CLIMATE CHANGE PROJECTIONS: SOURCES AND MAGNITUDES OF UNCERTAINTY Tom Wigley, National Center for Atmospheric Research, Boulder, CO 80307, USA Presented at: NCAR Summer Colloquium on Climate and Health National 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

CONSTANT-C WARMING COMMITMENT

CONSTANT-E WARMING COMMITMENT

CONSTANT-C SEA LEVEL COMMITMENT

CONSTANT-E SEA LEVEL COMMITMENT

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

RELATIVE IMPORTANCE OF DIFFERENT GASES

SRES CARBON DIOXIDE (CO 2 ) PROJECTIONS (emissions and concentrations)

IPCC TAR GLOBAL-MEAN TEMPERATURE PROJECTIONS

MAGICC 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)

SOURCES OF UNCERTAINTY IN GLOBAL-MEAN TEMPERATURE CHANGE

INPUT REQUIREMENTS FOR PRODUCING GLOBAL-MEAN TEMPERATURE PDFs (from Wigley & Raper, Science 293, , 2001) Emissions pdf (based on SRES) Climate sensitivity (  T2x) pdf Ocean mixing (Kz) pdf Aerosol forcing pdf Carbon 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.

PROBABILISTIC PROJECTIONS FOR THE RATE OF FUTURE GLOBAL-MEAN WARMING

MAGICC projections in the IPCC TAR

PDFs FOR DECADAL WARMING TRENDS

CONFIDENCE LIMITS FOR DECADAL WARMING TRENDS

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)

KEY POINTS

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

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 ADAPTATION

EFFECT OF NON-CO2 EMISSIONS REDUCTIONS

REVISED PDF FOR TARGET CO2 LEVEL

HOW WILL CO2 STABILIZATION AFFECT FUTURE GLOBAL-MEAN WARMING?

EFFECTS OF CO2 STABILIZATION ON TEMPERATURE PDFs

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

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 pathways

CH4 stabilization pathways

N2O stabilization pathways

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)

TEMPERATURE AND SEA LEVEL RESULTS

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

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

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

CONCLUSIONS 1: Commitments Concentrations stabilized 0.11 to 0.48 o C warming by 2050 sea level rises at 2–27cm/century Emissions stabilized warming at 0.8 to 2.0 o C/century sea level rises at 8–54cm/century

CONCLUSIONS 2: NO-POLICY (1) 90% C.I. for warming 1.5–4.7 o C (2) 90% C.I.s for warming rates … s: 0.16–0.65 o C/decade 2090s: 0.02–0.58 o C/decade [cf. 20 th century warming at 0.07 o C/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 1 o C 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.

Normalized annual-mean temperature and precipitation changes in CMIP2 1%/year CO 2 increase experiments Normalized temperature change Normalized precipitation change

MODELS GIVE A WIDE RANGE OF RESULTS FOR PROJECTED PRECIPITATION CHANGES

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