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Short-lived promise?
Short-lived climate pollutants, cumulative carbon
and emission metrics
MYLES ALLEN
Environmental Chan...
Key findings of the IPCC 5th Assessment
• Cumulative emissions of carbon dioxide largely
determine global mean surface tem...
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
Cumu...
“near-term emission
control measures (on
methane and soot),
together with measures to
reduce CO2 emissions,
would greatly ...
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...
Why the timing of SLCP and CO2
mitigation matter for the 2oC goal
CO2 emission scenarios
2020 2040 2060 2080 2100
Year
0
2...
Impact of immediate SLCP mitigation
Impact of CO2 and SLCP cuts
2020 2040 2060 2080 2100
Year
1.0
1.5
2.0
2.5
Warming(o
C)...
Impact of delayed SLCP mitigation
Impact of delayed emission cuts
2020 2040 2060 2080 2100
Year
1.0
1.5
2.0
2.5
Warming(o
...
What they should have said about SLCPs
“near-term emission
control measures (on
methane and soot) would
would greatly impr...
How should we prioritize short-lived v.
long-lived climate pollutants?
Impact of "equivalent" emissions
0 20 40 60 80 100
...
GWP100 really means GTP20-40: relevant
to temperatures in the 2050s
0 20 40 60 80 100
Time-horizon
1
10
100
a) Methane met...
GWP100 really means GTP20-40: relevant
to temperatures in the 2050s
0 20 40 60 80 100
Time-horizon
1
10
100
1000
10000
b) ...
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,
...
So how should governments interpret
GWP100?
• Accept it is a short-term metric and rename it GTP40.
– If governments want ...
Comparing pulse emissions of LLCPs
with sustained emissions of SLCPs
0 20 40 60 80 100
Years after start of emission
0.00
...
The only valid use of GWP100 for “CO2-
equivalence” between LLCPs & SLCPs
• Compare pulse emissions of long-lived gases li...
The meaning of “net zero”
• Net zero emissions of long-lived climate pollutants.
• Constant emissions of short-lived clima...
Measuring progress to “net zero
by 2oC.”
• http://safecarbon.org
An index of anthropogenic
warming
1900 1950 2000
Year
-0.5
0.0
0.5
1.0
Warmingfrom1861-1880(K)
a
Otto et al, to appear, 20...
Anthropogenic warming is 5%
closer to 2oC since 2009
1980 1990 2000 2010 2020
Year
0.2
0.4
0.6
0.8
1.0
Warmingfrom1861-188...
www.oxfordmartin.ox.ac.uk
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Allen m 20150708_1730_upmc_jussieu_-_amphi_24

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Allen m 20150708_1730_upmc_jussieu_-_amphi_24

  1. 1. 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 myles.allen@ouce.ox.ac.uk THE OXFORD MARTIN SAFE CARBON INVESTMENT INITIATIVE
  2. 2. 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.
  3. 3. The most important image of AR5 Reserves Resources
  4. 4. A surfeit of 2oC budgets Meinshausen et al (2009) & Ekins & McGlade (2015) budget to 2050
  5. 5. A surfeit of 2oC budgets IPCC (2014) likely below 2oC total anthropogenic warming
  6. 6. A surfeit of 2oC budgets IPCC (2014) unlikely below 2oC total anthropogenic warming
  7. 7. A surfeit of 2oC budgets IPCC (2014) likely below 2oC CO2- induced warming & Allen et al (2009)
  8. 8. A surfeit of 2oC budgets IPCC (2014) as likely as not below 2oC CO2-induced warming
  9. 9. How much will ratio of total-warming : cumulative-carbon increase in future?
  10. 10. How much will ratio of total-warming : cumulative-carbon increase in future? No change
  11. 11. How much will ratio of total-warming : cumulative-carbon increase in future? 2x
  12. 12. How much will ratio of total-warming : cumulative-carbon increase in future? 3x
  13. 13. 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.
  14. 14. “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)
  15. 15. The reason everyone is excited about SLCPs using GWP100 or RF metrics 10% cut in CO2-equivalent emissions
  16. 16. “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
  17. 17. Why the timing of SLCP and CO2 mitigation matter for the 2oC goal CO2 emission scenarios 2020 2040 2060 2080 2100 Year 0 20 40 60 80Billiontonnesperyear(GtCO2/yr) High emissions Ambitious mitigation Delayed mitigation Allen, Oxford Martin School, 2015
  18. 18. Impact of immediate SLCP mitigation Impact of CO2 and SLCP cuts 2020 2040 2060 2080 2100 Year 1.0 1.5 2.0 2.5 Warming(o C) CO2 & SLCP cuts CO2-CH4 offset CO2 only SLCP onlyNo emission cuts Period of SLCP cuts Allen, Oxford Martin School, 2015
  19. 19. Impact of delayed SLCP mitigation Impact of delayed emission cuts 2020 2040 2060 2080 2100 Year 1.0 1.5 2.0 2.5 Warming(o C) Both early Delayed SLCP Delayed CO2 Both delayed Early cuts Delayed cuts Allen, Oxford Martin School, 2015
  20. 20. 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.”
  21. 21. How should we prioritize short-lived v. long-lived climate pollutants? Impact of "equivalent" emissions 0 20 40 60 80 100 Years after time of emission 0 1 2 3 4 5 Globaltemperaturechange(o C) 1000 GtCO2-e under GWP100 Carbon dioxide Methane Nitrous Oxide HFC-134a HFC-152a Black Carbon Allen et al, in prep, 2015
  22. 22. GWP100 really means GTP20-40: relevant to temperatures in the 2050s 0 20 40 60 80 100 Time-horizon 1 10 100 a) Methane metric value GTP GWP Allen et al, in prep, 2015
  23. 23. GWP100 really means GTP20-40: relevant to temperatures in the 2050s 0 20 40 60 80 100 Time-horizon 1 10 100 1000 10000 b) Organic & black carbon metric value GTP GWP Allen et al, in prep, 2015
  24. 24. GWP100 really means GTP20-40: relevant to temperatures in the 2050s
  25. 25. 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.
  26. 26. 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.
  27. 27. Comparing pulse emissions of LLCPs with sustained emissions of SLCPs 0 20 40 60 80 100 Years after start of emission 0.00 0.01 0.02 0.03 Globaltemperaturechange(o C) 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
  28. 28. 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.
  29. 29. 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.
  30. 30. Measuring progress to “net zero by 2oC.” • http://safecarbon.org
  31. 31. An index of anthropogenic warming 1900 1950 2000 Year -0.5 0.0 0.5 1.0 Warmingfrom1861-1880(K) a Otto et al, to appear, 2015
  32. 32. Anthropogenic warming is 5% closer to 2oC since 2009 1980 1990 2000 2010 2020 Year 0.2 0.4 0.6 0.8 1.0 Warmingfrom1861-1880(K) b Otto et al, to appear, 2015
  33. 33. www.oxfordmartin.ox.ac.uk

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