21
CLIMATE ROUNDTABLE - CHASING NEW IDEAS
Based on the modeling results shown in the figure above,
and other lines of evidence, the IPCC estimates that, to have
a better than 2-in-3 chance of keeping the warming they cause
to less than 2°C, total CO
2
emissions must be limited to less
than one trillion tonnes of carbon (3.7 trillion tonnes of carbon
dioxide). Emitting 1.2 trillion tonnes would give an even chance
of CO
2
-induced warming exceeding 2°C, while 1.5 trillion tonnes
would give a 2-in-3 chance of CO
2
-induced warming over 2°C.
Emissions over the past 250 years add up to over half a trillion
tonnes of carbon, and if current trends continue, will exceed
one trillion tonnes by the early 2040s.
Total reserves of fossil carbon already exceed the remaining
carbon budget consistent with the 2°C goal by at least a factor
of two, and developments in unconventional fuels are adding
to these reserves faster than they are being consumed.
A substantial fraction have relatively low extraction costs,
such that they are likely to remain economically competitive,
particularly for high energy-density applications such as air
travel, regardless of future developments in renewable or
nuclear power.
Climate policy therefore boils down to a simple choice
between three options:
– One, we emit more than one to 1.5 trillion tonnes of carbon knowing
this will impose more than 2°C warming on future generations.
– Two, we introduce an emission control regime so draconian
that no one, anywhere in the world, ever, is permitted to burn
fossil carbon once the cumulative budget is exhausted.
– Or three, we ensure that, once the budget is exhausted, all further
use of fossil carbon can be offset by permanent CO
2
removal.
Given the timescales involved in the carbon cycle, permanent
removal requires geological sequestration or re-mineralization,
not temporary storage in the biosphere through afforestation.
In my view, the only ethical option is three, which suggests
a significantly greater emphasis on carbon capture and
sequestration in climate mitigation policy.
In my view, the only ethical option is (...)
a significantly greater emphasis on
carbon capture and sequestration in climate
mitigation policy.
Myles Allen
Cumulative total anthropogenic CO
2
emissions from 1870 (GtC)
Cumulative total anthropogenic CO
2
emissions from 1870 (GtCO
2
)
Temperature anomaly relative to 1861-1880 (°C)
1890
Past emissions
0
0
1
2
3
4
5
500
1000 2000 3000 4000 5000 6000 7000 8000
1000
1500
2000
2500
Must be sequestered/recaptured to meet 2°C goal
CO
2
-induced warming
Total anthropogenic
warming
2°C budget
1980
1950
2000
2010
2030
2100
2050
2030
2050
2100
2050
2010
2050
2100
Warming in the CMIP-5 multi-model ensemble under the Representative
Concentration Pathway scenarios (coloured lines, with orange plume
showing the range of uncertainty) and under idealized CO
2
-only
scenarios (grey line and plume) plotted as a function of cumulative
total anthropogenic CO
2
emissions from 1870 onwards – figure SPM10
of IPCC (2013). Black horizontal bar shows historical emissions to
date; dark grey bar shows the approximate cumulative emission budget
consistent with limiting warming to 2°C; light grey bar shows fossil
carbon reserves that, if used, must be sequestered or recaptured if
the 2°C goal is to be met.
RCP 2.6
RCP 4.5
RCP 6.0
RCP 8.5
Historical
RCP range
1% yr
-1
CO
2
1% yr
-1
CO
2
range
RCP 2.6
RCP 4.5
RCP 6.0
RCP 8.5
Historical
RCP range
1% yr
-1
CO
2
1% yr
-1
CO
2
range
