This document summarizes insights from integrated assessment scenarios that model pathways to limit global warming to 1.5°C. It notes that remaining carbon budgets are tight, with estimates of 580 gigatons of CO2 or less left to stay below 1.5°C warming. Meeting the 1.5°C goal will require net zero greenhouse gas emissions by around 2050 and massive investments in energy efficiency, renewables, and carbon removal technologies like BECCS. Scenarios show that transformation is possible but will require rapid changes across all sectors of energy, industry, transport, buildings, and land use over the next decade to achieve the necessary emissions reductions.

1. Selected Insights from Integrated
Assessment Scenarios
THE FIRST OF A NEW SERIES OF OECD WORKSHOPS ON CLIMATE SCIENCE,
POLICY, REGULATION AND PRACTICE
“CLIMATE CHANGE: ASSUMPTIONS, UNCERTAINTIES AND SURPRISES”
4 September 2020, Virtual Workshop
Keywan Riahi
International Institute for Applied Systems Analysis (IIASA)
Technische Universität Graz
University of Amsterdam

2. Process-based Integrated Assessment
Models (IAMs)
Temperature, RF
Exogenous Assumptions
Population
Labor Productivity
Technology
Policy
Livestock
Crops & Forests
Electric & Refining
Primary Energy Supply
Economic Activity
Commodity Prices
Prices, Taxes, e.g. CO2
Outputs of IAMs
Resources
CO2, GHGs, aerosols, OGs
External Data
Economy
Energy
Agriculture
& Land Use
Water
Climate
Atmosphere
Oceans
Carbon
Cycle
The Model
Highly non-linear, strategic models
designed to consider global climate
forcing and climate impacts at decadal
time scales and regional disaggregation
ranging from dozens to hundreds
Source: Jae Edmonds
 IAMs are used to test the response of the system to different policies or other
system constraints.
 Scenarios by IAMs are thus no predictions of what will happen. They rather
provide answers to “what if” questions.
 Typical question: What needs to be done in order to achieve a climate goal and
how much does it cost?
 Process-based IAMs are different from the aggregate macroeconomic “cost-
benefit IAMs” that assess the social cost of carbon (DICE, PAGE, FUND, etc…)

3. The remaining carbon
budget is very tight
• 580 GtCO2 left (50% chance of 1.5°C)
420 GtCO2 left (66% chance of 1.5°C)
+- 250 GtCO2 depends on what is done on non-CO2
+- 400 GtCO2 geophysical uncertainty
• Currently, 42 +- 3 GtCO2/yr annually
• 200 GtCO2 budget differences are about 5 year of
current emissions and imply roughly a 10 year
variation in the mid-century timing of reaching net
zero CO2 emissions.
Joeri Rogelj - CLA Chapter 2 – IPCC SR1.5

4. 1950 1975 2000 2025 2050 2075 2100
GHGemissions(GtCO2e)
-20
0
20
40
60
80
100
2°C
1.5°C
Emissions Pathways for reaching
1.5˚C and 2˚C
Net zero GHG emissions
~ 2050
Source: CD-LINKS, McCollum et al, 2018
NDCs
country pledges
since Paris
Agreement

5. “Shared socioeconomic pathways”
allows the community to systematically
explore uncertainties
• Shared socioeconomic pathways
(SSPs) explore two dimensions:
challenges to mitigation and challenges
to adaptation with 5 reference—i.e. no-
climate policy—scenarios.
• Different populations
• Different economic development
• Different technologies
• Different institutions
• Each is a possible pathway to
explore implications of long-term
climate goals (eg, limiting
temperature or GHG concentrations) Sustainability
Fossil-fueled
Development
Regional Rivalry
Inequality
Middle
of the Road

6. Broadly Two Perspectives on Meeting 1.5°C
GHG Emissions Profiles
Overshoot as
supply-side options
scale slowly, but need massive
long-term deployment
for high demand scenarios
Negative emissions, e.g. BECCS
Rapid Transformation
driven by end-use changes
(efficiency & behavior)
“Grand Restoration”
sink enhancement via
returning land to nature
Granular, distributed supply side
options lead the way for scaling
other mitigation options, rapid change
under low demand
Inertia in policy,
social & technology
change
“Conventional scenarios” LED Scenario (Gruebler et al, 2018)

7. Keywan Riahi - LA Chapter 5 – IPCC SR1.5
Globale Investments
Average Annual Energy Investments 2016 bis 2050
Efficiency
Renewables
T&D, Storage
Nuclear & CCS
Fossiler Exraction
Fossil Power
1.5°C in comparison to Baseline
investmentdisinvestment
~820 billion US$
(0.8% of GDP)
Source: McCollum et al, 2018

9. 1.5°C: Possible impacts of mitigation actions on
the SDGs
co-benefitadverseeffect
Krey et al. (submitted) – under embargo, do not cite
Mitigation risks
Mitigation co-benefits
http://www.cd-links.org

10. 1.5°C Scenario Explorer
The underlying data is available on-line and hosted
by IIASA
Visit the Scenario Explorer at https://data.ene.iiasa.ac.at/iamc-1.5c-explorer
Huppmann,Kriegler,Krey,Riahi,Rogelj,Rose,Weyantetal,2018

11. Thank you!
riahi@iiasa.ac.at