Net-zero emissions will require carbon dioxide removal from the atmosphere
There has been a significant shift in commitments by countries and companies across the world. This shift has involved the commitment towards “net-zero” greenhouse gas emissions.
In short, this means that there are now commitments to transition to a future where human activity will no longer cause increases in the concentrations of CO₂, methane or other greenhouse gases to the atmosphere.
So, how hard can it be? We just need some low emissions technologies and all will be good, right?
Unfortunately not. Almost all industrial and energy activities by humans produce some greenhouse gas emissions.
Take solar photovoltaics, for example. These are commonly thought of as zero emissions, but that’s only the case while a solar PV panel is operational. Solar photovoltaics are mostly based on the element silicon. Silicon does not occur in nature on its own. Almost all of it is bonded to oxygen, usually as silica (SiO₂).
To make silicon from silica for solar panels, it is necessary to remove the oxygen from the silicon. This is typically done in a high temperature reaction with a carbon-containing substance (coal, coke or wood charcoal), driven by an electric arc furnace.
The combination of electricity and carbon help strip the oxygen away from the silicon, forming CO₂ as a byproduct:
SiO₂ + C + electricity -> Si + CO₂
In most cases, the carbon source is non-biogenic — usually coal. Additionally, much of the electricity used to make silicon comes from non-renewable sources. As such, there is CO₂ emitted as a result of making the silicon for solar PV panels.
To be clear, the amount of CO₂ produced per unit of power a cell makes over its lifetime is modest. A life cycle assessment study in the journal Solar Energy estimated that between 30 and 45 grams of CO2 are released per kWh generated by silicon-based solar panels.
This is a long way below the emissions for natural gas or coal-based power, but regardless of whose numbers you use, one thing is clear. Solar PV will never have zero emissions. The number will always be greater than zero. We should do everything we can to reduce the emissions of human activity, but that just won’t be enough.
The same problem arises from other technologies that require metals or concrete to make them: the emissions in making those materials also mean that the technology can never be zero emissions. This includes wind power, hydropower, solar thermal, and pretty much every other energy and industrial technology. Some may be low emissions, but low doesn’t equal zero.
Even if we successfully remove most of the GHG emissions from some sectors, we will still have “hard to abate” sectors — such as steelmaking, cement, and aviation — that are going to need many decades, if not longer, to get their greenhouse gas emissions down.
Basic mathematics dictates that if you want the sum of anything to be zero, and most of your options are positive numbers, you must have negative options to make the total equal zero.
This is what net-zero means. We must start deploying our options for removing CO₂ and other greenhouse gases from the atmosphere. It’s the only way net-zero will be possible.
Bodies like the International Energy Agency (IEA) and the Intergovernmental Panel on Climate Change (IPCC) have recognised this basic need for removing greenhouses gases from the atmosphere as a condition for acheiving net-zero emissions.
None of this negates the urgent need to reduce emissions from our existing industrial and energy systems. The more reductions we can make, the lighter the burden on negative emissions technologies will be.
My next post will cover some of the options available to us for carbon dioxide removal (CDR).
This post originally appeared at davidtkearns.com.
PS — what about methane and nitrous oxide emissions? Fortunately, methane is a short-lived gas in the atmosphere — around 12 years on average. If we can get methane emissions levels low enough (admittedly a tough challenge) we will reach a point where its natural rate of destruction will match what we’re emitting.
Nitrous oxide (N₂O) lasts much longer — over a century — but thankfully the quantity of nitrous oxide emissions (in CO₂-equivalent) is small and is already dropping as a result of improved combustion technology and NOx reduction technologies. A modest additional amount of CO₂ removal should be enough to balance the warming impact of any remaining nitrous oxide emissions.
So CO₂ reduction and negative emissions will be the main game over the coming decades.
