Arguments
Stop emissions, stop warming: A climate reality check
Posted on 18 December 2024 by Guest Author
This is a re-post from The Climate Brink by Andrew Dessler
One of the most important concepts in climate science is the idea of committed warming — how much future warming is coming from carbon dioxide that we’ve already emitted.
Understanding the extent of committed warming is vital because it informs our current climate situation. If there is a significant amount of committed warming already “locked in,” then we have much less ability to avoid the levels of warming that policymakers judge as dangerous.
In a previous post about what made me optimistic about the climate problem, I wrote:
When humans stop emitting greenhouse gases into the atmosphere, the climate will stop warming.
I received emails and comments from people who found that difficult to believe, so I thought I’d write a post about why this is true and shed light on the reasons behind the controversy surrounding it.
the 2000s
To understand why people are so confused about this, let’s step back to the 2000s. In the IPCC’s fourth assessment report (published in 2007), committed warming was defined to be:
If the concentrations of greenhouse gases and aerosols were held fixed after a period of change, the climate system would continue to respond due to the thermal inertia of the oceans and ice sheets and their long time scales for adjustment. ‘Committed warming’ is defined here as the further change in global mean temperature after atmospheric composition, and hence radiative forcing, is held constant. (from box TS.9)
Consider this simple example: humans emit CO2 until the year 2010, when the atmospheric concentration of CO2 reaches 400 ppm. After that point, the concentration of CO2 is held fixed at 400 ppm in perpetuity, as are all other components of the atmosphere (methane, aerosols, etc.).
In this scenario, maintaining a fixed atmospheric composition is analogous to setting a thermostat at a constant set point for the Earth's climate system. This is the resulting trajectory:
As you can see, the climate continues to warm well after concentrations are fixed (the vertical dashed line). The reason is the immense thermal inertia of the ocean. In much the same way that it takes a very long time for a hot tub filled with cold water to warm after you set the heater, the oceans will take a very very long time to fully warm to reach equilibrium with the fixed atmospheric composition.
a better understanding of cessation of emissions
In the late 2000s, scientists recognized that this was not the right way to think about this problem. The abstract of this 2010 paper says:
The perception that future climate warming is inevitable stands at the centre of current climate-policy discussions. We argue that the notion of unavoidable warming owing to inertia in the climate system is based on an incorrect interpretation of climate science. Stable atmospheric concentrations of greenhouse gases would lead to continued warming, but if carbon dioxide emissions could be eliminated entirely, temperatures would quickly stabilize or even decrease over time.
These two emissions scenarios are demonstrated in this figure:
The blue line shows the emissions time series for constant concentrations (e.g., holding CO2 fixed at 400 ppm). The red line shows emissions going entirely to zero. This can also be achieved by reducing emissions of CO2 as much as possible and then balancing any remaining emissions with CO2 removal (e.g., direct air capture), which is referred to as “net zero”.
Under a zero emissions scenario, temperatures stop rising after emissions cease, which is quite different from the scenario with fixed atmospheric concentration:
The reason is that, when emissions stop, atmospheric CO2 will begin to decline as it is absorbed by the ocean and land biosphere. This in turn reduces heating of the climate system — e.g., turns down the heater on the climate hot tub.
The figure below shows what happens when emissions stop in year zero of a set of global climate models. The left panel shows atmospheric abundance of CO2 starts to decline as soon as we stop emitting CO2. After 100 years, CO2 has dropped around 100 ppm.
The right panel shows temperature after emissions stop. Some models show a few tenths of a degree of warming and others a few tenths of a degree of cooling. However, the central estimate is that the global average temperature does not change much once emissions stop.
About 10 years ago, this was formalized in a nice way by Ricke and Caldeira, who showed that the maximum heating from a slug of CO2 emitted to the atmosphere occurred about a decade after you emit it. After that, enough of the slug has been removed from the atmosphere that the heating from the slug starts to decline (but the fall off is slow and it continues to heat the climate for a very long time).
Thus, climate scientists no longer routinely talk about committed warming in the same way they did in the 2000s because it’s not as relevant a quantity as previously thought. This is very good news.
Hansen
So why do people argue so much about this? I think that Jim Hansen’s recent paper, Global Warming in the Pipeline, is responsible for a lot of the confusion1.
In the abstract, they write:
Equilibrium global warming for today’s GHG amount is 10°C, which is reduced to 8°C by today’s human-made aerosols.
In other words, if we maintain today’s atmospheric amounts of greenhouse gases until the system reaches equilibrium (many thousands of years), we would get an enormous amount of warming. Their estimate is based on a climate sensitivity that is higher than most other estimates (4.8C), which I think it unlikely, but it’s not an entirely unreasonable calculation.
But then things go south. They write that “Equilibrium warming is not ‘committed’ warming”. This is confusing since these are literally the same thing: the IPCC defined committed warming to be the equilibrium warming from the constant atmospheric composition.
But the next sentence clarifies what they mean: “rapid phaseout of GHG [greenhouse gas] emissions would prevent most equilibrium warming from occurring.” Thus, we are not actually committed to this 8-10C of warming. If we can reduce emissions, then we can avoid most of this warming.
I think this is where the confusion arises. Most people read Hansen’s paper as saying that we are already committed to 8-10C of warming and there’s nothing we can do about it, but that’s clearly a misreading of the paper.
The sooner we can get emissions to (net) zero, the sooner we stabilize our climate system. And, as I said in this post, we have the technology to largely do that today. Whether we do so or not is a political decision, not a technical or scientific decision.
Nerd time
[skip this section if you don’t want a more technical description]
In case you want a slightly nerdier description of why temperatures stop rising, you’re in luck. When we stop emitting greenhouse gases, the future behavior of Earth’s temperature depends on the relative speed of two critical processes:
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The rate at which CO? is removed from the atmosphere (through absorption by oceans and land).
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The rate at which heat is transferred from the surface ocean (a layer with small heat capacity) into the deep ocean (which has very large heat capacity).
To illustrate this, consider two extreme scenarios:
Scenario 1: CO2 removal is very slow, ocean heat transfer is very fast. Under this scenario, the Earth’s temperature would continue to rise long after emissions stop. This happens because the persistent CO2 in the atmosphere would keep trapping heat, while rapid heat mixing into the deep ocean would increase the effective heat capacity of the surface ocean. A higher surface heat capacity means it would take longer for the surface ocean to reach equilibrium for any given CO2 level. Thus, when emissions cease, the surface layer is much cooler than equilibrium with atmospheric CO2 and it must warm over the following centuries to reach equilibrium.
Scenario 2: CO2 removal is very fast, ocean heat transfer is very slow. Under this scenario, CO2 levels drop quickly after emissions stopped. The slow mixing limits the heat capacity of the surface ocean, causing the temperature of the surface layer to remain very close to equilibrium with atmospheric CO2 levels. As CO2 concentrations decrease, the surface ocean will therefore cool to stay near equilibrium.
In our actual climate system, these two processes happen at roughly comparable rates. The cooling effect of declining CO2 levels tends to offset the warming caused by heat transfer into the deep ocean. This balance means that, after emissions stop, global temperatures are expected to remain relatively stable instead of significantly rising or falling.
Other stuff
Twitter/X is dead for climate science. If you were someone who went to Twitter to read about climate, head over to Bluesky — most of the climate community I engage with are and, as an added bonus, it’s not a Nazi bar. Read more about Bluesky on Andrew Rumbach’s recent substack post. I’m almost exclusively on bluesky; you can find me here.
If you can’t get enough of my perspectives on climate and energy, I gave a “fireside chat” at the Texas A&M Innovation Forward conference a few weeks ago. You can watch it here:
Although I agree with the theoretical aspects of no warming after reaching net-0 emissions, the danger I see with the underlying message in this paper is that we are broadcasting the concept that the future is in our hands. This is not only an arrogant position, but may backfire. The average person is not reading SkS and is not grounded in legitimate climate science, but may be getting a fuzzy, positive feeling when they see the number of solar panels, wind turbines, and EVs on the road increasing. They see what looks like great progress deploying renewable energy and EVs, and therefore conclude wrongly that we're decreasing CO2 emissions, and so now can relax and rest assured that the future will be fine. As long as they continue to see the deployment of renewable energy projects and EVs, they are satisfied that we are doing what is needed. Now that we've got the climate back on track, let's go elect leaders to get the economy back on track.
This at a time when CO2 concentrations are increasing at a rate of 2.5 ppm/year and fossil-fuel use continues to increase year after year.
In my opinion it will never work to broadcast that the future is in our hands and that we just need to get to Net-0 emissions to stabilize the climate. The message is arrogant and really just a concept that we cannot possibly hope to effectively quantify. In my opinion, achieving it will require more than we've ever demonstrated we're capable of.
I hope my opinion is wrong!
Having said this, I'm still not sure what the best messaging is. I think what SkS is doing is critically important because it is helping people understand what is happening and why. So I offer my comments in an effort to put the message of this paper into context and to temper what I see as an overly optimistic message.
The message that I prefer to give people is this.
"The current CO2 concentration is 420 ppm. That concentration is sufficient to warm the planet to 1.7C if we don't bring it down. Every time we emit CO2 we are actively destroying Earth's life-support systems. We need to reduce our greenhouse gas emissions as fast as possible and to support local, national, and global initiatives that do that."
This is a message that is consistent with climate science and refers people back to the Keeling Curve to monitor how we're doing. If 420 ppm is enough to take us to 1.7C, then anything higher will take us to a higher temperature. The message is also that any level of emissions is bad, and that we need to do all that we can to reduce GHG emissions.
My understanding is that the committed / equilibrium warming is apparently zero, provided emissions drop abruptly to zero in some given year, as opposed to trailing off slowly (?). But it very unlikely that emissions will abruptly drop to zero, so for all practical purposes we have some committed warming!
However assuming purely for the sake of argument that the committed warming is Hansens 10 degrees, that will take many thousands of years to evolve and so won't affect humanity significantly for a very long time, so its not a reason for us to give up on reducing emissions.
I hear what Evan is saying. There is perhaps also a tendency for people to assume things like emissions trading schemes or carbon taxes are fixing the problem when they are not doing this adequately. However counter balancing this people must also be aware progess reducing emissions is going too slowly, given its been in the media often enough. So I'm not sure that too many people would assume the problem is being adequately solved.
Evan says "The message is also that any level of emissions is bad, and that we need to do all that we can to reduce GHG emissions."
Exactly.
Skeptical Science asks that you review the comments policy. Thank you.
[snip]
TODAY!......... Climate Shift. ..??.. DID the Climate Fanatics of the 1970’s CAUSE the Climate Crises of Today?
Funny as the ''Climate Craze'' back in the 1970's was the New Ice Age..... Yes ''they'' said that Pollution (partials) were being thrown up into the upper atmosphere and causing the suns light to be reflected back into space., This was causing a New Ice Age to destroy the earth.
(Scientists say sprinkling diamond dust into the sky could offset almost all of climate change so far — but it'll cost $175 trillion)
Story by Sascha Pare 12-19-2024
Today those same people (Rainmakers) are selling yet another climate ''Crises''.
Note; To STOP this New Ice Age, the USA went 'seriously' into protecting the 'Environment' way back in the 1970's with President Nixon signing the Environmental Protection Act (EPA) into law.
Today the USA only produces as much steel as it did in 1950, this is as an example of those EPA efforts.
High Gas Prices?.. EPA will Not allow a New Oil refinery to be built in America.
And this is also a major reason for the loss of Millions of very good paying jobs, I might add. 'clean', comes with a very steep 'price'.
Even IF the ‘Clean’ is ONLY here and all that pollution was just Moved to China, along with all the Jobs.
Good thing we don’t use the same Air as the Chinese. Otherwise it would ALL have been a waste of time and Money.
TODAY!......... Climate Shift. ..??.. DID the Climate Fanatics of the 1070 CAUSE the Climate Crises of Today?
(Scientists say sprinkling diamond dust into the sky could offset almost all of climate change so far — but it'll cost $175 trillion)
Story by Sascha Pare 12-19-2024
Sprinkling diamond dust into the atmosphere could offset almost all the warming caused by humans since the industrial revolution and "buy us some time" with climate change, scientists say.
New research indicates that shooting 5.5 million tons (5 million metric tons) of diamond dust into the stratosphere every year could cool the planet by 1.8 degrees Fahrenheit (1 degree Celsius) thanks to the gems' reflective properties. This extent of cooling would go a long way to limiting global warming that began in the second half of the 19th century and now amounts to about 2.45 F (1.36 C), according to NASA.
The research contributes to a field of geoengineering that's looking for ways to fight climate change by reducing the amount of energy reaching Earth from the sun.
https://www.msn.com/en-us/weather/topstories/scientists-say-sprinkling-diamond-dust-into-the-sky-could-offset-almost-all-of-climate-change-so-far-but-it-ll-cost-175-trillion/ar-AA1w6MuP?ocid=msedgntp&pc=HCTS&cvid=2dfb5c2f1669448799854ec819ce98bf&ei=43
[BL] Apart from getting facts wrong, this is essentially yet another uninformed political rant.
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AdriantheHistorian said: "Today the USA only produces as much steel as it did in 1950, this is as an example of those EPA efforts."
Not necessarilly. This is from "History of the iron and steel industry in the United States" on Wikipedia: "US production of iron and steel peaked in 1973, when the US industry produced a combined total of 229 million metric tons of iron and steel. But US iron and steel production dropped drastically during the recession of the late 1970s and early 1980s. From a combined iron and steel production of 203 million tons in 1979, US output fell almost in half, to 107 million tons in 1982. Some steel companies declared bankruptcy, and many permanently closed steelmaking plants. By 1989, US combined iron and steel production recovered to 142 million tons, a much lower level than in the 1960s and 1970s. The causes of the sudden decline are disputed. Among the many causes alleged have been: dumping of foreign imports below cost, high labor costs, poor management, unfavorable tax policies, and costs of environmental controls."
It seems most likely that the EPA contributed to a relatively small part of the stagnation in steel production if anything. I think is a price worth paying to look after the environment and have clean air and water and so forth. It's a values issue.
AdriantheHistorian @4,
In addition to reviewing the comments policy, the SkS home page helpfully offers 3 big boxes near the top for people who are relatively unaware or lack a reasonable understanding of the issue:
A very helpful part of the Newcomers, start here page (linked here) is the section: Good starting points for newbies.
After becoming more familiar with the issue you should understand and appreciate the lack of legitimacy, lack of merit, lack of value, of the beliefs you shared in your comment @4.
Obvious questions about constantly pumping massive amounts of diamond dust, or other materials, into the atmosphere are:
Since my opinion is in agreement with Evan, I too hope we are both wrong.
"When humans stop emitting greenhouse gases into the atmosphere, the climate will stop warming." Not just keep co2 emissions from increasing but stop any GHG pollution entering our air.
Here is part of why I think "committed warming" is the real world norm.
Take for example the USA,www.wri.org/insights/interactive-chart-shows-changes-worlds-top-10-emitters while its per capita co2 emissions has peaked, historically this one country has put 25% of the co2 in our atmosphere! It is number 2 in the current yearly co2 emitters and to get some perspective-this is the equivalent to the mass of 6,300+ small cars x a million in this year alone!
The worst top three add 46% of climate change pollutants with the worst 10 making this amount to over 66%.
I'm tired of the hopium of scaled co2 "scrubbers", of a wake-up of humanity and forcing our leaders to think decades ahead and to get the transition moving more quickly.
The map for 2017 ourworldindata.org/contributed-most-global-co2 shows the large inequalities of contribution across the world that the first treemap visualization has shown. The USA has emitted the most to date: more than a quarter of all historical CO2 — twice that of China, which is the second largest contributor.
In contrast, most countries across Africa have been responsible for less than 0.01% of all emissions over the last 266 years.
What becomes clear when we look at emissions across the world today is that the countries with the highest emissions over history are not always the biggest emitters today. The UK, for example, was responsible for only 1% of global emissions in 2017. Reductions here will have a relatively small impact on emissions at the global level – or at least fall far short of the scale of change we need. This creates tension with the argument that the largest contributors in the past should be those doing the most to reduce emissions today. This is because a large fraction of CO2 remains in the atmosphere for hundreds of years once emitted.3
This inequality is one of the main reasons why it’s so challenging to find international agreement on who should take action.
I think future warming is inevitable because of our flawed human nature..