The world after net-zero: Part 1
Is there another tunnel at the end of the tunnel ?
Everybody has heard about “net-zero emission”, the answer to the current climate crisis. It is the state at which all produced greenhouse gases are removed from the atmosphere via natural and artificial sinks.
However, one must wonder what will really happen the day we reach that goal. Are human saved ? Would we be able to crawl out of our bunker right away ? (unless you manage to score a place on Mars with Elon and Jeff, underground seems like the only affordable option left).
As this is a complex topic touching different facets of the natural ecosystem (ocean, land, vegetation,…), in this series we focus on the most fundamental metric: the global surface temperature.
The (very) hot frying pan
Different theories circulate in the media about the post zero-emission scenarios. For some people, the surface temperature will keep raising up because of the already accumulated emissions. Others are convinced that, just like a hot pan after cooking, the Earth will gradually cool down.
Even if we adhere to the latter optimistic view, as we are talking about a 4.5-billion-year-old rock, the notion of “gradually” can be, hm, pretty long. In the meantime, irreversible events like melting glaciers and raising sea levels will continue. Therefore if the improvement only happen at centuries or millennia timescales, we won’t leave our bunker any time soon.
There are thus two important questions once net-zero is reached:
- How will the surface temperature evolve ?
- How long will it take for the earth to reverse all the previous warming caused by human ?
We will reason around the notion of Zero Emissions Commitment (ZEC) [4]. It is defined as the amount of global surface temperature change that is still expected to occur after a complete cessation of net CO2 emissions.
The results presented here are based on the recent work of MacDougall, Andrew H., et al. [1].
18 models to rule them all
In order to answer the questions above, the authors bring together 18 climate simulation models to compare how they behave under different emission pathways and different cumulative CO₂ levels reached before net-zero emissions are achieved.
Emission pathways can be understood as the pattern of the emission before reaching net-zero.
In the paper they employ two pathway scenarios:
- Type A (left): the emissions raise with a 1% annual rate to a desired level of cumulative CO2 concentration (750/1000/2000 Petagrams of Carbon, PgC). Then the zero-emission event kicks in abruptly.
- Type B (right): the emissions follow a bell curve pathway and gradually decline to the zero-emission point. Once again, they simulate the scenario with three different thresholds of cumulative CO2 concentration. This is what’s more likely to happen in practice.
Baby It’s (finally) Cold Outside ?
750 PgC of cumulative emissions roughly corresponds to a warming of 1.5°C, an objective that’s quickly getting out of reach [5]. Therefore we can expect that the day zero-emission is reached, the cumulative emissions would be closer to 1000 PgC. Let’s examine the results of the 1000-PgC simulations.
We first take a look at this graph of projected ZEC type A vs type B in the 200 years following the zero-emission events according to different climate models.
As the type B bell-shape-emission scenario is closer to the reality, we will focus on them (dashed lines):
- There are significant variations among model’s results, but in general the average temperature shows a small gradual declining trend. We are far from reversing the 1°C of warming in the first two centuries though.
- In the first few decades right after the cessation of net CO2 emissions, the global temperature will likely stabilise (an estimated ZEC close to O°C).
- To note that a few models (GFDL ESM2M, LOVECLIM 1.2) give extremely unstable ZEC projections with periods of warming and cooling alternating.
We dwell further into the simulation results at three specific “short-term” dates: 25 years, 50 years and 90 years after the cessation of net CO2 emissions.
On average, in the first century after the net-zero milestone, the climate models expect a slight decline in the range of [0.01°C; 0.1°C] with a standard deviation around 0.2.
Depending on the use case, these orders of magnitude might be considered quite significant. Take for example the situation where we want to leverage this study of ZEC to compute the carbon target to stay under 2°C. Given that we have burnt through around 1.07 °C of warming [5], an uncertainty in ZEC of ± 0.2°C already leads to a substantial variation in the estimation of the remaining carbon budget. It is the equivalent of 120 PgC of CO2 emissions, or 3 to 4 years of current emission rates [6].
Need for Speed
One final important piece of information is the degree of gravity if we reach net-zero at 2° instead of at 1.5°. In others words, we want to study the relationship between the ZEC and the cumulative emissions.
The graph below shows the estimated ZEC of different climate models in the 50th year after the cessation of net CO2 emissions with three levels of cumulative CO2.
Most models agree on the general directions:
- For a cumulative emissions in the range of 750 to 1000 PgC, the temperature will stabilise with an estimated ZEC around zero as we have already seen in the previous section.
- If we wait till 2000 PgC to reach zero-emission, the global surface temperature will probably continue to increase for at least the first 50 years.
Every percent counts
In this first blog post of the series, we have discussed the results of an extensive study on the evolution of surface temperature after the cessation of net CO2 emissions. Without ignoring the variations in the model’s responses, we can conclude that if net-zero is reached soon enough, the surface temperature will immediately stabilise in the next few decades before starting to decline. Otherwise, more committed warming might be locked in.
This again calls for urgent action from nations around the world. We need a rigorous and actionable plan, something much better than what these future french leaders propose.
Because every freaking percent of a degree of warming counts.
References
- MacDougall, Andrew H., et al. “Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO 2.” Biogeosciences 17.11 (2020): 2987–3016.
- Rogelj, Joeri, et al. “Estimating and tracking the remaining carbon budget for stringent climate targets.” Nature 571.7765 (2019): 335–342.
- Allen, Myles, et al. “IPCC special report: global warming of 1.5 C.” Intergovernmental Panel on Climate Change (2018).
- Jones, Chris D., et al. “The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) contribution to C4MIP: quantifying committed climate changes following zero carbon emissions.” Geoscientific Model Development 12.10 (2019): 4375–4385.
- Allan, Richard P., et al. “IPCC, 2021: Summary for Policymakers.” (2021).
- Friedlingstein, Pierre, et al. “Global carbon budget 2020.” Earth System Science Data 12.4 (2020): 3269–3340.
