Embracing Overshoot

In the context of climate change, overshoot is the idea that we might temporarily exceed a temperature target. For instance, warming might peak at 1.8°C and then subside to 1.5°C. In this scenario, we have settled at 1.5°C but only after overshooting to 1.8°.

It's a controversial topic. Proponents argue that overshoot provides flexibility: landing at 1.5° will be easier if we allow for scenarios where we exceed that level temporarily. Detractors argue that this provides an excuse for inaction: if we can mitigate emissions later, there's less urgency to reduce them now. Also, cooking the planet on a temporary basis is still cooking the planet, and not all of the damage will be reversible.

In this post, I will argue that both sides are correct, but regardless of how we feel about it, overshoot is inevitable. More precisely: our emissions peak, whatever it might be, will be followed by a period of net-negative emissions. I don't know whether we will overshoot 1.5°C (though I fear we will), but I do argue that by anticipating the net-negative future, we can turn it to our (planet's) advantage. In particular, we should embrace short-term solutions if they get us to net zero sooner, confident that longer-term solutions will be around the corner.

Disclaimer: I am not a climatologist, or any other sort of ologist. I'm just trying to wrap my head around the climate challenge and find opportunities for effective action. Corrections, reactions, and feedback are welcomed! Drop me a line at steve [at] snewman [dot] net.

Overshoot is bad

If you define overshoot as "temporarily exceeding a given warming target", and compare it to "never exceeding that target", then obviously the latter is preferable. Temporary harms are still harms; and not all harms will be temporary. If warming exacerbates a flood that costs me my home, and later temperatures come back down, my misfortune is not reversed.

Overshoot is good

If you define overshoot as "temporarily exceeding a target", and compare it to "permanently exceeding that target", then obviously the former is preferable. If we fail to hold the peak to (say) 1.5°C, we can still aim to come back down to that level, or lower. A few years of excess storms, floods, fires, heat waves, and drought is preferable to an eternity of the same.

I am not a number, I am a planet

The contradiction stems from trying to encapsulate warming in a single number. Climate follows a curve. The curve will have a peak, but the peak is not the whole story.

To make this clear, consider the following (exaggerated) scenarios:

The Overshoot scenario is clearly preferable, despite peaking much higher than the Flat scenario. These exact scenarios are not plausible; I have exaggerated for effect. The point is that it is a drastic oversimplification to use a single number to characterize the multi-decade trajectory of a planet.

What goes up will come down

The day will come when we achieve net zero greenhouse emissions. What happens the next day?

Getting to net zero is not like raking leaves, where you aren't finished until you've laboriously chased down the last straggler. We're not going to creep up on it asymptotically, eliminating smaller and smaller emissions sources until the last one, a wood-fired stove in some rural hut, is gone. Instead, we're going to plow through net zero like a freight train.

The key is the term net zero. The coming years will see both a reduction in emissions, and an increase in removal – taking greenhouse gases out of the atmosphere. Net zero will arrive when the rate of removal equals the rate of remaining emissions. To understand what that looks like, we need to consider the components separately, and then consider how they add up.

Gross emissions will probably decrease according to an S curve. In the short term, as societal impetus builds and technological options increase, we hope to see emissions flatten, and then start to come down at an accelerating rate. Eventually, the curve will flatten (gross emissions can never drop below zero), creating the second half of the S.

Meanwhile, removal of CO2 from the atmosphere will accelerate, again due to improvements in technology coupled with increased societal impetus. Putting these trends together, and we get a trajectory like this:

These are made-up figures, but they illustrate the idea. As an increasing amount of CO2 removal comes on line, and emissions decrease, the net rate of change in atmospheric GHGs smashes through zero and plows into negative territory. The effect holds up across a range of assumptions regarding the rate at which we reduce emissions and/or increase CO2 removal.

The trends that are necessary to bring us to net zero will propel us straight on through to negative emissions. Net zero is an aggregate property of the myriad processes that drive emissions; it does not represent any special milestone for those processes individually, and we should not expect an inflection point.

It has been noted that we always overestimate the change that will occur in the short term and underestimate the change that will occur in the long term. Getting to net zero will be harder than we think; the aftermath will be better than we expect.

Some details

Various natural processes pull greenhouse gases out of the atmosphere. The ocean absorbs CO2, atmospheric methane degrades, etc. As our emissions decrease, these compensating processes will also slow. Presumably there is some lag, meaning that for some time after Net Zero Day, natural processes will continue to tug GHG levels down.

As we approach net zero, the societal impetus to reduce net emissions may decrease. However, global society overshoots everything. Just as we were slow to react to the reality of climate change, we will be slow to react to the reality of net zero emissions. New projects will still be in the works, laggards will continue to deploy new clean technologies simply because they are cheaper, pollution concerns unrelated to climate will continue to drive green technologies. And once it becomes apparent that we have the ability to bring CO2 levels down toward pre-industrial levels, there will be impetus to do so, especially if technological developments continue to reduce the cost.

A new paper from a group at Oxford provides an idea of what this might look like. It projects a rapid continued fall in the cost of renewable energy. During times of (near-)peak sunlight, there will be ample amounts of near-free energy available to drive carbon removal. Matthew Yglesias recently touched on this in a column discussing applications for the coming glut of cheap renewable energy.

When we reach net zero emissions, global temperatures will continue to rise for a while. Ocean surface temperatures provide a certain amount of inertia to the system, causing temperatures to lag behind where they "ought" to be for a given level of greenhouse gases. Once GHGs plateau, temperatures will continue rising until they reach the equilibrium point. A 2020 article on climate.gov, cites an IPCC report in saying that this might add another 0.5°C. Presumably, negative net emissions could counteract some of this "momentum" warming.

Implications

The prospect of negative net emissions has interesting, actionable implications.

It's fine to include short-term solutions in our path to net zero; for instance, planting trees even if they're only likely to hold onto their carbon for a few decades, or temporarily employing solar radiation management to counteract warming. By the time the short-term solutions give out, we'll have made further progress elsewhere. Solutions with a longer time horizon will have a place consolidating our gains post-net-zero, but for now we should focus on whatever gets us over the line.

Any sacrifices made in service of net zero will be short-lived. We'll be able to gradually replace less-desirable solutions (high cost, high land use, etc.) with more-desirable solutions.

Conclusion

We may or may not overshoot any particular warming target. What is inevitable is that our warming peak, whatever it might be, will be temporary, and we should plan accordingly. We should be pessimistic about the path from here to net zero, avoiding complacency, and rely on whatever mix of solutions – including short-term workarounds – gets us there sooner. We should be optimistic about the subsequent future, and anticipate that we will be able to consolidate our gains and move on to negative net emissions with much less effort than was required to reach net zero.