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Short-lived promise? Short-lived climate pollutants, cumulative carbon and emission metrics
MYLES ALLEN
Environmental Change Institute, School of Geography & the Environment and Department of Physics, University of Oxford
THE OXFORD MARTIN SAFE CARBON INVESTMENT INITIATIVE
Key findings of the IPCC 5th Assessment
• Cumulative emissions of carbon dioxide largely determine global mean surface temperature by the late 21st century and beyond.
• All current GHG emissions [& other pollutants] affect the rate and magnitude of climate change over the next few decades…
• Emissions of non-CO2 forcers are often expressed as “CO2-equivalent emissions”, but the choice of metric to calculate these emissions, […], depends on application, policy context, and … value judgments.
The most important image of AR5
Reserves Resources
A surfeit of 2oC budgets
Meinshausen et al
(2009) & Ekins &
McGlade (2015)
budget to 2050
A surfeit of 2oC budgets
IPCC (2014) likely
below 2oC total
anthropogenic
warming
A surfeit of 2oC budgets
IPCC (2014) unlikely
below 2oC total
anthropogenic
warming
A surfeit of 2oC budgets
IPCC (2014) likely
below 2oC CO2-
induced warming &
Allen et al (2009)
A surfeit of 2oC budgets
IPCC (2014) as
likely as not below
2oC CO2-induced
warming
How much will ratio of total-warming : cumulative-carbon increase in future?
How much will ratio of total-warming : cumulative-carbon increase in future?
No change
How much will ratio of total-warming : cumulative-carbon increase in future?
2x
How much will ratio of total-warming : cumulative-carbon increase in future?
3x
Reasons the ratio of total-warming : cumulative-carbon might increase
1. Uncertainty in Transient Climate Response to Cumulative Carbon Emissions (TCRE): precautionary budgets.
2. Ratio of non-CO2-induced warming to CO2-induced warming is expected to increase:
– Stringent mitigation scenarios envisage as much or more future non-CO2-induced than CO2-induced warming.
– Meeting the 2oC goal will require action on non-CO2 climate pollutants like methane and soot.
– But how much action, and when? The metrics problem.
“near-term emission control measures (on methane and soot), together with measures to reduce CO2 emissions, would greatly improve the chances of keeping Earth’s temperature increase to less than 2˚C.” UNEP/WMO report & Shindell et al, 2012
Enthusiasm for action on short-lived climate pollutants (SLCPs)
The reason everyone is excited about SLCPs using GWP100 or RF metrics
10% cut in CO2-equivalent emissions
“That said, the scientific community must speak out against recommendations — explicit or implicit9, 10 — to exclude SLCPs from discussions of climate-change mitigation or to delay their reduction. Tens of millions of lives are at stake…” Schmale et al, Nature, 2014
In case you are tempted to take a nap because I’m talking about GHG metrics
Why the timing of SLCP and CO2 mitigation matter for the 2oC goal
CO2 emission scenarios
2020 2040 2060 2080 2100Year
0
20
40
60
80B
illio
n to
nnes p
er
ye
ar
(GtC
O2/y
r)High emissions
Ambitious mitigation
Delayed mitigation
Allen, Oxford Martin School, 2015
Impact of immediate SLCP mitigation
Impact of CO2 and SLCP cuts
2020 2040 2060 2080 2100Year
1.0
1.5
2.0
2.5
Wa
rmin
g (
oC
)
CO2 & SLCP cuts
CO2-CH4 offset
CO2 only
SLCP onlyNo emission cuts
Period of SLCP cuts
Allen, Oxford Martin School, 2015
Impact of delayed SLCP mitigation
Impact of delayed emission cuts
2020 2040 2060 2080 2100Year
1.0
1.5
2.0
2.5
Wa
rmin
g (
oC
)
Both early
Delayed SLCP
Delayed CO2
Both delayed
Early cutsDelayed cuts
Allen, Oxford Martin School, 2015
What they should have said about SLCPs
“near-term emission control measures (on methane and soot) would would greatly improve the chances of keeping Earth’s temperature increase to less than 2˚C provided that CO2 emissions are significantly reduced at the same time.”
How should we prioritize short-lived v. long-lived climate pollutants?
Impact of "equivalent" emissions
0 20 40 60 80 100Years after time of emission
0
1
2
3
4
5
Glo
bal te
mp
era
ture
cha
ng
e (
oC
) 1000 GtCO2-e under GWP100
Carbon dioxide
Methane
Nitrous Oxide
HFC-134a
HFC-152aBlack Carbon
Allen et al, in prep, 2015
GWP100 really means GTP20-40: relevant to temperatures in the 2050s
0 20 40 60 80 100Time-horizon
1
10
100
a) Methane metric value
GTP
GWP
Allen et al, in prep, 2015
GWP100 really means GTP20-40: relevant to temperatures in the 2050s
0 20 40 60 80 100Time-horizon
1
10
100
1000
10000
b) Organic & black carbon metric value
GTP
GWP
Allen et al, in prep, 2015
GWP100 really means GTP20-40: relevant to temperatures in the 2050s
So, if your government is using GWP100 to quantify its INDC, then either…
• It doesn’t care what happens after the 2050s,
• It is confident temperatures will be approaching stabilization by the 2050s, or
• It still thinks “100-year Global Warming Potential” is a measure of relative importance of emissions for global warming over the next 100 years.
So how should governments interpret GWP100?
• Accept it is a short-term metric and rename it GTP40.
– If governments want to prioritize warming over the next 40 years, they should at least say so.
• Accept it is a conditional metric, to be used when CO2 emissions are falling.
– “CO2 first”: the simplest strategy to limit peak warming.
– As long as CO2 emissions are still rising, we cannot quantify the time to peak warming, so no amount of methane or black carbon is equivalent to another tonne of CO2.
• Use it to compare one-off emissions of long-lived pollutants with sustained emissions of SLCPs.
Comparing pulse emissions of LLCPs with sustained emissions of SLCPs
0 20 40 60 80 100Years after start of emission
0.00
0.01
0.02
0.03G
lob
al te
mp
era
ture
cha
ng
e (
oC
) b) 0.38GtCO2-e/yr sustained emissions
Pulse emission of CO2 in year 1
Same CO2-e sustained emission of SLCPs
Similar temperature
response
Allen et al, in prep, 2015
The only valid use of GWP100 for “CO2-equivalence” between LLCPs & SLCPs
• Compare pulse emissions of long-lived gases like CO2 (residence time >> metric time horizon) with sustained emissions of short-lived gases like CH4 (residence time << metric time horizon).
• E.g. GWP100 of methane is 28, so 28 tonnes of CO2 today is approximately equivalent to 1 tonne of CH4 spread over 100 years, or a sustained reduction in methane emission rates of 1/100th tonne/year.
The meaning of “net zero”
• Net zero emissions of long-lived climate pollutants.
• Constant emissions of short-lived climate pollutants.
• Any remaining one-off emissions of LLCPs offset by sustained reductions in rates of emissions of SLCPs and vice versa.
An index of anthropogenic
warming
1900 1950 2000Year
-0.5
0.0
0.5
1.0W
arm
ing
fro
m 1
861
-188
0 (
K)
a
Otto et al, to appear, 2015
Anthropogenic warming is 5%
closer to 2oC since 2009
1980 1990 2000 2010 2020Year
0.2
0.4
0.6
0.8
1.0W
arm
ing
fro
m 1
861
-188
0 (
K)
b
Otto et al, to appear, 2015
www.oxfordmartin.ox.ac.uk
