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Environmental Science and Policy 133 (2022) 193–202
Available online 11 April 2022
1462-9011/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Navigating the political: An analysis of political calibration of integrated
assessment modelling in light of the 1.5 ◦C goal
Lisette van Beek
a
,
b
,
*
, Jeroen Oomen
b
, Maarten Hajer
b
, Peter Pelzer
c
, Detlef van Vuuren
d
a
Copernicus Institute of Sustainable Development, Faculty of Geosciences Utrecht University, Utrecht, The Netherlands
b
Urban Futures Studio, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands
c
Department of Human Geography and Spatial Planning, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands
d
PBL Netherlands Environmental Assessment Agency, The Hague, The Netherlands
ARTICLE INFO
Keywords:
Integrated Assessment Models
Science-policy interface
Political calibration
1.5 degrees
Climate policy
Climate mitigation
ABSTRACT
Some of the most inuential explorations of low-carbon transformations are conducted with Integrated Assess-
ment Models (IAMs). The recent attempts by the IPCC to look for pathways compatible with the 1.5 ◦C and 2 ◦C
temperature goals are a case in point. Earlier scholarship indicates that model-based pathways are persuasive in
bringing specic possible future alternatives into view and guiding policymaking. However, the process through
which these shared imaginations of possible futures come about is not yet well understood. By closely examining
the science-policy dynamics around the IPCC SR1.5, we observe a sequence of mutually legitimising interactions
between modelling and policy making through which the 1.5 ◦C goal gradually gained traction in global climate
politics. Our ndings reveal a practice of ‘political calibration’, a continuous relational readjustment between
modelling and the policy community. This political calibration is indicative of how modellers navigate climate
politics to maintain policy relevance. However, this navigation also brings key dilemmas for modellers, between
1) requirements of the policy process and experts’ conviction of realism; 2) perceived political sensitivities and
widening the range of mitigation options; and 3) circulating crisp storylines and avoiding policy-prescriptiveness.
Overall, these ndings call into question the political neutrality of IAMs in its current position in the science-
policy interface and suggest a future orientation in which modellers aim to develop additional relations with
a broader set of publics resulting in more diverse perspectives on plausible and desirable futures.
1. Introduction
Delimiting climate change in line with the Paris Agreement (2015)
implies the need for low-carbon transformations in energy, agriculture
and transport systems (Geels, Berkhout and Vuuren, 2016). Model-based
scenarios form an important tool to explore these low-carbon trans-
formations. Such scenarios are typically made using Integrated Assess-
ment Models (IAMs), computer simulations that couple socio-economic,
technical and biophysical systems (Van Vuuren et al., 2011; Weyant,
2017). This modelling of complex interactions enables the systematic
comparison of the costs and effectiveness of alternative climate miti-
gation strategies as well as the scope and timing of required emission
reductions consistent with global temperature goals (Geels et al., 2016).
Over the past decades, IAMs
1
have become increasingly prominent in
the climate science-policy interface, co-evolving with global climate
politics (McLaren and Markusson, 2020; van Beek et al., 2020; Bosetti,
Abbreviations: IAM, Integrated Assessment Model; IPCC, Intergovernmental Panel on Climate Change; NETs, Negative Emissions Technologies; ToF, Techniques of
Futuring; SR1.5, Special Report on 1.5 ºC; STS, Science and Technology Studies; UNFCCC, United Nations Framework Convention on Climate Change; AOSIS,
Alliance of Small Island States; COP, Conference of the Parties; SED, Structured Expert Dialogues; CLA, Coordinating Lead Author; CA, Contributing Author; LA, Lead
Author; IIASA, International Institute for Applied Systems Analysis; SPM, Summary for Policymakers; BECCS, Bioenergy with Carbon Capture and Storage; WG,
Working Group; LED, Low Energy Demand.
* Correspondence to: Princetonlaan 8a, 3584 CB Utrecht, The Netherlands.
E-mail address: l.m.g.vanbeek@uu.nl (L. van Beek).
1
We use ‘IAMs’ to describe process-based Integrated Assessment Models, that include a detailed representations of the human and climate system and their
interlinkages. These models are often used to assess cost-effective climate change mitigation pathways under global temperature targets. Cost-benet IAMs constitute
a different IAM type that are used to asses economically optimal levels of abatement given future climate impacts and typically include a simplied representation of
both the human and climate system (Wilson et al., 2021 for more details on process-based IAMs).
Contents lists available at ScienceDirect
Environmental Science and Policy
journal homepage: www.elsevier.com/locate/envsci
https://doi.org/10.1016/j.envsci.2022.03.024
Received 28 June 2021; Received in revised form 22 March 2022; Accepted 29 March 2022
Environmental Science and Policy 133 (2022) 193–202
194
2021 on the history of IAMs). While scattered over different institutions,
together the IAM modellers constitute a globally organised epistemic
community with a leading role in scenarios underlying Working Group
III of the Intergovernmental Panel on Climate Change (IPCC), which is
dedicated to mitigation (Cointe et al., 2019). As such, IAMs provide a
critical tool to explore mitigation pathways towards the 1.5 ◦C and 2 ◦C
temperature goals in IPCC reports.
In recent years, the IPCC has moved from providing scientic evi-
dence for climate change’s cause and existence towards a more solution-
oriented mode (Beck and Mahony, 2017; Guillemot, 2017). As such, the
capacity of IAMs to explore mitigation options has become increasingly
central to inform climate policy (van Beek et al., 2020). IAM scenarios
quantify a range of alternative climate policy pathways (Edenhofer and
Kowarsch, 2015). They can, however, only present a subset of possible
climate actions due to their mathematical structures and bias towards
technical feasibility and cost-effectiveness (Forster et al., 2020; Keppo
et al., 2021). As such, IAM scenarios are inuential in bringing specic
alternatives into the imagination of policymakers while foreclosing
other potentially crucial ways to mitigate climate change (Beck and
Mahony, 2018b). For instance, alternatives that are not part of the IAM
repertoire are 100% renewable energy scenarios (Hansen et al., 2019),
degrowth scenarios (Keyßer and Lenzen, 2021) or relying strongly on
ecosystem restoration (Roe et al., 2019; see Keppo et al., 2021 for
overview of limitations). By rendering particular possibilities more
thinkable or actionable, IAM pathways inuence the imagined ‘corridor
of climate mitigation’, structuring the deliberation of political actors on
future climate action (Beck and Mahony, 2017, 2018a, 2018b; Beck and
Oomen, 2021).
Given their central role in the climate science-policy interface, a
detailed understanding of the practice of IAMs is critical to further both
the scientic and societal debate. IAM pathways have been found to be
inuential in shaping policy commitments, such as in establishing the
feasibility of the 2 ◦C degrees target (L¨
ovbrand, 2011; Beck and Mahony,
2017, 2018a, 2018b; McLaren and Markusson, 2020). More recently, the
1.5 ◦C goal has become the new symbol for climate action – despite
serious doubts about its feasibility (Livingston and Rummukainen,
2020). IAMs again played a signicant role, as showcased by the
world-wide adoption of policy commitments towards ‘net-zero by 2050′
emissions targets and the deployment of negative emissions technolo-
gies (NETs), both originating from IAM-based 1.5 ◦C pathways (Thoni
et al., 2020). Although these observations indicate an inuential role of
IAMs, we still have only a limited understanding of the pattern of
science-policy interactions through which such policy commitments
emerge and gain traction.
The current study aims to address this gap. We study integrated
assessment modelling using the concept of ‘Techniques of Futuring’
(ToF; Hajer and Pelzer, 2018; Oomen et al., 2021), analysing the
sequential and contextualised practices through which visions of
possible futures become collectively shared. We analyse how the 1.5 ◦C
goal increasingly gained traction by reconstructing the science-policy
dynamics around the Special Report on 1.5 ◦C (SR1.5) (IPCC, 2018a).
Our reconstruction captures the 2015–2020 period, from the adoption of
the 1.5 ◦C in the Paris Agreement to a few years following the aftermath
of the SR1.5. To this end, we conducted 22 semi-structured interviews
with IPCC authors and policymakers (Appendix A and B), a quantitative
literature analysis and reviewed IPCC and UNFCCC documentation
(Appendix C and D). We selected interviewees based on ensuring a
comprehensive view on science-policy dynamics from the diverse
viewpoints of key actors, including IPCC authors, government repre-
sentatives and expert reviewers (Appendix A and B). The selection of
IPCC SR1.5 authors was based on their role in chapters relevant to
climate mitigation (chapter 2, 4 and 5) as well as to ensure a balanced
view on the role of IAMs, selecting IAM modellers as well as authors
representing other scientic communities (e.g. bottom-up modelling). In
the following paragraphs, we rst elaborate on our conceptual approach
(Section 2), which guides our reconstruction. We then provide
background information on the emergence of the 1.5 ◦C target (Section
3). Section 4 presents our analysis on how and why the 1.5 ◦C gained
traction. In Section 5, we reect upon this analysis and discuss impli-
cations for the use of IAMs to explore low-carbon transformations.
2. Analysing the sequence of events through which images of
the future gain traction
Taking a constructivist perspective on science-policy dynamics, our
analysis is framed by a co-productionist approach of STS research
(Latour, 1993; Jasanoff, 2004). This epistemological stance regards
scientic practice not as neutral knowledge-making but as a performa-
tive endeavour that always ‘co-produces’ ideas about what to govern
and how, whether intentionally or unintentionally. This means that we
are particularly interested in the performative effects of projections. As
revealed by a growing scholarship, collectively shared images and vi-
sions of the future inuence political, economic, and technological de-
cisions and developments. Scholarship on the collective imagination, for
example, shows how “collectively shared, institutionally stabilised, and
publicly performed visions of desired futures” animate future-oriented
policy and technology development (Jasanoff and Kim, 2015, p. 4)
and how ‘ctional expectations’ enable actors to make decisions under
uncertainty based on the shared assumptions about some future state
(Beckert, 2013, 2016).
In the context of environmental science and policy, model-based
representations in authoritative scientic assessments such as the IPCC
are powerful in shaping political deliberations about future climate ac-
tion (Beck and Mahony, 2017; Beck and Oomen, 2021). However, little
effort goes into understanding how and why particular images of the
future become persuasive. To understand the relational process of
science-policy dynamics through which such future visions become
performative, we use the concept ‘Techniques of Futuring’ (ToF),
dened as “practices bringing together actors around one or more
imagined futures and through which actors come to share particular
orientations for action” (Hajer and Pelzer, 2018, p. 225). Rather than
taking IAMs or their pathways as the objects of analysis, the ToF lens
brings into focus the relational process of mutually adjusting expecta-
tions among actors around the plausibility and desirability of possible
futures (Oomen et al., 2021). As theorised by Oomen et al. (2021), this
involves a “sequence of events [of] step- by-step braiding of knowledge,
images of the future and legitimacy” (p. 12). This theoretical lens
informed our detailed reconstruction of the sequence of events through
which shared expectations around the 1.5 ◦C emerged. We took an
interpretative approach to analyse the interviews and other data,
revealing shifting perspectives and expectations regarding the 1.5 ◦C
goal and the role of IAMs among different actors involved in the IPCC
SR1.5 (Appendix A and B).
3. Background: the origins of the 1.5 ◦C degrees goal
(2009–2015)
While science-policy discussions on the level of dangerous anthro-
pogenic interference and long-term global goals can be traced back to
the late 1980 s (Tschakert, 2015; Morseletto et al., 2017), the 1.5 ◦C goal
rst emerged at the UNFCCC negotiations during the 15th Conference of
the Parties (COP) in Copenhagen in 2009. At that time, the Alliance of
Small Island States (AOSIS) claimed that the projected sea-level rise
related to a 2 ◦C warming implied that their islands would be wiped off
the map (Guillemot, 2017; Tschakert, 2015; Livingston and Rummu-
kainen, 2020). AOSIS and the Least Developed Countries (LDC) alliances
emphasised the need to lower the global temperature goal to 1.5 ◦C
(IISD, 2009). Although an international agreement could not be reached
in Copenhagen, most countries supported the Copenhagen Accord,
where the 2 ◦C was adopted in the negotiation document (UNFCCC,
2009). Under the pressure of the LDC and AOSIS, the Copenhagen
Accord explicitly called for strengthening this goal: “consideration of
L. van Beek et al.
Environmental Science and Policy 133 (2022) 193–202
195
strengthening the long-term goal [.] including in relation to temperature
rises of 1.5 degrees Celsius” (UNFCCC, 2009, emphasis added). At COP16
in Cancun, the ‘well below’ 2 ◦C was formally agreed upon, but also to
periodically review the long-term global goal (UNFCCC, 2010). Despite
little response from the scientic community (Schleussner et al., 2016),
a review process was initiated: so-called Structured Expert Dialogues
(SEDs) involving face-to-face interactions between UNFCCC parties and
experts addressing the adequacy of the temperature goal and the overall
progress towards these goals (UNFCCC, 2011). The difference between
1.5 ◦C and 2 ◦C was a central topic during the SEDs. However, the
meaning of this temperature difference was difcult to assess due to a
lack of research (Tschakert, 2015). The nal report of the SEDs in 2015
concluded: “While the science on the 1.5 ◦C warming limit is less robust,
efforts should be made to push the defence line as low as possible”
(UNFCCC, 2015a). Shortly before COP21 in Paris, the Marshall Islands
launched a High Ambition Coalition which demanded an explicit
reference to 1.5 ◦C as a prerequisite for an agreement. Before and during
COP21 in Paris, they rallied support from NGOs and more than 100
countries (Guillemot, 2017). A potential shift of the long-term global
temperature goal from 2 ◦C to 1.5 ◦C was a key topic during the nego-
tiations (IISD, 2015). The High Ambition Coalition managed to convince
more and more countries of the need for a shift to 1.5 ◦C, whereas some
countries remained sceptical and supported only a “well below 2 ◦C”
goal (IISD, 2015b; Brun, 2016). Finally, in the Paris Agreement, coun-
tries compromised to: “Holding the increase in the global average tem-
perature to well below 2 ◦C above pre-industrial levels and pursuing
efforts to limit the temperature increase to 1.5 ◦C above pre-industrial
levels” (UNFCCC, 2015b, Art 2.1). Obviously, this compromise pro-
vided all parties with the ability to claim a victory. Many factors
explaining the success of Paris are outlined elsewhere (e.g. Brun, 2016;
Christoff, 2016; Guillemot, 2017). A key reason for the adoption of the
1.5 ◦C specically was that it provided a bargaining chip for vulnerable
countries who could not accept the 2 ◦C, while the agreement remained
lenient regarding nancial or legal obligations to developed countries
for loss and damages of vulnerable countries (Guillemot, 2017; in-
terviews 21 and 22, government representatives at COP21).
4. A reconstruction: how the 1.5 ◦C became the new guardrail of
climate action (2015–2020)
This section starts from the adoption of the ‘pursuing effort to 1.5 ◦C′
goal in Paris to reconstruct the science-policy interactions around the
IPCC SR1.5 between 2015 and 2020. We identify three phases through
which the 1.5 ◦C goal gradually went from being perceived as unrealistic
to becoming the new symbol of climate action. In each phase, this
involved an iterative process between modelling and policy, in which
model ndings and policy targets legitimised each other (see Fig. 2):
•Phase 1 2015–2016 (4.1): the initial post-Paris emerging interac-
tion between the modelling and policy shifted the 1.5 ◦C goal from
being perceived as unrealistic towards ‘achievable with NETs’,
relying on newly modelled 1.5 ◦C IAM pathways;
•Phase 2 2016–2018 (4.2): the IAM community then helped to
further establish the perceived feasibility of the 1.5 ◦C through a
series of readjustments of ‘acceptable’ levels of NETs and overshoot
during the SR1.5 writing process;
•Phase 3 2018–2020 (4.3): nally, these published pathways shaped
policy commitments to limit global warming to 1.5◦Cin the after-
math of the IPCC SR1.5.
4.1. Phase 1: the 1.5 ◦C goal shifted from perceived as ‘unrealistic’ to
‘achievable with NETs’ (2015–2016)
In the Paris Agreement, the UNFCCC invited the IPCC “to provide a
special report in 2018 on the impacts of global warming of 1.5 ◦C above
pre-industrial levels and related global greenhouse gas emission path-
ways” (UNFCCC, 2015b, decision 1/21, para21). The initial idea was to
invite the IPCC to draft a Special Report on the impacts of 1.5 ◦C vs. 2 ◦C,
but during the negotiations the assessment of how to achieve this target
was also emphasized. This focus on the ‘how’ was important to convince
some governments on the feasibility of necessary actions to achieve the
1.5 ◦C (IISD, 2015a; interview 21 and 22, government ofcials attending
COP21). Although the 1.5 ◦C target had been debated in previous ne-
gotiations, its adoption in Paris still came as a surprise to many scientists
(Livingston and Rummukainen, 2020). Modellers, in particular, had
previously considered 1.5 ◦C mitigation pathways irrelevant because
they thought a 1.5 ◦C goal was not realistic, either politically or socie-
tally (interview 2, 6; cf. Livingston and Rummukainen, 2020). As stated
by an IAM modeller “We talked about [1.5 ◦C] but never seriously. It felt so
unrealistic and infeasible that the models were not applied to this.” (inter-
view 2, CA IPCC SR1.5).
Despite lingering doubts of the feasibility of this target, the focus of
modelling studies shifted from 2 ◦C to 1.5 ◦C degrees after Paris
(interview 2, 12, 20). According to one of the (non-IAM) CLAs of the
SR1.5, “the scientic debate was still centred around 2 ◦C degrees. […] Only
after the target emerged during COP21, various modelling studies appeared
that could solve for 1.5 ◦C degrees.” (interview 1).
Moreover, the explicit request of the IPCC report to show how to
achieve the 1.5 ◦C target created a demand for research showing if and
how the goal might be achieved. Being well-organized (cf. Cointe et al.,
2019; van Beek et al., 2020), the IAM community could rapidly develop
1.5 ◦C pathways (see Fig. 1). As described by an IPCC Bureau member:
“[The IAM community] took the models […] and turned up the volume to 11
as it were, to run the models again with 1.5 ◦C.” (interview 5). This rapid
increase in 1.5 ◦C pathways shows the ability of the IAM community to
adjust the model focus towards a newly established target. The sheer size
of the output and number of pathways from different IAM teams also
helped to legitimise the achievability of this new goal.
This reveals an empirical example of ‘calibrating’ the model analysis
in view of relevance: despite the personal conviction of realism of some
of the modellers at the time, modelling efforts were redirected from
exploring 2 ◦C pathways to those limiting warming to 1.5 ◦C. The
alternative would have been to say that the 1.5 ◦C goal was infeasible
according to modelling results. However, this would disregard small
island states (interview 5, IPCC co-chair). In fact, if the IPCC would have
concluded that the 1.5 ◦C was unrealistic, Paris negotiators might even
have had to go back to the negotiation table (interview 22, COP21
negotiator). On the other hand, the shift from 2 ◦C to 1.5 ◦C implied
faster emissions reduction, in which the rapidly appearing 1.5 ◦C sce-
nario literature relied on NETs to an even more signicant degree
(interview 2,3,6,15). As explained by one modeller: “I am not more
condent that we can reach it, but I am more condent that we can model it.
[…] we would never have to say it would not be achievable, we just put more
negative emissions in” (interview 18).
Essentially, UNFCCC’s knowledge demand to understand if and how
the new target could be achieved was answered by IAM research with:
“yes – using NETs”. At the same time, NETs remain an issue of heated
academic debate: their assumed scale in IAM scenarios is debated as well
as the potential risks and ethical considerations (e.g. Vaughan and
Gough, 2016; Forster et al., 2020). Others argue that counting on NETs
in the future risks undermining near-term climate action (Markusson
et al., 2018). Responding to UNFCCC’s request for 1.5 ◦C pathways and
showing it was ‘feasible with NETs’, IAMs came to play a legitimising
role for the 1.5 ◦C target. This role was not inevitable. We observe three
main reasons why IAMs could play this role: 1) the high degree of or-
ganization of the IAM community; 2) the more structural legitimacy of
quantitative and system-wide future-oriented knowledge in the climate
science-policy interface; and 3) the analytical qualities of IAMs.
4.1.1. Organization
First, modellers often work closely together in large-scale modelling
L. van Beek et al.
Environmental Science and Policy 133 (2022) 193–202
196
intercomparison projects, harmonise their assumptions through shared
scenario frameworks and develop scenario databases to compare and
analyse modelling outputs (Cointe et al., 2019). This high degree of
collaboration and synchronisation in IAM research and the intimate ties
between the major modelling groups facilitates the adoption of IAM
outputs in IPCC reports (interview 1, 2, 12, 15, 18). These organizational
capacities are exemplied by the 1.5 ◦C scenario database hosted by the
International Institute for Applied Systems Analysis (IIASA). IIASA has
served as IAM ‘community hub’ for decades (interview 3; Hughes and
Paterson, 2017). The database resulted in a ‘robust’ set of scenarios
assessed across different assumptions and models (interview 4), making
IAM studies convenient to assess in an IPCC report compared to other
types of literature that are more difcult to systematically compare
(interview 1, 4, 7, 20). Although IIASA’s call to submit 1.5 ◦C scenarios
were meant to be “as broad as possible” (IIASA, 2017), the inclusion
criteria of the database – e.g. covering all sectors and projecting towards
2100 – were such that it matched the usual model output of the six most
established IAMs. As a result, these six IAM groups were at an advantage
in getting their pathways assessed at the expense of less established IAM
teams and bottom-up modellers (or other disciplines, for that matter):
“If you start from zero, it takes some time to upload it, it might take a
couple of months. The IAM community uses that format for their
daily use and their models spit out the scenarios in that format. So
the other modelling teams have a much higher hurdle to be included”
(interview 4, CLA IPCC SR1.5, IAM modeller).
“They are like a great football team. […] When you’re playing
against an IAM team, it becomes 5–0 very quickly before half-time.
Because it’s a consistent community.” (interview 10, CLA IPCC
SR1.5).
4.1.2. Structural legitimacy
Second, the reliance on IAMs to demonstrate the feasibility of the
1.5 ◦C goal relates to a more structural legitimacy of quantitative,
global, and system-wide future-oriented knowledge in the climate-
science policy interface. By default in environmental science and pol-
icy, the climate is approached as a global interconnected system, a view
that has been shaped by the IPCC (Miller, 2004; Turnhout et al., 2016).
Legitimacy of quantitative knowledge can be traced back to a much
longer history of ‘trust in numbers’ among policymakers (Porter, 1995;
cf. van Beek et al., 2020) as well as the emergence of computer
modelling as the key epistemic approach to understand the past, present
and future of the climate (Edwards, 2010). The privileged position of
IAM analyses in the SR1.5 was not uncontroversial due to its biases,
calling for more diversity in scientic disciplines in IPCC reports
(Hansson et al., 2021; interview 5). Although the IPCC Bureau suc-
cessfully brought in a much broader set of disciplines in the SR1.5
compared to previous reports, the Summary for Policymakers (SPM) –
the most politically inuential part of the report – still predominantly
contained gures based on IAMs:
“The main advantage of IAMs is their rigorous quantitative framing
and systems perspective. This quantitative systems perspective helps
you to illustrate points with numbers. […] And since the SPM is
usually trying to assess and quantify the order of magnitude of
Fig. 1.. Number of academic peer-reviewed literature on 1.5
◦C published over time between 2009 and 2020. Data derived from Scopus (Appendix D for
methodology).
L. van Beek et al.
Environmental Science and Policy 133 (2022) 193–202
197
changes that need to happen they traditionally rely a lot on the IAM
results” (interview 6, LA IPCC SR1.5).
4.1.3. Analytical qualities
Third, a key analytical strength of IAMs is to connect climatic (e.g.
global temperature) and societal dynamics across sectors (e.g. energy
supply and demand). The question of whether the 1.5 ◦C was a feasible
global goal was thus tailor-made for an IAM approach, in contrast to for
instance sectoral or nationally oriented approaches. As our interviews
revealed, modellers as well as non-modellers struggle to identify viable
alternative to IAMs:
“If we did not have IAMs, we’d have to invent them because they are
the only way of getting between human activity on climatic changes
on a century scale” (interview 2, IPCC Bureau member).
“Even when I am critical of IAMs and throw them all out of the
window, if I sit tomorrow at my desk, I would still build a new IAM.
One that understands how decisions in land use or building affect
how much mitigation we need and how much land we need.”
(interview 4, CLA IPCC SR1.5, IAM modeller)
4.2. Phase 2: becoming persuasive: how the 1.5 ◦C gained traction despite
emerging criticism on NETs (2017–2018)
In this second phase, covering the lead-up to the publication of the
SR1.5, the 1.5 ◦C goal increasingly gained traction. At the same time, the
specic corridor towards 1.5 ◦C projected by IAMs was highly contro-
versial. This contention emerged already before Paris when IAMs
asserted that 2 ◦C was possible (only) under the condition of substantial
implementation of NETs. Several scholars warned in high-prestige aca-
demic journals that policymakers, unaware of the assumed scale and
implications of NETs, may nd “betting on negative emissions” more
appealing than near-term emission reduction, risking a lock-in into a
fossil-fuel-dependent society (Fuss et al., 2014; Anderson and Peters,
2016). We observe three key mechanisms through which the 1.5 ◦C as a
feasible target could gain traction despite this criticism: 1) a tightening
interdependence of modelling and policy around the acceptable level of
overshoot in 1.5 ◦C pathways; 2) IPCC SR1.5 authors’ attempts to
harmonise cross-chapter discrepancies around the feasibility of NETs;
and 3) efforts of the modelling community to expand their range of
mitigation options towards demand-side mitigation.
4.2.1. Tightening interdependence
The rst mechanism relates to science-policy negotiations around
the acceptable level of “overshoot” in scenarios. In the First Order Draft
of Chapter 2 (IPCC et al., 2017a): all 191 IAM scenarios compatible with
the 1.5 ◦C were ones that temporarily exceeded 1.5 ◦C warming before
returning to that level in 2100 – meaning that they all relied on NETs.
The absence of non-overshoot scenarios in the rst draft of the SR1.5
was ercely criticised by expert reviewers and civil society organisations
(IPCC, 2017b, 28 comments; interview 8). In response, the authors
included non-overshoot scenarios in the subsequent draft (IPCC et al.,
2017c), albeit very few (only 10 out of 578 scenarios). Again, critics
commented on the extent to which scenarios exceeded the 1.5 ◦C,
viewing high levels of overshoot as inconsistent with the Paris Agree-
ment (interview 3, 4; IPCC, 2017d).
“a lot of [scenarios] overshoot the target. Some delegations would
then say: this is not what we would dene as a 1.5 ◦C degree target as
we have the water up to our necks by then.” (interview 7, LA IPCC
SR1.5).
Excluding all overshoot scenarios, however, would basically
disqualify all the underlying scenario literature (interview 3) – and
hence present the 1.5 ◦C goal as unrealistic. Eventually, it was agreed
that overshoot to 2 ◦C degrees (but not higher) would be acceptable
(interview 3,4). This compromise showcases the tightening interde-
pendence between modelling and climate politics: the UNFCCC and
IPCC relied on IAMs to present the 1.5 ◦C goal as realistic, and IAMs
simply relied on NETs, resulting in an agreement on the acceptable level
of overshoot – and hence accepting a signicant use of NETs. Here we
again observe a process of ‘calibration’ of the focus of analysis based on
the societal debate: the acceptable level of overshoot and use of NETs in
IAM pathways was readjusted to establish a sufcient number of path-
ways to hold the 1.5 ◦C goal attainable as well as avoiding high levels of
overshoot that were feared by vulnerable countries.
4.2.2. Harmonizing discrepancies
A second mechanism through which 1.5 ◦C pathways attained their
persuasiveness despite criticism was through resolving discrepancies
between Chapter 2 and Chapter 4. These struggles involved the feasi-
bility of Bioenergy with Carbon Capture and Storage (BECCS). Chapter
2, based on IAMs, assumed much higher potentials of BECCS (67–130
EJ/year) than Chapter 4, based on bottom-up studies (maximum of 100
EJ/year) (IPCC, 2018a). The signicant use of NETs in IAM pathways
was already under re due to concerns about feasibility, land-use pres-
sures and biodiversity loss. Again, it attracted erce criticism from
expert reviewers of the SR1.5, civil society organisations and govern-
ment representatives (IPCC, 2017d, 2017c, interview 1; cf. Hansson
et al., 2021). Despite agreement about Chapter 4 ndings being more
accurate, BECCS featured centrally in the SR1.5′s ‘Illustrative Path-
ways’, the four IAM-based archetype 1.5 ◦C scenarios that were high-
lighted visibly in the SPM (interview 1, 7):
“Essentially in Chapter 4, we said: what is stated in Chapter 2 is
impossible […]. But no one really found this problematic. We knew
that models are just one version of reality, which is not the real
world. What is problematic, however, is that the Illustrative Path-
ways suggest it is possible, while in Chapter 4 we convey that it isn’t”
(interview 1, CLA IPCC SR1.5).
To harmonize discrepancies, the authors developed a feasibility
assessment, crosschecking a range of mitigation options between
Chapters 2 and 4 as a ‘reality check’ of IAM assumptions (interview 1,
10). Yet while this table was included in the report’s nal draft sent to
governments for the line-by-line approval session, it did not make it into
the nal SPM (interview 9,10). Negotiations about the table were seen as
jeopardising the approval of the full report (interview 10), as the
country-specic information in the table might conict with IPCC’s
mandate to provide ‘non-policy-prescriptive’ knowledge (interview 6,
10). In contrast, the Illustrative Pathways caused only minor disagree-
ment among member states (IISD, 2018). As a result, only the Illustrative
Pathways - some of which assuming high levels of NETs - were elevated
in the SPM (Figure SPM.3b, IPCC, 2018a). IAM’s quantitative,
system-wide, and global orientation appeared crucial to align with
IPCC’s mandate to provide ‘non-policy-prescriptive’ information.
Moreover, the overlap of IPCC WGIII authors and the IAM community
blurs the distinction between providing and assessing literature (interview
5, cf. Corbera et al., 2016; Hughes and Paterson, 2017). This double role
as both author and reviewer within the IPCC has also taught IAM
modellers how to netune their output and anticipate policymakers’
knowledge questions:
“the challenges that we encounter in the IPCC, we try to solve. The
community learns from that and tries to anticipate and create
knowledge that can be useful in IPCC reports that can be used for the
arising questions” (interview 4, CLA IPCC SR1.5, IAM modeller).
4.2.3. Expand range of mitigation options
A third mechanism that rendered the 1.5 ◦C target persuasive despite
criticism on NETs was that modellers expanded their range of options
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198
towards demand-side mitigation. Traditionally, the IAM community is
more supply-side oriented. Changes in supply-side technology are easier
to quantify in economic and mathematical equations than more complex
choices in end-use regarding efciency and lifestyle change that often
involve a heterogeneity of people, perspectives, attitudes, and motiva-
tions (interview 2, 3, 4, 6). The IAM community had started to address
this challenge in the context of the 2 ◦C goal (e.g. van Sluisveld et al.,
2016), but the 1.5 ◦C goal gave a strong push to further expand their
options in this direction (interview 2, 4, 6, 7, 8):
“The 1.5 degrees made us think about other radical changes that we
had not taken into consideration before, including radical lifestyle
changes. […]. So we went beyond what we would normally thought
was possible” (interview 2, CA IPCC SR1.5, IAM modeller).
Notably, the emerging demand-side pathways could explicitly ach-
ieve the 1.5 ◦C with no or limited use of NETs, for instance, by assuming
low energy use and dietary shifts (Grubler et al., 2018; van Vuuren et al.,
2018). Even though the majority of 1.5 ◦C pathways still relied heavily
on NETs, the ‘Low Energy Demand’ (LED) scenario (Grubler et al.,
2018), was selected as one of the four illustrative pathways presented in
the SPM, which appeared crucial to respond to growing criticism:
“It was very exciting whether [the LED scenario] would be published
in time. It came just in time, just a few days before the literature
deadline. […] The message was that it would be possible without
BECCS, but it would then require behaviour changes much earlier.”
(interview 1, CLA IPCC SR1.5).
“The LED scenario that came out right before the end and made a
huge splash, being one of the Illustrative Pathways. The scenario
made quite a career in a very short time.” (interview 8, civil society
representative).
The inclusion of this ‘no NETs’ scenario as one of the archetype
scenarios was well received by critics, including civil society organisa-
tions (interview 8). This illustrates a recurring mechanism: motivated by
criticism on NETs by experts and civil society organisations, modellers
explored pathways that relied more on demand-side mitigation.
4.3. The 1.5 ◦C as the new guardrail for climate action: the uptake of IAM
pathways in the aftermath of the SR1.5 (2018–2020)
In the third phase, the 1.5 ◦C goal became the new guardrail for
climate action as IAM pathways in the SR1.5 became translated into
policy commitments to limit global warming to 1.5 ◦C (cf. Hermansen
et al., 2021). Interviewees indicated that the SR1.5 was ‘incredibly
inuential’ (interview 14) in policy and public debates, if not ‘the most
important report the IPCC ever produced’ (interview 20). This is also
reected in its massive wave of media coverage (Boykoff and Pearman,
2019). This had various reasons. For one, the IPCC had changed their
communication strategy, replete with visualisation experts and a head of
communications (interview 5, 10). Secondly, the report was eagerly
anticipated by a growing activist movement such as the #FridayforFu-
tures movement (Hermansen et al., 2021), with Greta Thunberg
imploring the world to ‘listen to the science’ (interview 10, 14). The
impacts of climate change were also becoming increasingly visible
(interview 14). Such contextual factors and charismatic spokespeople
are what Morgan, 2011 calls ‘good companions’ that allow facts to
‘travel well’. The (non-IAM) chapters on climate impacts between 1.5 ◦C
and 2 ◦C raised the urgency of climate action (interview 7, 10,14).
Regarding the chapters on mitigation, two IAM-based messages reso-
nated in particular: 1) the need to reach net-zero emissions in 2050 and
2) the necessity of NETs to achieve the 1.5 ◦C target (interview 7, 8, 10,
12, 14, 16, 20).
The need to reach net-zero around mid-century already appeared in
Article 4.1 of the Paris Agreement, albeit more ambiguously: “to achieve
a balance between anthropogenic emissions by sources and removals by sinks
of greenhouse gases in the second half of this century” (UNFCCC, 2015b).
The SR1.5 and the crisp and clear messaging from IAMs imprinted the
necessity to reach ‘net-zero in 2050′on governments (interview 7, 14,
18, 20). This message was once more elevated by the IPCC co-chairs
during the press release (IPCC, 2018b) and quickly became the new
‘catchy number’ reiterated in all government speeches in the following
climate negotiations (interview 19, UNFCCC secretariat).
Apart from the contextual factors, two key reasons why IAM path-
ways resonated were the simplicity of their storylines and, as outlined in
previous phases, their quantitative character:
“The thing about pathways is that it is very simple. […] at the end of
the day, if Greta can’t communicate your idea to half a million young
people, then in the world of action, it is not very much used.”
(interview 10, CLA IPCC SR1.5)
“We know that 1.5 is better than 2, even a kid would tell you that, but
they could now justify this with some numbers.” (interview 19,
UNFCCC secretariat).
The simplicity of the message, however, can invite mis-
understandings and have unintended effects. An obvious example is that
the emissions reductions by 2030 were interpreted by inuential media
such as The Guardian, CNN and The Independent as ‘we only have 12
years left’ (Boykoff and Pearman, 2019). Although this ‘climate dead-
linism’ has arguably raised urgency, it also risks opening the door for
backstop technologies such as geoengineering and inducing fear and
helplessness among the public (Asayama et al., 2019; Boykoff and
Pearman, 2019). Moreover, there are many misconceptions about both
the meaning of net-zero emissions as well as the scale and timing of the
implementation of NETs among policymakers (McLaren et al., 2019).
This dilemma between communicating clearly and becoming more
prescriptive than intended was also visible with the Illustrative Path-
ways, which were interpreted as ‘recipes for the future’ (interview 7):
“that pathways diagram is an incredibly useful communication de-
vice for me. Policymakers get it straight away.” (interview 5, IPCC
Bureau)
“It was a lot of work to always say: it’s just an illustrative pathway,
it’s just to demonstrate there are different pathways and we’re not
saying that one is superior to the other […]. It was a key insight: how
powerful those pathways are. It gives a lot of responsibility to the
IAM community.” (interview 7).
In all, in our reconstruction of science-policy interactions between
2015 and 2020 we identied three phases that were characterized by a
tightened interdependence between modelling and climate policy and
through which pathways towards the 1.5 ◦C became solidied (Fig. 2).
4.4. Political calibration
Throughout these phases, we observed that the 1.5 ◦C target grad-
ually gained traction through a process of mutually legitimising in-
teractions between modelling and policymaking, in terms of informing,
cooperating and exploring pathways that had a t to the policy de-
liberations at a particular time. We refer to this process as ‘political
calibration’, given the analogy with the more formal ‘model calibration’.
We dene political calibration as: ‘a process of iterative readjustment be-
tween modellers and policymakers, in which the t and focus of the model
analysis and the requirements of the policy community are negotiated. With
this, we do not mean an adjustment based on the acceptability of model
outcomes but rather on their policy relevance. Of course, the analogy
with model calibration is only partial. The term calibration in modelling
practices usually refers to a process of manipulating model parameters
to obtain a match between observed historic data and model simulations
in order to evaluate the ‘epistemic adequacy’ of models (Oreskes et al.,
1994). The extent to which model behaviour reproduces historic or
L. van Beek et al.
Environmental Science and Policy 133 (2022) 193–202
199
near-term observations is one of the methods to evaluate process-based
IAMs (Wilson et al., 2021). With ‘political calibration’, we refer not to
the epistemic but to the political adequacy of models. As described by
Oreskes et al. (1994), model calibration usually involves multiple steps
of renement until model simulations adequately reproduce observed
data. Likewise, we see political calibration as a sequential process of
continuously rening the t between modelling and policy re-
quirements. As we show in the nal section below, this process of ‘po-
litical calibration’ is delicate and reective, posing several dilemmas for
modelers.
5. Reection: understanding the role of IAMs in policy
commitments to limit climate change to 1.5 ◦C
In our reconstruction we observed that IAMs played a key role in the
shift of the 1.5 ◦C goal from an unrealistic target to the new guardrail for
climate action. The role of IAMs in policy commitments was not inevi-
table. By analysing science-policy interactions through the Techniques
of Futuring lens (Hajer and Pelzer, 2018; Oomen et al., 2021), we
explained the role of IAM modelling in the (political) legitimation of the
1.5 ◦C goal. This analysis relies on relational, discursive and structural
elements:
•the analytical qualities that rendered IAMs tailor-made for this
particular policy question (phase 1);
•the advantageous material and organisational capacities of the IAM
community for modellers compared to less experienced and more
dispersed scientic communities, through which 1.5 ◦C pathways
could rapidly be established (phase 1, cf. Cointe et al., 2019; van
Beek et al., 2020);
•the legitimisation of global, system-wide quantitative projections
over qualitative and country-specic future-oriented knowledge
(phase 1 and 2, cf. Miller, 2004; Edwards, 2010; Turnhout et al.,
2016; van Beek et al., 2020); and,
•the communicative power of concrete numbers and powerful visu-
alisations that helped shape policy commitments (phase 3).
The continuous readjustment of modelling efforts to requirements of
the policy community, the process of political calibration, was a key
mechanism through which the 1.5 ◦C could gain traction in policy
making and politics. Calibrating the focus of analysis based on ongoing
political discussions appeared as an important strategy for modellers to
remain policy relevant. However, the signicant role of IAMs in climate
politics also brings their political neutrality into question. We identied
three key dilemmas that modellers face when navigating climate poli-
tics: 1) between the personal assessment of feasibility and the re-
quirements of the policy process; 2) between respecting political
sensitivities and widening the range of mitigation options; and 3) be-
tween furthering crisp storylines and avoiding policy-prescriptiveness.
The three dilemmas are interrelated, reecting a tension between pol-
icy relevance, and shaping policy commitments. The dilemmas have
several implications for the usage of IAMs in the climate science-policy
interface.
5.1. Dilemma 1: policy relevance vs legitimising an unrealistic policy
commitment
With the adoption of the 1.5 ◦C goal in the Paris Agreement and the
invitation to develop 1.5 ◦C pathways, the IPCC and IAM modellers
faced a conundrum. Policymakers expressed interest in showing how to
achieve 1.5 ◦C. Presenting 1.5 ◦C pathways, however, would automati-
cally provide a perceived degree of feasibility – while many analysts at
the time would assess the 1.5 ◦C to be infeasible (phase 1). The only
possible route would imply large-scale deployment of negative emis-
sions, possibly at a scale that would be hard to achieve in the real world.
Moreover, concerns were raised regarding the risks of temporarily
overshooting the 1.5 ◦C regarding potential impacts of NETs (phase 2).
In other words, presenting the 1.5 ◦C as infeasible or presenting it as
feasible with NETs both had direct policy implications. This shows how
the often-reiterated boundary of ‘policy-relevant’ versus ‘policy-pre-
scriptive’ is far more uid in actual practice.
Fig. 2.. Overview of the sequence of science-policy interactions around the IPCC SR1.5 between 2015 and 2020 through which the 1.5
◦C goal increasingly
gained traction.
L. van Beek et al.
Environmental Science and Policy 133 (2022) 193–202
200
5.2. Dilemma 2: exploring radical solutions vs staying close to policy
discussions
A second dilemma concerns the exploration of mitigation options. On
the one hand, modellers aim to explore a wide range of policy options.
The community refers to themselves as ‘mapmakers’ showing possible
pathways that policymakers can use to navigate policy options
(Edenhofer and Kowarsch, 2015; Beck and Oomen, 2021). On the other
hand, modellers are aware of dominant discourses in international
climate politics and avoid anticipated ‘policy no-go’s’. For instance, in
the context of the IPCC SR1.5, modellers explored more demand-side
mitigation options to reduce the use of NETs. However, more trans-
formative changes such as radical lifestyle changes and discontinued
economic growth were not part of this expansion. Modellers’ continuous
anticipation and adjustment to existing policy discourses contribute to
their policy relevance but also implies that they explore their solutions
space within the discursive context in which they are situated (cf.
Ellenbeck and Lilliestam, 2019). Hence modellers face the risk of what
political scientist Carl Friedrich (1937) once described as the power of
the ‘anticipated reaction’; actors refrain from raising an issue, assuming
it will be refuted (cf. Lukes, 1974). A potential risk is that modellers
exclude transformative pathways that contain politically challenging
but potentially crucial low-carbon strategies.
5.3. Dilemma 3: quantitative and crisp storylines vs avoiding policy-
prescriptiveness
Clear and consistent storylines, concrete numbers and visualisations
help modellers to get their messages across. The quantitative nature of
the storylines, such as ‘net-zero by 2050′, aid the credibility of their
projections (cf. van Beek et al., 2020; Porter, 1995). Moreover, the
storylines are short, specic, and autonomous and hold a certain level of
‘sturdiness’ that explains their travels in policy and media (cf. Morgan,
2011). On the other hand, these characteristics also risk model-based
results to become ‘rounded off’: they might lose important details or
nuance during these travels (cf. Morgan, 2011). For instance, the
communicative power of the illustrative pathways invited an interpre-
tation as ‘recipes for the future’ and the 45% emissions reductions by
2030 resulted in the ‘only 12 years left’ narrative (phase 3). Their
persuasiveness gives the IAM community a signicant responsibility
regarding their messaging and the range of options they explore.
6. Conclusion
Our ndings reiterate that rather than a neutral knowledge practice,
IAMs intrinsically shape ideas around how climate change should be
governed (Edwards, 1996; Beck and Mahony, 2017; Beck and Oomen,
2021). On the one hand, the shift towards a solution-oriented mode of
scientic assessments on climate mitigation implies that IAM analysis
becomes increasingly policy-relevant given their capacity to explore the
costs and effectiveness of mitigation options. On the other hand, the
direct political implications of IAM analysis in political and public
spheres brings the political neutrality of IAMs into question. Our anal-
ysis highlights that IAMs are not neutral ‘map-makers’ but are powerful
in shaping the imagined corridor of climate mitigation (cf. Beck and
Mahony, 2018b; Beck and Oomen, 2021). As such, IAM pathways may
not be policy-prescriptive in a strict sense, but they are certainly poli-
cy-shaping to a degree beyond policy relevance. Importantly, our nd-
ings suggest that the boundaries of this imagined corridor of climate
mitigation are not merely shaped by model capabilities or biases in
expert judgments (see e.g. Beck and Krueger, 2016; Keppo et al., 2021).
It is also the result of political calibration, the continuous readjustment
of the focus of key model questions to maintain policy relevance.
The worldwide resonance of the IPCC SR1.5 indicates that IAM
outputs have become relevant to inform deliberations on possible low-
carbon transformations beyond the science-policy interface. Since
Paris, non-state actors and substate actors such as civil society organi-
sations, industry and local governments are increasing involved in the
UNFCCC (B¨
ackstrand et al., 2017). Climate mitigation has become a
central topic of public debate. This prominence implies the need
broaden the constituency of IAM scenarios to a much more diverse set of
actors. IAM modelling teams are mostly situated in the Global North and
their projects are often funded by the EU (Cointe et al., 2019). This may
hinder the diversication of relevant publics and may preclude more
diverse and perhaps more radical perspectives on mitigation. In other
words, there is a need to ‘calibrate’ to the needs of societal actors beyond
policymakers. Perhaps IAMs should be shaped to function in the broader
‘science-society interface’ and be judged accordingly. In so doing, IAMs
could explore a greater variety of possible pathways. Perhaps they could
also correct for the bias that is inherent to the political calibration
necessary for operating in close proximity of the policymaking world.
CRediT authorship contribution statement
Lisette van Beek: Conceptualization, Data curation, Formal anal-
ysis, Investigation, Methodology, Visualization, Writing – original draft,
Writing – review & editing. Jeroen Oomen: Conceptualization, Data
curation, Writing – original draft, Writing – review & editing. Maarten
Hajer: Conceptualization, Supervision, Writing – review & editing.
Peter Pelzer: Conceptualization, Supervision, Writing – review & edit-
ing. Detlef van Vuuren: Conceptualization, Supervision, Writing – re-
view & editing.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgements
The authors thank all interviewees for their valuable time and in-
sights. Through the CLIMAGINARIES project, our work beneted from
the Swedish Research Council for Sustainable Development (FORMAS).
Appendix A. Supporting information
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.envsci.2022.03.024.
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