Climate Confusion

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

Graphs of expected warming for ECS = 3 and another graph showing measured warming up to 2021

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 CO2, the relationship between atmospheric CO2 concentration and equilibrium temperature, ΔTeq, is ΔTeq=3log(CO2/280)/log(2). Table 1 summarizes some typical values of ΔTeq vs CO2.

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

CO2 [ppm] ΔTeq [ºC]
350 1.0
400 1.5
445 2.0
500 2.5
560 3.0

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.

Warming in the pipeline due to difference between equilibrium and committed temperature anomalies

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.

Posted by Evan on Monday, 27 June, 2022

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