storage of CO2,
it creates negative emissions. DAC technologies are sometimes called artificial
trees whilst unlike a real forest, they would need little land.
DAC is not an alternative to conventional, point-source CCS, but is
can be used to complement compensating CO2 emission from extremely
hard-to-abate sources as mentioned earlier. However, finding a safe site with
enough capacity to store CO2 permanently is still an open question
for all CCS applications.
Estimations suggest direct air capture could remove all the CO2 currently emitted each year, but we need to ensure whether Direct Air capture can live up to its promise? Currently, the barrier to its commercial realization are its financial and practicality. The atmospheric concentration of CO2 is nearly 0.04% whilst the CO2 concentration in the flue gas of a gas or coal power plant is 100 and 300 times higher, respectively, this would make CO2 capture using DAC expensive. Currently, the cost of CO2 capture by DAC is around $200-1000 per tonne of CO2. This technology needs to scale to bring its cost down, by reaching the economy of scale the cost of CO2 captured by DAC is expected to reach below $80 by 2050.
As mentioned earlier, today the level of CO2 is higher
than any time in human history mainly due to human-caused reasons. Climate
scientists widely agree the earth’s surface average temperature has already
increased by 1°C since the Industrial Revolution, and without proper actions to
control and limit emissions, the atmospheric CO2 emission could
reach above 500 ppm by 2050, leading to an increase in the average temperature to
more than 3°C, a level that climate
scientists say would cause extreme weather and sea level rise that would
endanger global food supplies, cause disruptive mass migrations, and even
destroy the Amazon rainforest through drought and fire.
Lots of grand promises have been made about capturing and removing CO2 and now it is the right time to sort through reality from promise. The goal is using the right mix of carbon removal solutions as a part of humanity’s response to climate crisis. Now the big question is what form of carbon capture at what scale is going to make the best and the most socially just contribution? There is no single solution, but we need to bring everything we can to the table to combat climate crisis before it leaves behind irreversible and catastrophic scars.