CLIMATE CHANGE PROJECTIONS: SOURCES AND MAGNITUDES OF

UNCERTAINTY 

Tom Wigley,National Center for Atmospheric Research,

Boulder, CO 80307, USA(wigley@ucar.edu)

  Presented at:

NCAR Summer Colloquium on Climate and HealthNational Center for Atmospheric Research,

Boulder, CO.  

July 22, 2004 

OUTLINE OF TALK

Goal: To provide information about future global-mean temperature and sea level change and rates of temperature change, and their uncertainties, for both no-climate-policy and policy (concentration stabilization) emissions scenarios.

OUTLINE OF TALK

Warming and sea level commitments  No-climate-policy projections (probability density functions

for temperature and rates of change of temperature)  CO2 concentration stabilization: Concentration profiles and implied CO2 emissions Article 2 and choosing a CO2 stabilization target: Effects of adaptation and non-CO2 gases Effects of CO2 stabilization on future warming and sea level Multi-gas stabilization (CO2, CH4 and N2O) Effects of CH4 and N2O on CO2 emissions, warming and sea level

 

Future climate change depends on:

Perturbations already imposed on the climate system (because of oceanic thermal inertia, the effects of these perturbations have not yet been fully realized); and

Perturbations we may impose in the future.

The latter depends on what policies we introduce to limit future change.

FUTURE CLIMATE CHANGE: THREE CASES

(1) Changes already in the system – the ‘warming commitment’

(2) ‘No climate policy’ emissions scenarios

(3) Policy (concentration stabilization) scenarios

CASE 1: WARMING COMMITMENTS (changes already in the system)

(a) If we were able to stabilize atmospheric composition at today’s (year 2000) level[constant-C commitment]

(b) If we stabilized all emissions at today’s levels[constant-E commitment]

COMMITMENT UNCERTAINTIES….. are due to …..

(1) uncertainties in past natural and anthropogenic forcing (mainly aerosol forcing)

(2) gas-cycle and climate model uncertainties ….. • carbon cycle feedbacks • climate sensitivity • ocean mixing

FORCING BREAKDOWN IN 2000

-2

-1.5

-1

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1

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2.5

3

FOR

CIN

G (

W/m

**2)

CO2

CH4

N2O

TROP. O3

HALOS

AEROSOL

UNCERTAINTY

2.68

-0.23

-1.91

range used in analyses

CONSTANT-C WARMING COMMITMENTCONSTANT-Q WARMING COMMITMENT: DT2x AND QAER EFFECTS

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L-M

EAN

TEM

PER

ATU

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CH

AN

GE

(deg

C)

DT2x = 1.5 degC

DT2x = 2.6 degC

DT2x = 4.5 degC

H

M

L

CONSTANT-E WARMING COMMITMENTCONSTANT-E WARMING COMMITMENT: DT2x AND QAER EFFECTS

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L-M

EAN

TEM

PER

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CH

AN

GE

(deg

C)

DT2x = 1.5 degC

DT2x = 2.6 degC

DT2x = 4.5 degC

H

M

L

CONSTANT-C SEA LEVEL COMMITMENTCONST-Q SEA LEVEL RISE COMMITMENT: DT2x, QAER & MELT EFFECTS

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L-M

EAN

SEA

LEV

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ISE

(cm

)

DT2x = 1.5 degC; QAER = HIGH; MELT = LOW

DT2x = 4.5 degC; QAER = LOW; MELT = HIGH

H

M

L

KEY : DT2x = 1.5, 2.6, 4.5 degC

CONSTANT-E SEA LEVEL COMMITMENTCONST-E SEA LEVEL RISE COMMITMENT: DT2x, QAER & MELT EFFECTS

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GLO

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SEA

LEV

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ISE

(cm

)

DT2x = 1.5 degC; QAER = HIGH; MELT = LOW

DT2x = 4.5 degC; QAER = LOW; MELT = HIGH

H

M

L

KEY : DT2x = 1.5, 2.6, 4.5 degC

CASE 2: CLIMATE CHANGE IN THE ABSENCE OF CLIMATE MITIGATION POLICIES

PREDICTING FUTURE CLIMATE CHANGE

  Predict future socioeconomic changes  Use these to predict future emissions  From these predict changes in atmospheric composition  Use these results to drive a climate model

Question: How do we do this probabilistically?

[Results in this presentation use the MAGICC climate model. MAGICC can be downloaded from www.cgd.ucar.edu] 

THE SRES EMISSIONS SCENARIOS (The basic drivers for future climate change)

The Intergovernmental Panel on Climate

Change (IPCC) has sponsored production of a new set of ‘no-climate-policy’ emissions scenarios for GHGs, sulfur dioxide, and other gases

These scenarios are based on a range of

assumptions regarding future population, economic growth, energy technology growth, etc.

The scenarios are published in a Special Report

on Emissions Scenarios – hence the acronym SRES

(Special Report on Emissions Scenarios, eds. N.Nakicenovic and R. Swart, Cambridge University Press, 2000)

FUTURE EMISSIONS IPCC SPECIAL REPORT ON EMISSIONS SCENARIOS (SRES) GASES CONSIDERED: CO2 CH4 N2O SO2 Reactive gases (CO, NOx, VOCs) Halocarbons (CFCs, HCFCs, HFCs, PFCs, SF6)

RELATIVE IMPORTANCE OF DIFFERENT GASES

FORCING CONTRIBUTIONS : A1B EMISSIONS

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1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

YEAR

RADIA

TIVE

FO

RCIN

G (

W/m

**2)

CO2

AEROSOLS

N2OHALOS

TROP O3CH4

SRES CARBON DIOXIDE (CO2) PROJECTIONS

(emissions and concentrations)

SRES DECADAL FOSSIL-FUEL CO2 EMISSIONS

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1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100YEAR

FOSS

IL C

O2

EMIS

SIO

NS (

GtC

/yr)

A1 - REDB1 - BLACKA2 - GREENB2 - BLUE

MEAN - MAGENTA

SRES RANGE OF CO2 PROJECTIONS

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950

1000

1050

1100

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100YEAR

CO

2 C

ON

CEN

TRA

TIO

N (

ppm

)

IPCC TAR GLOBAL-MEAN TEMPERATURE PROJECTIONS

MAGICC projections in the IPCC TARMAGICC projections in the IPCC TAR

PREDICTING FUTURE CLIMATE CHANGE

  Predict future socioeconomic changes  Use these to predict future emissions  From these predict changes in atmospheric composition  Use these results to drive a climate model

Question: How do we do this probabilistically?

 

A PROBABILISTIC PROJECTION IS ONE THAT …

quantifies uncertainties by …

(1) giving confidence intervals, or(2) presenting results in the form of a probability density function (p.d.f)

p.d.f. FOR GLOBAL-MEAN WARMING OVER 1990-2100

0

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0.35

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0 1 2 3 4 5 6 7 8

1990-2100 TEMPERATURE CHANGE (degC)

PRO

BA

BIL

ITY

DEN

SITY

(de

gC**

-1)

AREA = 0.05 AREA = 0.0590% CONFIDENCE INTERVAL

1.7 4.9

SOURCES OF UNCERTAINTY IN GLOBAL-MEAN TEMPERATURE

CHANGE

KEY SOURCES OF UNCERTAINTY FOR GLOBAL-MEAN TEMPERATURE

(1) Future emissions (2) The climate sensitivity* (3) Heat flux into the ocean (4) Radiative forcing due to aerosols (5) Carbon cycle/climate feedbacks (6) Changes in ocean circulation

* The climate sensitivity determines how much the climate will change for a given change in atmospheric composition. It is usually expressed as the eventual global-mean warming for a doubling of the CO2 concentration, and lies in the range 1.5-4.5oC with approx. 90% confidence.

INPUT REQUIREMENTS FOR PRODUCING GLOBAL-MEAN TEMPERATURE PDFs

(from Wigley & Raper, Science 293, 451-454, 2001)

Emissions pdf (based on SRES)Climate sensitivity (T2x) pdfOcean mixing (Kz) pdfAerosol forcing pdfCarbon cycle parameter pdfs

NOTE: Other climate model parameters are also altered (land-ocean sensitivity ratio, exchange coefficients, THC slowdown rate), but the values are tied to T2x based on AOGCM results.

0

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0 1 2 3 4 5 6 7GLOBAL-MEAN TEMPERATURE CHANGE FROM 1990 (oC)

PRO

BA

BIL

ITY

DEN

SITY

((o C

)-1)

PROBABILISTIC PROJECTIONS OF GLOBAL WARMING

TAR RANGE

1990-2030

1990-2070

1990-2100

PROBABILISTIC PROJECTIONS FOR THE RATE OF FUTURE GLOBAL-MEAN WARMING

MAGICC projections in the IPCC TARMAGICC projections in the IPCC TAR

PDFs FOR DECADAL WARMING TRENDSPROBABILITY DENSITY FUNCTIONS FOR DECADAL TRENDS

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8

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GLOBAL-MEAN TEMPERATURE TREND (degC/decade)

PRO

BABI

LITY

DEN

SITY

(de

cade

/deg

C) 1991-2000

2051-2060

2091-21002.6%

CONFIDENCE LIMITS FOR DECADAL WARMING TRENDSTREND UNCERTAINTIES AS PERCENTILES : SRES EMISSIONS

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1991 2001 2011 2021 2031 2041 2051 2061 2071 2081 2091

DECADE (1991 = 1991-2000, etc.)

GLO

BAL-

MEA

N TE

MPE

RATU

RE T

REND

(deg

C/de

cade

)99%

95%

50% (=median)

5%

1%

Trend over 1900-1999

CASE 3: CLIMATE MITIGATION POLICIES

ARTICLE 2 OF THE UNFCCC

Our objective should be …“stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the

climate system ….. within a time-frame sufficient to allow ecosystems to adapt naturally

to climate change, to ensure that food production is not threatened and to enable

economic development to proceed in a sustainable manner”.

 

POLICY CASES (CO2 CONCENTRATION STABILIZATION)

CO2 CONCENTRATION STABILIZATION PATHWAYS

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1990 2010 2030 2050 2070 2090 2110 2130 2150 2170 2190 2210 2230 2250YEAR

CO

2 C

ON

CEN

TRA

TIO

N (

ppm

)

WRE450

WRE550

WRE650

WRE750

P50 BASELINE

CONST EFOSS(2000)

KEY POINTS

CO2 CONCENTRATION STABILIZATION PATHWAYS

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800

1990 2010 2030 2050 2070 2090 2110 2130 2150 2170 2190 2210 2230 2250YEAR

CO

2 C

ON

CEN

TRA

TIO

N (

ppm

)

WRE450

WRE550

WRE650

WRE750

P50 BASELINE

CONST EFOSS(2000)

(1) Stabilizing emissions does not stabilize concentrations (magenta line in plot)

(2) These concentration stabilization pathways

depart from the ‘no-policy’ baseline (black P50 line in plot) in 2005 (450ppm stabilization), 2010 (550ppm), 2015 (650ppm) and 2020 (750ppm)

(3) A future departure date does not mean ‘do

nothing’ until then – it means setting in place now the mechanisms for future (substantial) emissions reductions below the no-policy case

EMISSIONS REQUIREMENTS FOR

CO2 CONCENTRATION STABILIZATION

(Note: stabilization of emissions does not stabilize concentrations, but leads to steadily increasing concentrations – at around 100 ppm/century.)

CO2 EMISSIONS TO ACHIEVE STABILIZATION

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TOTA

L C

O2

EMIS

SIO

NS

(GtC

/yr)

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WRE550

WRE650

WRE750

P50 BASELINE

KEY POINTS CO2 EMISSIONS TO ACHIEVE STABILIZATION

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TOTA

L C

O2

EMIS

SIO

NS

(G

tC/y

r)

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P50 BASELINE

After peak emissions, rapid reductions in emissions are

required to achieve stabilization, implying a rapid transition to non-fossil energy sources and/or a rapid reduction in carbon ‘intensity’ (CO2 emissions per unit of energy)

Eventually, emissions must fall substantially below current

levels

Note that these results are for CO2 alone – in practice the effects of other greenhouse gases must also be accounted for

Except for the 450ppm case, emissions can rise substantially above present levels and still allow concentration stabilization to be achieved

WHAT SHOULD THE CO2 STABILIZATION TARGET BE?

(What does ‘dangerous interference’ mean?)

[From Wigley, ‘Choosing a stabilization target for CO2’, Climatic Change, in press]

INPUT PDFs : CO2 STABILIZATION

CONCENTRATION IS CONTROLLED BY WARMING LIMIT,

CLIMATE SENSITIVITY AND NON-CO2 FORCING

INPUT PDF FOR CLIMATE SENSITIVITY

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0 1 2 3 4 5 6CLIMATE SENSITIVITY, T2x (degC)

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ITY

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(de

gC**

-1)

INPUT PDF FOR GLOBAL WARMING LIMIT (from 2000)

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0 1 2 3 4 5 6GLOBAL WARMING LIMIT (degC)

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DEN

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(de

gC**

-1)

INPUT PDF FOR NON-CO2 FORCING

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-1 0 1 2 3 4

NON-CO2 FORCING (W/m**2)

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BA

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ITY

DEN

SITY

(m**

2/W

)

CO2 CONCENTRATION STABILIZATION TARGET

0

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0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

CO2 CONCENTRATION (ppm)

PRO

BABI

LITY

DEN

SITY

(pp

m**

-1)

HIGH SENSITIVITYHIGH NON-CO2

FORCING

LOW SENSITIVITY

LOW NON-CO2 FORCING

Median (536 ppm)

17%

LOW WARMING LIMIT

HIGH WARMING LIMIT

WHAT CAN BE DONE TO RELAX THE CO2 TARGET?

(….. AND SO REDUCE THE CO2 MITIGATION LEVEL AND COSTS)

EFFECTS OF ADAPTATION AND NON-CO2 GASES

EFFECT OF ADAPTATIONPDFs FOR DANGEROUS INTERFERENCE WARMING

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GLOBAL-MEAN WARMING FROM 2000 (degC)

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(d

egC

**-1

)

BASE CASE

WITH ADAPTATION

EFFECT OF NON-CO2 EMISSIONS REDUCTIONS

PDFs FOR NON-CO2 FORCING

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RADIATIVE FORCING (W/m**2)

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SITY

(m

**2/

W)

BASE CASE

REDUCED EMISSIONS

REVISED PDF FOR TARGET CO2 LEVELSTAB TARGET PDFs : EFFECTS OF ADAPTATION AND NON-CO2 REDUCTIONS

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CO2 CONCENTRATION TARGET (ppm)

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(pp

m**

-1)

BASE CASE

ADAPTATION

NON-CO2 REDUCTIONS

BOTH

370

HOW WILL CO2 STABILIZATION AFFECT FUTURE GLOBAL-MEAN

WARMING?

EFFECTS OF CO2 STABILIZATION ON TEMPERATURE PDFsNO-POLICY, WRE450 AND WRE550 PDFs

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1990 to 2100 GLOBAL-MEAN TEMPERATURE CHANGE (degC)

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BABI

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(de

gC**

-1)

NO POLICY (SRES)

450ppm CO2 STABILIZATION

550ppm CO2 STABILIZATION

EFFECT OF CO2 STABILIZATION ON SEA LEVEL[WRE profiles; other gases follow median emissions to 2100, then constant emissions]

SEA LEVEL PROJECTIONS FOR STABILIZATION PROFILES

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ISE

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)

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THE IMPORTANCE OF NON-CO2 GASES

In the following, CO2, CH4 and N2O concentrations are stabilized. The emissions reductions required to do this are balanced between the gases in order to minimize

the total cost (‘cost optimization’). This is done using the energy-economics model MERGE developed by

economists Alan Manne and Richard Richels

CO2 stabilization pathwaysCO2 CONCENTRATION STABILIZATION PROFILES

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550

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750

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YEAR

CO

2 C

ON

CEN

TRA

TIO

N (

ppm

)BASELINE (P50)

WRE450

WRE550

550 to 450

CH4 stabilization pathwaysCONCENTRATIONS FOR COST-EFFECTIVE METHANE EMISSIONS REDUCTIONS

500

750

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1500

1750

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2750

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YEAR

MET

HA

NE

CO

NC

ENTR

ATI

ON

(pp

b)No-policy baseline (P50)

If CO2 target is 550ppm

If CO2 target is 450ppm(including overshoot case)

N2O stabilization pathwaysNITROUS OXIDE CONCENTRATIONS

310

320

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410

2000 2050 2100 2150 2200 2250 2300 2350 2400

YEAR

N2O

CO

NC

ENTR

ATI

ON

(pp

b)No-policy baseline (P50)

If CO2 target is 550ppm

If CO2 target is 450ppm

(including overshoot case)

Reducing CH4 and (to a lesser extent) N2O concentrations reduces future warming and so reduces the magnitude of climate feedbacks on the carbon cycle.

As a consequence, the CO2 emissions required to follow a given concentration pathway can be higher than otherwise.

CO2 emissions: with CH4 and N2O reductions (full lines) compared with the no CH4/N2O reduction case (dashed lines)

CO2 EMISSIONS WITH (BOLD) AND WITHOUT (DASHED) CH4/N2O REDUCTIONS

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YEAR

FOSS

IL C

O2

EMIS

SIO

NS

(GtC

/yr)

WRE550

550 to 450

WRE450

TEMPERATURE AND SEA LEVEL RESULTS

Global warming: with CH4 and N2O reductions (full lines) compared with the no CH4/N2O reduction case (dashed lines)

TEMPERATURES WITH (BOLD) AND WITHOUT (DASHED) CH4/N2O REDUCTIONS

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WRE550

550 to 450

WRE450

Sea level change: with CH4 and N2O reductions (full lines) compared with the no CH4/N2O reduction case (dashed lines)

SEA LEVEL WITH (BOLD) AND WITHOUT (DASHED) CH4/N2O REDUCTIONS

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ISE

(cm

)WRE550

550 to 450

WRE450

Warming rate: with CH4 and N2O reductions (full lines) compared with the no CH4/N2O reduction case (dashed lines)

dT/dt WITH (BOLD) AND WITHOUT (DASHED) CH4/N2O REDUCTIONS

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2000 2050 2100 2150 2200 2250 2300 2350 2400

YEAR

WA

RM

ING

RA

TE (

degC

/dec

ade)

WRE550

550 to 450

WRE450

CONCLUSIONS 1: Commitments Concentrations stabilized 0.11 to 0.48oC warming by 2050 sea level rises at 2–27cm/century

Emissions stabilized warming at 0.8 to 2.0oC/century sea level rises at 8–54cm/century

CONCLUSIONS 2: NO-POLICY

(1) 90% C.I. for 2000-2100 warming 1.5–4.7oC

(2) 90% C.I.s for warming rates ….. 2050s: 0.16–0.65oC/decade 2090s: 0.02–0.58oC/decade [cf. 20th century warming at 0.07oC/decade]

(3) 3% probability of cooling in the 2090s

CONCLUSIONS 3: STABILIZATION

(1) CO2 emissions must eventually drop well below present levels

(2) Based on ‘dangerous interference’, there is a 17% chance that the CO2 stabilization target should be less than the present level (absent adaptation and non-CO2 emissions reductions) (3) Multi-gas concentration stabilization: • CO2 emissions targets less stringent [for a given conc. profile] • Asymptotic warming is reduced by almost 1oC • Sea level rise is reduced by up to 15cm • Maximum warming rate is reduced by 2% to16%

POST SCRIPT

Lead in to Nychka/Tebaldi presentation.

RESULTS FOR PATTERNS OF CLIMATE CHANGE

(per 1oC global-mean warming)

Normalized annual-mean temperature and precipitation changes in CMIP2 1%/year CO2 increase experiments

Normalized temperature change

Normalized precipitation change

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MODELS GIVE A WIDE RANGE OF RESULTS FOR

PROJECTED PRECIPITATION CHANGES

SPANNING THE RANGE OF POSSIBLE FUTURES(blue = better models)

DJF PRECIP CHANGES vs DJF TEMP CHANGES (Control drift removed)

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NORMALIZED TEMPERATURE CHANGE (degC/degC)

NO

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PR

ECIP

ITA

TIO

N C

HA

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E (%

/deg

C)

Sth. CA: 30 to 35N, 115 to 120W

Model mean

CCC1BMRC

GISS

CCSR

PCMCSM

HadCM2

ECHAM3

ECHAM4

HadCM3

GFDL

CSIRO2

MRI

W&M

LMD

IAP

CERF

FURTHER INFORMATION

IPCC Third Assessment Reports published by Cambridge University Press (see www.ipcc.ch). These reports are quite technical. Summaries are downloadable from the web site. Pew Center on Global Climate Change (www.pewclimate.org). The Pew Center has published many plain language reports on various facets of the global warming issue; downloadable. The MAGICC (Global-mean temperature and sea level)/SCENGEN (Regional details for temperature and precipitation) software can be downloaded from ….. www.cgd.ucar.edu