Posted on 27 June 2022 by Evan

Climate Confusion

I periodically see the phrase, "when we reach net-zero emissions," as though it's a foregone conclusion. It is not. What if the best we can do over the next 100 years is no better than stabilizing CO₂ concentration. What then?

Current climate models indicate that future warming is a function only of future emissions, and not current atmospheric CO₂ concentration (read here). However, if CO₂ stabilization is the best we can do, then the minimum warming we will experience is defined by current CO₂ concentrations. These two views are compatible, because to stabilize CO₂ concentration at current levels and hold it there over the next 100 year requires some level of continuing emissions. CO₂ stabilization therefore implies some level of future emissions that would be unavoidable. A world where the best we do is to stabilize CO₂ has, for all intents and purposes, "warming in the pipeline", something that does not occur if and when we reach net-zero emissions.

Some people seem so confident that we will achieve net-zero emissions that they no longer consider current CO₂ concentrations to represent a minimum commitment temperature. This is dangerous. Considering that as of 2022 CO₂ is still accelerating upwards, it is prudent to consider what happens if the best we can do is CO₂ stabilization.

Here is the specific event that prompted me to write this piece.

I recently used Fig. 1 in one of my posts. The top curve shows the expected warming corresponding to measured atmospheric CO₂ concentrations for an Equilibrium Climate Sensitivity (ECS) = 3ºC/doubling CO₂. The bottom curve shows measured temperature anomalies. The dotted line labeled "Ocean Time Lag" indicates that the oceans delay the warming because of the time required to heat up the top layers of the oceans. This plot shows measured data only: there are no modeling results, other than showing expected warming based on ECS = 3ºC/doubling CO₂. 

Figure 1. Top curve (solid orange points) shows expected warming corresponding to atmospheric CO₂ concentration (shown by the open circles) and an ECS of 3ºC/doubling CO₂. The bottom curve shows the GISS measured Land-Ocean temperature anomalies (read here).

In response, a reader commented

No... although the ocean lag is plausible and is still often cited, it was questioned a decade ago and is now much in doubt...

I believe the commenter was well intentioned, because they cited a credible source by Zeke Hausfather (read here), but drew the wrong conclusion about what it says about climate science. There are three important physical principles that I would like to emphasize. No modeling study I'm aware of has modified the efficacy of the following.

Principle 1: For given CO₂ concentration, there is a corresponding equilibrium temperature that indicates when the Earth is in energy balance.

Principle 2: If the current temperature is lower than the equilibrium temperature, Earth will warm until the two temperatures are equal.

Principle 3: The oceans delay the time to achieve energy balance due to the time required to warm the oceans.

Principle 3 is often embodied as the concept of "warming in the pipeline" associated with the time to warm the oceans. For ECS = 3ºC/doubling CO_2, the relationship between atmospheric CO₂ concentration and equilibrium temperature, ΔT_eq, is ΔT_eq=3log(CO₂/280)/log(2). Table 1 summarizes some typical values of ΔT_eq vs CO₂.

Table 1. ΔT_eq vs CO₂ concentration, rounded to the nearest 0.1ºC.

Figure 2 shows one way to visualize these three physical principles. According to recent climate models (read here), if and when we achieve, and indefinitely maintain, net-zero emissions, the situation is straightforward: atmospheric temperatures stabilize at that point and there is no significant change in temperatures for centuries. The more complicated situation occurs if we do not achieve net-zero emissions, but that the best we do is to stabilize CO₂ at some level. Even though stabilizing CO₂ at current levels is "easier" than achieving net-zero emissions, stabilizing CO₂ is itself very difficult. By writing about these two pathways, I am not implying anything about the likelihood that either will be achieved.

Figure 2 shows the hypothetical situation where CO₂ is stabilized at 2022 levels for about 100 years. To stabilize CO₂ concentrations, Hausfather indicates that GHG emissions must initially drop about 70%. To maintain stable CO₂ concentrations, GHG emissions must decrease to 0 over about 100 years and then remain at net-zero thereafter. Indefinitely. The exact profile by which emissions must decrease to maintain stable CO₂ concentrations is complicated and not important for this discussion. What matters is that if our emissions over the next 100 years result in stable CO₂ concentrations, then for all intents and purposes, there is "warming in the pipeline" of a magnitude represented by the difference between the equilibrium and current temperatures. Think of this temperature difference as a spring, pulling the current temperature up to the equilibrium temperature.

Figure 2. Temperature trajectory assuming atmospheric CO₂ stabilization near 2022. To achieve this trajectory, CO₂ emissions must first drop about 70% and then continually decrease until they reach net-zero emissions about 100 years later. About 100 years are required for the temperature to stabilize because of the time required to warm the oceans.

Dangerous Confusion

The problem is that many people, including, I believe, the commenter to my post, believe so strongly that we will achieve net-zero emissions, that any talk about levels of "warming in the pipeline" due to current atmospheric CO₂ levels is old science and incorrect. The path where future emissions allow us to cap warming at current temperatures is very difficult. Although we can talk boldly about what future we will "choose", this is not like choosing what clothes to wear or what to have for dinner. A much more likely emissions pathway is one where current equilibrium temperature anomalies represent minimum, committed temperatures.

The pragmatic point is this. As long as CO₂ concentrations are increasing, by any amount, for all intents and purposes, there is warming in the pipeline. Even if we manage to stabilize CO₂ at some level, there will still be warming in the pipeline for at least 100 years. And to maintain that level of CO₂ indefinitely, we will have to reach net-zero emissions at the end of that 100-year period.

If we do not achieve net-zero emissions, there will always effectively be "warming in the pipeline", delayed by the time required to warm the oceans.