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Four Representative
Concentration Pathways
(RCPs) present a range of
climate futures, running
from hot to cool: from very
high emissions in RCP 8.5
down to what was then
considered an ambitious
mitigation pathway, RCP
2.6. The numbers labelling
each RCP give the climate
forcing in 2100, that is, the
Earth’s energy imbalance
owing to human greenhouse
gas emissions, in watts per
square meter.
On the other axis of the
matrix, the Shared Socioeconomic
Pathways (SSPs) tell of five future
worlds. SSP1 is a sustainable, equitable
path, with a focus on wellbeing over
wealth. SSP2 is a middle-of-the road tale,
with moderate improvements in efficiency
and sustainability, but with trends mainly
following historical patterns. SSPs 3, 4, and 5
respectively tell of regional rivalry, inequality, and
fossil-fueled development.
To develop a detailed scenario, you choose one
SSP (the setting of the story) and constrain emissions
according to one RCP (the story arc). Then a model can
try to work out the most cost-effective set of mitigation
options that fit both pathways.
Sometimes, that is impossible. For example, there
is no way to force the SSP3 regional rivalry story to be
compatible with RCP2.6. This is a crucial insight from
the matrix era: a wrong turn in global socioeconomic
development would make deep mitigation impossible.
Alarms and excursions
The Paris Agreement gave scenario-makers a clear
and specific goal: limit global warming to well below
2°C, preferably 1.5°C. But by aiming at the Paris
goals only at the end of the story, in 2100, almost all
recent pathways and scenarios have taken us on a
narrative rollercoaster. They overshoot temperatures
in the middle of the century, then swing back down
by requiring that global emissions go net-negative.
This requires carbon removal technologies, such as
bioenergy with carbon capture and storage (BECCS),
on a huge scale. This is a risky plot device, letting the world stray
into danger and then expecting the cavalry to ride
in and save us. While carbon removal is likely to be
an essential mitigation tool, it might turn out to be
unfeasible at the scale required by this narrative.
Meanwhile, there would be serious impacts during
the overshoot, increasing climate hazards for millions.
A temporary temperature rise could also lead to
permanent shifts in climate or ecology. Once a species
or an ecosystem has gone extinct, no amount of
negative emissions will bring it back.
Such an adventure into unknown territory may make
for an exciting story, but that’s not what the planet
needs right now. In 2019, a team led by IIASA researcher
Joeri Rogelj set out principles for telling more soothing
stories, which pay attention to the journey as much as
the ending. In their framework, the goal is to limit peak
temperature. Emissions cuts bring the world to net-
zero without breaking the carbon budget, which means
there is no temperature overshoot.
As well as reducing climate hazards, this could
bring economic benefits. Avoiding overshoot needs
ambitious, rapid mitigation in the short term, and that
takes a lot of investment – but it pays off in the long-
run, according to a study by IIASA Energy, Climate, and
Environment Program Director, Keywan Riahi and
colleagues. Scenarios avoiding overshoot lead to higher
global GDP by the end of the century. That is without
even allowing for the economic impacts of climate
change, which would be worse in overshoot scenarios.
Recent scenarios confirm a more general point:
action is urgent. The study found that unless countries
strengthen their short-term climate commitments
(Nationally Determined Contributions), then 1.5°C is out
of reach. Moving fast may also be more feasible. A study
by IIASA researcher Elina Brutschin and colleagues
examined pitfalls that could derail mitigation, such as
ineffective governance and stranded assets, and found
that rapid early mitigation lessens such concerns.
The moral of these new stories
is clear: don’t waste time.
Further information:
pure.iiasa.ac.at/6101
pure.iiasa.ac.at/16075
pure.iiasa.ac.at/17259
climatescenarios.org/toolkit Elina Brutschin brutschin@iiasa.ac.at
Nebojsa Nakicenovic naki@iiasa.ac.at
Keywan Riahi riahi@iiasa.ac.at
Joeri Rogelj rogelj@iiasa.ac.at
11Optionswww.iiasa.ac.at
Winter 2021
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Volume winter 2021
- Title
- options
- Volume
- winter 2021
- Location
- Laxenburg
- Date
- 2021
- Language
- English
- License
- CC BY-NC 4.0
- Size
- 21.0 x 29.7 cm
- Pages
- 32
- Categories
- Zeitschriften Options Magazine