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:

temperature change over time; atmospheric concentration is held fixed starting in 2010, at the dashed line. adapted from Damon Matthews plot.

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:

from presentation by Damon Matthews

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:

temperature change over time for two different scenarios; the solid yellow line shows emissions ceasing in 2010, at the dashed black line. the red dashed line shows constant atmospheric concentrations beginning in 2010. adapted from Damon Matthews plot.

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.

Changes in (a) atmospheric CO₂ concentration and (b) evolution of global surface air temperature (GSAT) following cessation of CO₂ emissions in year zero. Individual models are the gray lines, the multi-model mean is the black line. From Fig. 4-39 of the IPCC AR6 WG1 report.

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).

Figure 1 of Ricke and Caldeira, 2014. Temperature increase due to emission of carbon dioxide (CO₂). Time series of the marginal warming in milliKelvin = 0.001 K) per GtC (=10¹⁵ g carbon) as projected by 6000 convolution-function simulations for the first 100 years after the emission. Maximum warming occurs a median of 10.1 years after the CO₂ emission event and has a median value of 2.2 mK GtC⁻¹. The colors represent the relative density of simulations in a given region of the plot.

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:

1. The rate at which CO? is removed from the atmosphere (through absorption by oceans and land).

2. 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:

Related posts

Warming in the pipeline: Decoding our climate commitment