Stabilization and Global Climate
Policy in a Multi-Gas WorldMarcus C Sarofim*, Chris E Forest*,David M Reiner†, John M Reilly**Joint Program on the Science and Policyof Global Change, MIT†Judge Institute of Management Studies,Cambridge University
NIES and EMF Stabilization Workshop
Tsukuba, Japan, Jan. 22-23, 2004

Aim of the Research• To examine the issues involved in current
discussions of stabilization policy given amulti-greenhouse gas world– To encourage tighter definition ofstabilization in academic and political
discussion.
– To reemphasize the importance of non-CO2
greenhouse gases for effective, inexpensive
temperature reduction on the two centurytime scale.
– To examine stabilization under uncertainty.

Article 2 of the Framework
Convention on Climate Change• “The ultimate objective of this Convention... isto achieve... stabilization of greenhouse gas
concentrations in the atmosphere at a levelthat would prevent dangerous anthropogenic
interference with the climate system. Such alevel should be achieved within a time-frame
sufficient to allow ecosystems to adapt... toensure that food production is not threatened,
and to allow economic development toproceed in a sustainable manner.”

Definitions of Stabilization• Many anthropogenic greenhouse gases exist:– CO2, CH4, N2O, SF6, HFCs, etc.
– Not including “climatically important substances” such
as SO2, black carbon, ozone precursors, etc.

• Stabilize CO2 only? (EU 550 ppm position)– What are assumptions about other gases?
– SRES A1B is often used for non- CO2 gases.

• Stabilize overall radiative forcing?
– Separate targets for each gas? CO2 equivalents?

• Trading between gases?
– The use of Global Warming Potentials (GWPs) isincompatible with stabilization.

• When? 2100? 2150? 2500? 3000?

Nature of Two Scenarios:
CO2ONLY and GHGTRADE• Estimate cumulative CO2 emissions to 2100
consistent with ‘stabilization’ of CO2 at 550 ppm– Actually 530 ppm in 2100 to allow for gradualstabilization after 2100.

• Allocate CO2 reductions optimally over time.
– Discounted marginal abatement equalized over time—
price rises at the discount rate.

• Expand constraint to Other GHGs
– Allow GHG trading using 100-year GWP to achieve
reductions equal to CO2 only case– Considered proportional reduction case (not shown)Considerations• Emissions path consistent with a frequently
discussed policy target, reinterpreted inmultigas terms.
– Other interpretations possible.

• Economic rationale for initial allocation of
reductions over time
– but once expanded to other GHGs its no longer
quite true

• Known concentration and climate outcomes
associated with these emissions scenario• Economic and climate outcomes are true for
EPPA/MIT IGSM—not necessarily for othermodels.

Further Considerations• Initial CO2 path was achieved with a globally
uniform carbon tax– equal marginal costs.
• After adding other GHGs, reinterpreted as aquantity constraint• Concentration and climate effects depend
little on the regional allocation– Regionally reallocate global totals as desired, andstill be consistent with concentration and climateresults.

EPPA: An Economic/Emissions
Model• CGE model of the world economy with all humanactivities and all CIS’s.
– GHGs: CO2, CH4, N2O, SF6, PFC, HFC .
– Other air pollutants: NOX, SOX, CO, NMVOC,
NH3 and carbonaceous particulates.
– Activities: Energy combustion and production,agriculture and land use, industrial processes,waste disposal (sewage & landfills).

• Designed for the 100 year time scale.

Emission paths (550 ppm)
Methane Path

Carbon Path
3010009002515

GHGTRADE

Tg C H 4

GtC/year

700

BAU

20

10

CO2ONLY

600
500
400300GHGTRADE

200

CO2ONLY
5100
00
2000

BAU

800

2020

2040

2060

Year

2080

2100

2000

2020

2040

2060
Year

2080

2100

The MITIGSM
A coupled chemistry,climate, ocean, andecosystem model.

Some Aspects of the MIT IGSM
• Natural systems (ocean and terrestrial) integrated
part of the coupled atmosphere-ocean model– ocean and terrestrial biology of C uptake– natural CH4, N2O, C affected by climate andatmospheric concentrations of CO2• Carbon from human land use assumed to be
neutral over the century• Active and integrated atmospheric chemistry
resolved for urban and rural conditions– Tropospheric ozone as an additional warming effect– Sulfate aerosols as cooling effect– Oxidation of CH4 explicit so lifetime is endogenous∆ total forcing since 1990 (W/m 2)

Total GHG Forcing(change since 1990)
876BAU

5

CO2ONLY43

GHGTRADE

2
10
2000

2020

20402060Year2080

2100

Results in 2100
Temperature
Reduction(From 2.8 ºC)

Reduction inNet PresentConsumption

CO2ONLY

0.75 ºC

1.2%

GHGTRADE

1.18 ºC

0.5%

3

1.22.5

120.8

1.5

0.6

10.4
CO2ONLY Temperature
GHGTRADE Temperature
CO2ONLY Sea Level Rise
GHGTRADE Sea Level Rise

0.502000

2050

2100

2150

Year22002250

0.202300

Sea level rise since 1990 (m)Temperature change since 1990 (ºC)

Long Term Studies (to 2300)

Uncertainty in Policy Costs
Cumulative probability density

10.9
0.8
0.7
0.6CO2ONLY: Mean 1.1
5/95 bounds: 0.02 / 3.9

0.5
0.4
0.3

GHGTRADE: Mean 0.67
5/95 bounds: 0.01 / 2.6

0.2
0.100123456

Percentage Loss of Net Present Consumption

Uncertainty in Climate System Parameters
1.8

0.9

1.6

0.8

1.4

0.7

Probability Density

Probability Density (x 10 -2)

550 ppm CO2ONLY emissions scenario

1.2
1
0.8
0.6
0.4

0.6
0.5
0.4
0.3
0.2

0.2
0.1

00490

540590CO2 concentration (ppm)0123

Temperature Rise from 1990 (°C)

4Carbon Uptake
550 ppm CO2ONLY emissions scenario
10600
CO2 concentration

500

8Pg C/year

7

Carbon emissions

400

65300

421Total uptake

Ocean uptake

3CO2 emissions

200
100

Terrestrial uptake

02000

2050

2100

2150
Year

2200

2250

02300

ppm CO29CAVEATS• Modeling– Results depend heavily on abatement curves
and technology assumptions in model.
– Discount rate impacts emissions path and costcalculations: Absolute numbers but notconclusions are sensitive to choice of rate.

• Policy Implementation– Non-CO2 sources are hard to monitor.
– Reducing CO2 emissions may require capital
investments which should be started early.

Future Work• Uncertainty:– Impact of other gas emission uncertainty on
global temperature change results.
– Determining carbon emissions pathways given
carbon uptake uncertainty.
– Tradeoffs between cost and damages.

• Policy Improvements– Devising an “optimum cost over time” all-GHG
policy.
– More realistic policies: developing countries
should have differentiated goals.

Conclusions• Stabilization of carbon dioxide concentrations
can be met at reasonable costs. However,these costs will be much less if trading isallowed between all gases. Additionally, an
all-gas policy is much more effective thanCO2 only policies on the two century scale.
• Uncertainty in costs and uncertainty inimpacts should be incorporated into thedetermination of appropriate targets.
• Imprecision in language should be addressed
before creating long term policy frameworks.

Carbon-Equivalent Prices
2005-2050
Cost of Carbon (1995 US $/ton C eq)

300
250

Year 2005 Prices:$30.48$1.46

200
150100All GHG
Proportional

CO 2 only500200020102020

2030
Year

2040

2050

Carbon equivalents by GWP (Gt C)

Total GHG Emissions, GWP
weighted3530

Reference

25
20
CO2 only

15
10All GHG
Proportional Reduction

502000GWP Equiv.

2020

2040

2060
Years

2080

2100

Total Carbon Emissions
CO2 emissions (Gt C)

25
Reference

20

15
GWP Equivalent
10
CO2 only
ProportionalReductions

502000

2020

2040

2060
Year

2080

2100

Total Methane Emissions
CH4 emissions (MT)

1000
900

Reference

800
700
600
500
400

CO2 only
All GHGs
GWP equivalent

300
20010002000All GHGs
Proportional Reductions

2020

2040

2060
Year

2080

2100

CO2 Concentrations
CO2 Concentration (ppm)

800
700
All GHGs
GWP Eq.

600
Reference

500

CO2 only

400
All GHGs Proportional

300
2000

2020

2040

2060
Year

2080

2100

Methane Concentration
CH4 Concentration (ppm)

6Reference

54

CO2 only
3All GHGs
GWP equivalent

2
All GHGs
Proportional Reductions

102000

2020

2040

2060
Year

2080

2100

Total N2O Emissions
N2O Emissions (Tg)25
20

Reference

15
CO2 only
10
All GHGs
Proportional reduction

502000All GHGs
GWP equivalent

2020

2040

2060
Year

2080

2100

N2O Concentration
N2O Concentration (ppb)

500
480
460
440

Reference

420
400

CO2 only

GWP eq.

380360All GHGs
Proportional reduction

340
320
300
2000

2020

2040

2060
Year

2080

2100

Ozone Concentration
O3 concentration

55
50
45
40
35
30
2000

2020

2040

2060
Year

2080

2100

Global Mean Temperature Change:
2000-2100
Mean Temperature Change
from 1990 (°C)

32.5
Reference

2CO2 only

1.5
1All GHGs
Proportional Reduction

0.502000

2020

2040

2060
Year

2080

2100