Carbon Brief - Zeke Hausfather
 |
Civil Society 1.5C protest at COP21. Credit: Takver.
|
In the 2015
Paris Agreement
on climate change, nearly every country on Earth pledged to keeping
global temperatures “well below” 2C above pre-industrial levels and to
“pursue efforts to limit the temperature increase even further to 1.5C”.
However, at the time, scientists had only modelled energy system and
carbon mitigation pathways to achieve the 2C target. Few studies had
examined how the world might limit warming to 1.5C.
Now a paper in
Nature Climate Change
presents the results from a new modelling exercise using six different
“integrated assessment models” (IAMs) to limit global temperatures in
2100 to below 1.5C.
The results suggest that 1.5C is achievable if global emissions peak
in the next few years and massive amounts of carbon are sucked out of
the atmosphere in the second half of the century through a proposed
technology known as bioenergy with carbon capture and storage (
BECCS).
Defining the 1.5C target
One challenge with the goal of limiting warming to 1.5C above pre-industrial levels is that it was
not clearly defined in the text of the Paris Agreement. For example, scientists disagree on what, exactly, pre-industrial temperatures were and
how best to define them, as well as
what dataset to use.
There is also not a clear consensus if the target should be to aim to
have even odds of the world reaching 1.5C warming by 2100, or seek to
try and avoid having temperatures exceed 1.5C by aiming for an even
lower warming amount. Because
uncertainties in climate sensitivity
mean that we could have anything between 1.5C and 4.5C warming per
doubling of CO2 emissions, scientists tend to plan to avoid the worst
case where climate sensitivity ends up being on the higher end of the
range.
In the case of the 2C target, the Paris Agreement’s “well below”
language has been interpreted as ensuring that there is no more than a
33% chance of exceeding 2C – and, therefore, a 66% chance of staying
below it. But the 1.5C target
could be interpreted
as either aiming for a 50% chance of staying below 1.5C, or a 66%
chance similar to the 2C target. This may sound like a small
distinction, but it has large impacts on the
resulting carbon budget and ease of meeting the target.
In their new paper, a team of 23 energy researchers choose the
stricter interpretation of the target, aiming for a 66% chance of
avoiding more than 1.5C warming in the year 2100. However, they allow
for temperatures to exceed 1.5C over the course of the century as long
as they fall back down to below 1.5C by the year 2100. This is known as
an “overshoot” scenario.
1.5C only possible in some future pathways
To assess viable pathways to limit warming to 1.5C, the researchers use the new
Shared Socioeconomic Pathways
(SSPs) developed in preparation for the next Intergovernmental Panel on
Climate Change (IPCC) assessment report due early next decade. These
SSPs – which Carbon Brief will explore in more depth in the coming weeks
– present five possible future worlds that differ in their population,
economic growth, energy demand, equality and other factors.
Each world could have multiple different climate trajectories, though
some will have a much easier time reducing emissions than others. The
new climate trajectory associated with avoiding more than 1.5C warming
in 2100 is called Representative Concentration Pathway 1.9 (“RCP1.9”),
which is a world where the radiative forcing from greenhouse gases is
limited to no more than 1.9 watts per meter squared (W/m2) above
pre-industrial levels. This is lower than the range of RCPs previously
used by climate modellers, which went from 2.6 up to 8.5W/m2.
The six IAMs all find viable 1.5C scenarios in SSP1, which is a
pathway that focuses on “inclusive and sustainable development”. Four of
the six models find pathways in SSP2, which is a middle of the road
scenario where trends largely follow historical patterns. No models show
viable 1.5C pathways in SSP3, which is a world of “regional rivalry”
and “resurgent nationalism” with little international cooperation.
Finally, only one of the models has a 1.5C pathway in SSP4, which is a
world of “high inequality”, while two models have viable pathways in
SSP5, a world of “rapid economic growth” and “energy intensive
lifestyles”.
Emissions must peak quickly
To limit warming to below 1.5C, all the models that the researchers
examined require that global emissions peak by 2020 and decline
precipitously thereafter. After 2050, the world must reduce net CO2
emissions to zero and emissions must be increasingly negative throughout
the second half of the 21st century.
Even with these rapid reductions, all the scenarios considered still
overshoot 1.5C warming in the 2040s, before declining to around 1.3-1.4C
above pre-industrial levels by 2100. Models with more rapid reductions –
generally associated with SSP1 – have less temperature overshoot than
those with more gradual reductions.
The figure below shows both CO2 emissions (left) and global warming
above pre-industrial (right) across all the 1.5C models examined. The
lines are coloured based on the SSP used.
CO2 emissions in gigatons (Gt) CO2 (left) and global mean surface
temperature relative to preindustrial (right) across all RCP1.9/1.5C
scenarios included in Rogelj et al 2018. Data available in the IIASA SSP database. Chart by Carbon Brief using Highcharts.
The models show a remaining 1.5C “
carbon budget” from 2018 to 2100 of between -175 and 400 gigatonnes of CO2 (GtCO2). This range is
consistent with estimates from the
IPCC’s 5th Assessment Report.
The wide range is largely a result of differences in emissions of
non-CO2 greenhouse gases, such as methane and nitrous oxide, which vary
by a factor of between two and three across the models by 2100. Some
models with higher non-CO2 emissions have a remaining carbon budget of
less than zero, requiring more CO2 to be removed from the atmosphere
than added by the end of the century. In these simulations, the carbon
budget for 1.5C has already been used up.
The central estimate across the models is that the remaining
2018-2100 carbon budget is around 230 GtCO2. At the current rate of
emissions, this would allow roughly six years until the entire 1.5C
budget is exhausted, with a range of zero to 11 years across all the
models.
Replacing fossil fuels with renewables
The study explores the different ways that global energy needs can be
met, while also cutting GHG emissions in order to meet the 1.5C goal.
Limiting warming to below 1.5C requires that the world rapidly phase out
all types of fossil fuels – or at least those without accompanying
carbon capture and storage
(CCS). At the same time, the world need to quickly ramp up the use of
zero and net-negative carbon energy sources – things such as BECCS that
generate energy while actually removing CO2 from the atmosphere.
The figure below shows the use of renewables (left), net-negative
BECCS (centre) and coal without CCS (right) across all the 1.5C models.
The colours show which SSPs the model simulations use.
 |
| Global primary energy use in exajoules (EJ) for non-renewable biomass
(left), BECCS (center), and coal without CCS (right) across all
RCP1.9/1.5C scenarios. Adapted from Figure 2 in Rogelj et al 2018. LARGE IMAGE |
In most models, overall energy use actually increases between 2018
and 2100, by between -22% and +83%, with a central increase of 22%.
However, the models also show that energy efficiency is quite
important in the short term – at least, while most energy comes from
fossil fuels. This is particularly important in the transportation and
building sectors, where rapid decarbonisation is more difficult than in
power generation.
The models show an estimated 60-80% of all energy coming from
renewables globally by 2050. Some models also show a much larger role
for nuclear power, though others do not.
To limit warming to 1.5C, coal use without carbon capture declines by
around 80% by 2040, with oil similarly mostly phased out by 2060. This
would require most petrol or diesel vehicles to be phased out by 2060,
with electric or other low-carbon alternative fuel vehicles making up
the vast majority of sales well before that date. Future natural gas use
is more mixed in the models, with some showing increases and some
decreases by mid-century.
Emissions must go negative
Negative emissions
are needed in the latter half of the century to pull the extra CO2 out
of the atmosphere. This is because emissions cannot fall fast enough in
the models to avoid exceeding the allowable carbon budget to avoid 1.5C
warming.
Most of the models emit roughly 50-200% more CO2 than the allowable
carbon budget over the course of the century, before pulling the extra
CO2 back out.
The models assume widespread adoption of BECCS starting between 2030
and 2040 and then rapidly scaling up. By 2050, many models have BECCS
producing more than 100 exajoules (EJ), roughly the same amount of
energy globally as coal provides today. By 2100, BECCS will be around
200EJ compared to 300EJ for all non-biomass renewable energy.
The figure below shows the amount of CO2 sequestered by CCS (both
from BECCS and fossil fuels) across all the models. Carbon capture ramps
up after 2020 and could be 20 GtCO2 or higher by the end of the
century, which is around half of global CO2 emissions in 2018.
 |
| Annual CO2 sequestered by carbon capture and storage in gigatons (Gt)
CO2 by year and SSP across all RCP1.9/1.5C scenarios. Adapted from
Figure 3 in Rogelj et al 2018. |
The models produce estimates of global forest cover changes between
-2% and 26% between today and 2100, with most models showing significant
increases in forest cover. Both BECCS and afforestation require a lot
of land. Most models show a decline in global cropland scenarios roughly
equal to the area currently used for agriculture across the entire
European Union.
However, most of the models used in the study do not include
afforestation as an explicit mitigation option, so afforestation and
other
“natural” negative emissions technologies
could potentially play a larger role in the future. The specific
technologies used for future negative emissions may be different and
somewhat less reliant on BECCS, but non-BECCS approaches are largely
excluded from the models due to remaining uncertainties in cost and
effectiveness at scale.
Similarly, the amount of BECCS used differs quite a bit between
models and across SSPs, with SSP1 requiring the least negative emissions
and SSP5 requiring the most due to its slower emissions reductions and
higher overall energy use.
Dr Joeri Rogelj, the paper’s lead author from the
International Institute for Applied Systems Analysis (IIASA) in Austria, tells Carbon Brief:
“This indicates that a focus on sustainable lifestyles that limit energy demand can strongly reduce the reliance on BECCS.”
One interesting consequence of the 1.5C target is a reduced use of
fossil fuels combined with CCS, compared to what is found in 2C
scenarios. This is because fossil fuels with CCS still results in
methane emissions from coal mining or gas handling, as well as CO2
emissions due to imperfect capture and leakage. These extra emissions
can become too important to allow at a large scale in a 1.5C world.
Much more difficult to reach 1.5C than 2C
In addition to exploring the details of what it would take to limit
warming to 1.5C, the paper also compares it to existing 2C scenarios
across a number of different categories. The figure below shows the
difference between 1.5C and 2C scenarios across both economic and CO2
reduction metrics. Each dashed line represents a 100% increase in cost
or effort in a 1.5C world compared to a 2C world.
 |
| Relative increases in cost and CO2 reduction metrics for 1.5C scenarios compared to 2C scenarios for various SSPs. Each dashed line represents a 100% increase in cost or reduction amount, up to a 500% increase. Taken from figure 4 in Rogelj et al 2018. LARGE IMAGE |
The largest increases are in carbon prices, which must be between
200% and 400% higher, and in near-term costs, which are 200% to over
300% higher. These increases in short-term costs are driven by the more
severe near-term emission reductions needed. Long-term costs are also
expected to be around 200% higher.
For CO2 reduction metrics, a 1.5C world requires approximately two to
three times larger reductions in CO2 from buildings and transport than
in a 2C world. These sectors are more difficult to decarbonise than
power generation as they involve the direct combustion of fossil fuels
that are less easily replaced.
Difficult, but possible?
The new scenarios in this study are important because they show that
there are possible trajectories and technological pathways that can
limit warming to below 1.5C in 2100. However, all of the models included
overshoot 1.5C of warming in the middle of the century. Most also rely
on massive amounts of
still-unproven negative emissions later in the century to allow a more feasibly gradual reduction in emissions in the near-term.As
Dr Glen Peters, a senior researcher at the
CICERO Center for International Climate Research in Norway who was not involved in the study, tells Carbon Brief:
“Limiting temperature to 1.5C is getting close to what
models can deliver, with only certain socioeconomic, technological and
resource assumptions amenable to 1.5C pathways. How to transform the
model results into a viable society transformation remains the elephant
in the room. The 1.5C scenarios require radical reductions in unabated
fossil fuel use, rapid expansion of non-fossil energy sources and
planetary-scale carbon dioxide removal. Failing to meet any of those
core building blocks will make 1.5C quickly infeasible.”
Links
