Stop emissions, stop warming: A climate reality check
Why future warming isn't as "locked in" as you might think
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:
The rate at which CO₂ is removed from the atmosphere (through absorption by oceans and land).
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:
Fantastic article! It’s refreshing to see such a balanced perspective, combining hope with the urgency of the challenge. The way you break down complex topics into digestible insights makes it accessible for a wide audience. Thank you for shedding light on such an important issue.
This description of what would happen if we stop emissions of CO2 oversimplifies the problem. The Arctic permafrost is melting and releasing methane in ever increasing amounts. If you stop CO2 emissions, a huge amount of permafrost melting is already committed so these methane emissions will continue to be emitted for decades. Additionally, there is only a finite amount of CO2 that the oceans can absorb and if Co2 concentrations in the atmosphere go down, the CO2 in the oceans will begin to be emitted to remain in equilibrium. Finally, the analysis ignores the heating concentrations of other greenhouse gases. The analysis completely fails if you miraculously stop CO2 emissions but don't stop methane from farms, rice paddies, and other sources, or the increasing CFCs, HCFCs and HFCs from the existing and increasing refrigeration in air conditioning particularly in the emerging economies like China, India, Brazil, Indonesia ...