Five Fundamental Truths About Decarbonization
Understanding Decarbonization, Part 2
This part is Part 2 in the THB classroom series Understanding Decarbonization. You can find Part 1 here. This series is in response to reader requests for less technical THB posts that focus on climate mitigation policy.1 I am writing these in the same way that I have taught energy and climate to upper level undergraduates. As always — questions, requests, feedback are most welcome! Class is in . . .
Consider a bathtub. Water comes out of a faucet and fills the tub. If the drain is plugged and there is no other way for water to exit the tub, then leaving the faucet on means that the tub will overflow and flood your house.
Accumulating carbon dioxide in the atmosphere can be thought of like water filling a bathtub.2 We emit carbon dioxide mainly through the burning of fossil fuels — coal, oil, and natural gas. Carbon dioxide stays in the atmosphere for a long time and accumulates, just like water filling a tub.
The accumulation of carbon dioxide in the atmosphere is very well understood. There are however longstanding debates over how much added carbon dioxide might be “dangerous,” in the language of the 1992 U.N. Framework Convention on Climate Change (to which the United States is a signatory).3 That is equivalent to debating how high the side of the bathtub is, and at which point the filling tub overflows and becomes a damaging house flood.
In the early 2000s, climate policy focused on stabilizing concentrations of carbon dioxide in the atmosphere, with 280 parts per million (ppm) characterized as the pre-industrial baseline. Variously, concentrations of 580ppm, 450ppm, and even 350ppm have been proposed as the metaphorical height of the bathtub. Currently the world is at about 425ppm.
About a decade ago, climate policy shifted from a focus on concentrations as a policy metric to projected global temperatures in 2100, a change that I have long thought to be unhelpful for both science and policy. The 2015 Paris Agreement identified 2 degrees Celsius as the point where the metaphorical bathtub overflows, and recommended that the world seek as well to strive to hit a 1.5C target. These temperature targets have been associated with corresponding amounts of cumulative carbon dioxide emissions — in the tub metaphor, equivalent to how much water the tub can hold before overflowing.
There are a few complexities, such as the fact that the oceans and the land absorb about half of the carbon dioxide that we emit — think of that like a small open drain in the bottom of the tub that lets some of the water out as it fills.
The bathtub metaphor makes the challenge of reducing emissions clear: In order to eliminate the possibility of overflowing the tub, the amount of water filling the tub must be reduced to be at least equal to the amount flowing out of the small drain. That would halt the increase of water levels in the tub.
If we want to not only stop the increasing accumulation of carbon dioxide in the atmosphere and we do not want to rely on the oceans and land to take up carbon dioxide, then the metaphorical faucet needs to be shut off completely — that’s net zero.4
Turning down the rate of water flowing from the faucet might buy some additional time before the tub overflows, but so long as the water is flowing into the tub, altering the rate of flow does not solve the fundamental problem that the tub will eventually overflow and flood the house. That is what makes accumulating carbon dioxide in the atmosphere a unique problem as compared to, say, conventional air pollution.
Let’s leave the bathtub behind and explore the tools in the policy tool box that we have to reduce carbon dioxide emissions, under a policy goal of ultimately trying to approach net zero.
The Kaya Identity provides an extremely useful framework for understanding the entire set of options available for reducing emissions.
The four elements of the Kaya Identity are:
Population;
GDP per capita;
Energy intensity of the economy;
Carbon intensity of energy;
Together, these four factors combine to result in carbon dioxide emissions. The figure below shows how each of the four elements of the Kaya Identity have changed from 2000 to 2023.
This brings us to a first fundamental truth about decarbonization:
In order for carbon dioxide emissions to decrease in absolute terms, the decrease in energy and carbon intensities must be greater than the combined increases in population and per capita GDP. More compactly, the rate of decrease in emissions intensity of the economy must be greater than GDP growth.
Although cumulative emissions are directly influenced by population and wealth, we are not going to take steps to cull humanity or intentionally degrow the economy, even though these fringe views are often found in and around climate policy discussions.
That leaves improvements in energy intensity and carbon intensity as the only options for reducing emissions. Let me emphasize that point as a second fundamental truth of decarbonization:
To contribute to reducing emissions, every proposed mitigation policy necessarily must have an effect on either energy or carbon intensity. There are no other options.
Improvements in energy intensity come from two main sources.
One is change in the composition of economic activity. For instance, imagine an economy that consists of two carbon-intensive steel plants, each contributing $1,000 to GDP. One steel plant is shut down and replaced with a much less carbon intensive bank that also contributes $1,000 to GDP. The energy intensity of the steel+bank economy will be much less than that of the steel+steel economy.
A second contributor to improving energy intensity is energy efficiency – getting the same or more economic output from less energy input. Such efficiencies can result from changes in energy consumption or from changes to technologies that consume energy, and often both.
As an example, the figure below shows the significant improvement in fuel efficiency of jet aircraft from1960 to 2015. That improvement in efficiency means that each aircraft can perform the same or better travel services while using less fuel, reducing costs, boosting profits, and contributing to economic growth. Improved efficiencies make good sense independent of climate policies.
One consequence of improved efficiencies however is often increased use of the more efficient technologies or processes. The figure below shows recent and projected air passenger traffic, which is projected to double over the next 25 years. Improved efficiencies can lead to a rebound effect that leads to greater energy consumption and greater absolute emissions, even as the economy decarbonizes.
This brings us to our third and fourth fundamental truths about decarbonization:
To date, the long-term trend in decarbonization has been driven primarily by improvements in energy intensity.
Going forward, as a matter of simple math, if deep decarbonization is to occur, then considerable improvements in the carbon intensity of energy will be necessary.
Improvements in the carbon intensity of energy come from adding low carbon energy consumption or, more significantly for moving toward net zero, reducing or replacing energy consumption from more carbon intensive energy sources with less intensive sources.
This leads to a fifth fundamental conclusion about decarbonization:
Adding low or zero carbon energy consumption will result in a reduction in the overall carbon intensity of energy, however, it does not directly reduce aggregate carbon dioxide emissions. Emissions reductions result from the retirement or replacement of fossil fuel consumption.
Consider coal. In a round number coal is about twice as carbon intensive as natural gas. For producing electricity, coal and natural gas are functionally equivalent. Thus, the carbon intensity of a coal-fired power plant can be cut (roughly) in half by replacing the burning of coal with natural gas.
The carbon intensity of that plant could be reduced to near zero by replacing the burning of coal with nuclear power (or if you prefer, utility-scale solar with batteries).
You can see right away that while natural gas offers the potential for significant cuts in emissions, to the extent that it replaces coal. However, natural gas cannot lead to zero emissions — unless technologies that can capture carbon at scale become technologically and economically feasible.
One reason why mitigation wonks focus on electrification — of cars, heating, stoves, ships, and more — is that we know how to produce vast amounts of electricity with low or zero carbon dioxide emissions.
We can get a good sense of the scale of the challenge of reducing the carbon intensity of energy through some simple numbers: According to the Global Energy Monitor there are almost 2,500 coal-fired power plants in operation around the world, with a total generation capacity of about 2,200 gigawatts.
In a round number, to replace this capacity would require about 1,000 new nuclear power plants (with two AP-1000 reactors each at 1.1 GW capacity).
If coal is responsible for about 40% of total carbon dioxide emissions (another round number), then replacement of coal with nuclear power would reduce global carbon intensity of energy by 40% (you can scroll up to the Kaya Identity decomposition figure above to eyeball what that would look like).
There are about 400 nuclear power plants in operation around the world today — 1000 new ones is thus not an unreasonable number, but their deployment would take many decades.
To Summarize
This post has described five fundamental truths about decarbonization.5 They are:
In order for carbon dioxide emissions to decrease in absolute terms, the decrease in energy and carbon intensities must be greater than the combined increases in population and per capita GDP. More compactly, the rate of decrease in emissions intensity of the economy must be greater than GDP growth.
To contribute to reducing emissions, every proposed mitigation policy necessarily must have an effect on either energy or carbon intensity. There are no other options.
To date, the long-term trend in decarbonization has been driven primarily by improvements in energy intensity.
Going forward, as a matter of simple math, if deep decarbonization is to occur, then considerable improvements in the carbon intensity of energy will be necessary.
Adding low or zero carbon energy consumption will result in a reduction in the overall carbon intensity of energy, however, it does not directly reduce aggregate carbon dioxide emissions. Emissions reductions result from the retirement of fossil fuel consumption.
Comments and questions welcomed!
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I realize that there are many THB readers who are experts in climate and energy. This series is targeted at the vastly larger numbers of THB readers who may not have that level of expertise. Let me know in the comments if this series hits the mark.
There are other greenhouse gases, of course, and as well, important non-GHG influences on the climate system. In this series I focus on carbon dioxide which is projected to be the dominant human-caused climate forcing at the global scale this century. For instance, under SSP2-4.5, Meinshausen et al. 2020 find that 79% of total radiative forcing to 2100 is due to carbon dioxide.
I have long argued that changes in climate resulting from human activities are a risk management problem, and that people will arrive at different and irreconcilable judgments about possible future risks. Some will say there are no risks and others will say that the risks are apocalyptic. You can find me in between these poles — risks are real and worth reducing. A focus on “no regrets” mitigation and adaptation policies takes us beyond debates about future risks.
In this analogy you can think of carbon capture and storage as someone using a bucket to remove water from the tub as it fills and dumping it into another reservoir, thereby slowing or stopping the water level from rising in the tub.
Paid subscribers have access to the full text of The Climate Fix, which offers a much deeper dive into decarbonization.
You state:
One reason why mitigation wonks focus on electrification — of cars, heating, stoves, ships, and more — is that we know how to produce vast amounts of electricity with low or zero carbon dioxide emissions.
I would be careful making that statement: The wind and the sun are renewable, but solar PV cells, wind turbines, transmission lines, etc. are not. The minerals have to be mined to create these things, they have to be processed, the products have to be manufactured, they have to be shipped and installed. All this takes energy. How much energy?
Well, there are a few groups in Europe who are working on this exact problem. The groundbreaking paper by Weibbach et al, 2013 who looked at energy returned over invested (EROI) The authors looked at all type of energy plants and calculated the energy cost of mining the materials, refining the materials, transporting the materials, building the plant, maintaining the plant and decommissioning the plant. They found that for a nuclear power plant you would get 75 times more energy than you invested over the lifetime of the plant. However, the numbers do not look nearly as good for wind and solar. For solar with battery backup in a temperate environment, that number declines to 1.6X.
A later work by Ferroni and Hopkirk 2016, observed that Weibbach et al., 2013 failed to take into account the energy cost of integrating solar energy with battery backup into a grid. When they did that, the number dropped to 0.82X. What this means is that the energy cost of a solar farm with battery backup is greater than the total amount of energy that that farm will produce during its lifetime. Since many of the components of a solar farm come from China, that means that coal and diesel were the primary sources of energy.
We are not reducing our CO2 emissions with the construction of wind and solar farms; we are simply outsourcing them to China. Last I checked the earth has only one atmosphere and what is emitted in China does not stay in China.
Weibbach et al, 2013 https://www.sciencedirect.com/science/article/abs/pii/S0360544213000492
Ferroni and Hopkirk
https://www.sciencedirect.com/science/article/pii/S0301421516301379
The bathtub analogy is fundamentally flawed. The assumption is that overflowing the tub ends up in catastrophic destruction of the house and the inevitable conclusion that you need to avoid overflow at any cost. In the case of CO2 and the atmosphere there is little basis for assuming that increasing CO2 content results in catastrophic destruction. It seems to me that this is just a tactic for positing that there is a cause and effect linkage between CO2 emissions and catastrophic climate change that can be avoided by turning the CO2 control knob.