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Net-Zero Does Not Mean What You Think it Does
A Guest Post from Climate Scientist Tom Wigley
Tom Wigley is a giant of the climate science community. He is one of the most cited scientists in IPCC reports and has contributed over the past 30 years to IPCC Working Groups 1, 2 and 3. Tom and I were colleagues at the National Center for Atmospheric Research from 1994-2001 and, along with Chris Green, published a widely cited paper in 2008 on assumptions of spontaneous decarbonization in IPCC scenarios. I am thrilled to have Tom here at THB.
Since the 2015 Paris Agreement on climate, the term ‘net-zero emissions’ has become a much-used phrase and a buzzword for an aspirational policy target. But the term is frequently misused. Neither net-zero greenhouse gas (GHG) emissions, nor net-zero carbon dioxide (CO2) emissions mean that carbon dioxide emissions need to drop to zero.
Over the years since the first IPCC report in 1990, there have been multiple sets of emissions scenarios produced by the Integrated Assessment Modelling (IAM) community, with the most commonly-used ones listed in the table below, which use a wide range of different socioeconomic assumptions.
A common feature of all of these exercises is that they are designed for forward calculations —That is, emissions to concentrations to forcings to climate change. They have two primary applications:
To drive coupled gas-cycle/climate models to project future changes in climate;
More recently, to assess the emissions trajectories consistent with meeting the 1.5°C and 2.0°C3 global-mean warming targets of Article 2 of the Paris Agreement on climate change.
What is somewhat alarming is that the warming projections for many emissions scenarios in the SRES, RCP and SSP sets significantly overshoot the Paris Agreement temperature targets.
What are the implications?
In some cases, specific and quite drastic new policies must be introduced if we are to meet the Paris targets. And, in addition to introducing additional mitigation policies, we will also likely need to put much greater effort into developing and applying measures of adaptation and vulnerability reduction.
Of course, a few of the scenarios in the SRES, RCP and SSP sets do lead to future warming similar to the Paris temperature targets. These scenarios provide a great deal of information about what is required to meet (or get close to) the Paris warming targets. However, there is an additional approach to gaining this insight.
An alternative approach to emissions scenario construction is to use inverse modelling. This approach is particularly appropriate in the context of assessing the emissions requirements for meeting the global-mean temperature goals of Article 2 of the Paris Agreement.
Instead of focusing on those emissions scenarios that meet or get close to the Paris targets, we can start with global-mean warming scenarios that meet the targets and work backwards via inverse calculations to determine what the corresponding emissions would have to be.
The first step in applying this approach is to specify warming trajectories that meet the Paris targets. With only the eventual targets specified in the Paris Agreement, there is clearly a range of temperature trajectories that could be followed that will be consistent with the targets, including what are called “overshoot” trajectories that exceed the targets and then decline to meet them.
In my 2018 paper on the Paris Agreement (Wigley, 2018) I used a hybrid approach employing both conventional IAM results and an inverse calculation to determine the CO2 emissions. I simplified the task by choosing a fixed set of IAM-derived emissions for all non-CO2 forcing components — namely, the most stringent policy scenario from the CCSP set.
To define temperature trajectories that were consistent with the Article 2 goals, I developed two trajectories that stabilized warming at 1.5°C, and one that stabilized at 2.0°C. In addition, I ran a large number of forward simulations and decided from these that it was virtually impossible to reach either warming target without an overshoot.
The likelihood that a warming overshoot was unavoidable was my considered opinion in 2016, but I noted at the time that the wording of Article 2 was sufficiently ambiguous that it was unclear whether it allowed or did not allow an overshoot. This is still a debatable point, but I note now that the community has come to realize that, for the 1.5°C target at least, a warming overshoot is almost certainly unavoidable.
Under the hybrid approach there is only one unknown — CO2 emissions — and it is a conceptually simple inverse modelling task to determine these emissions for any assumed temperature trajectory. The results depend on the value chosen for the climate sensitivity and on what is assumed for emissions of the non-CO2 gases.
The results are shown in the Figure below, which shows the two 1.5°C (labelled 15A and 15B) and single 2.0°C warming trajectories, the implied fossil CO2 emissions, and the corresponding CO2 concentration changes.
There are two important results here.
First, it is possible to meet the 1.5°C target without going to negative emissions, as the higher warming overshoot case (15A) demonstrates.
Second, meeting the 1.5°C target does not require totally eliminating CO2 emissions. This result may be surprising to some — perhaps many — but the reason is elementary.
The CO2 budget that determines future CO2 concentrations is the sum of sources — anthropogenic emissions and positive feedbacks — offset by sinks of CO2 into the ocean and terrestrial biosphere. These two sinks, no matter what the emissions, change only slowly over time and remain significant for centuries into the future, and this allows CO2 emissions in both 1.5°C cases to remain positive — of order 3.5 to 7 GtCO2/yr for all times after 2150 out to 2400.4
The emissions required to meet the temperature goals of UN FCCC Article 2 are addressed in Article 4.1. Here, I have considered only CO2 emissions for a specific, but realistic scenario for non-CO2 gases. CO2 emissions alone, or net-zero CO2 emissions, are only part of the picture — it is net-zero aggregate GHG emissions that Article 4.1 focusses on. As a follow-on question, we can ask: Is net-zero aggregate GHGs a useful metric to guide future mitigation?
I’ll discuss this question in a later essay.
While we may not need to reduce carbon dioxide emissions all the way to zero to meet the Paris Agreement targets, the required reduction in CO2 emissions is huge and rapid. It is an enormous and daunting challenge.
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As we have noted before, however, reference scenarios invariably include ‘spontaneous’ emissions changes due to technology developments (Pielke et al., 2008: examples are given in Wigley et al., 2021, Fig. 3 and Table 2). In other words, in most reference scenarios, there is a great deal of mitigation technology development that is not policy driven.
IPCC uses ‘reference’ to denote the fact that, in the scenario construction process, no future policy interventions directed towards reducing greenhouse gas emissions are made
Strictly, 2.0°C is not the goal, rather, the stated goal is “well below 2.0°C”, but investigating a 2.0°C warming case is useful because it might be considered to be a more realistically achievable target.
The Figure also quantifies the net-zero CO2 emissions points for CO2. For CO2, if net emissions are zero, this means that the rate of change of CO2 concentration (dC/dt) must be zero. For the 2.0°C target, dC/dt = 0, and hence net-zero CO2 emissions, occurs first in 2065 and then for all years after 2300. For the high overshoot case (15A), net-zero CO2 emissions occurs first in 2047 and then for all years after 2151. For the low overshoot case (15B), net-zero occurs first in 2034, then at 2131, and later for all years after 2350. I’ll leave it up to the reader to decide whether these are meaningful or even useful results.