MODELING CLIMATE CHANGE UNDER NO-POLICY AND POLICY EMISSIONS PATHWAYS

Transcription

1 MODELING CLIMATE CHANGE UNDER NO-POLICY AND POLICY EMISSIONS PATHWAYS Tom Wigley, National Center for Atmospheric Research, Boulder, CO 837, USA Presented at: OECD Workshop on the Benefits of Climate Policy: Improving Information for Policy Makers OECD Headquarters, Paris. December, SUMMARY Future global-mean warming no-policy cases Sources of uncertainty at the global-mean level Uncertainties at the regional level Policy cases stabilizing the climate system CO concentration stabilization Dangerous interference and choosing the CO target Conclusions

2 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 THESE ARE THE FIRST STEPS IN AN INTEGRATED ASSESSMENT OF THE CLIMATE PROBLEM PREDICTING FUTURE GLOBAL- MEAN TEMPERATURE CHANGE

3 FOSSIL CO EMISSIONS (GtC/yr) FOSSIL CO EMISSIONS : SRES MEAN AND RANGE 38 MAXIMUM MEAN MINIMUM YEAR 5 CO CONCENTRATION : SRES MEAN AND RANGE MAXIMUM CO CONCENTRATION (ppm) MINIMUM MEAN YEAR 3

4 8 FULL RANGE OF TEMPERATURE PROJECTIONS vs SAR BEST GUESS GLOBAL-MEAN TEMPERATURE CHANGE (degc) / HIGH EMS SAR.5 / LOW EMS SENSITIVITY/ EMISSIONS.5 / HIGH EMS.5 / AVE EMS.5 / LOW EMS YEAR PROBABILISTIC PROJECTIONS OF FUTURE GLOBAL-MEAN WARMING (from Wigley & Raper, Science 93, , ) UNCERTAINTIES ACCOUNTED FOR: () Emissions () Climate Sensitivity (3) Aerosol forcing (4) Ocean mixing rate (5) Carbon cycle 4

5 PROBABILISTIC PROJECTIONS OF GLOBAL WARMING PROBABILITY DENSITY (( o C) - ) GLOBAL-MEAN TEMPERATURE CHANGE FROM 99 ( o C) TAR RANGE RELATIVE IMPORTANCE OF DIFFERENT SOURCES OF UNCERTAINTY TWO DIFFERENT ASPECTS: REDUCING A SOURCE OF UNCERTAINTY MAY.. () SHIFT THE MEAN OF THE DISTRIBUTION () CHANGE THE SPREAD OF THE DISTRIBUTION 5

6 RELATIVE IMPORTANCE OF DIFFERENT SOURCES OF UNCERTAINTY METHOD Sequentially remove individual sources of uncertainty, and see how the mean and uncertainty range change. For example, if we know the distribution for factors A, B, C, D and E, then comparing this with the distribution for B, C, D and E gives the marginal uncertainty for factor A EFFECT OF REMOVING AEROSOL FORCING UNCERTAINTIES MEDIAN AEROSOL FORCING ONLY PROBABILITY DENSITY (degc**-) ALL UNCERTAINTIES TEMPERATURE CHANGE (degc) 6

7 .45.4 EFFECT OF REMOVING CARBON CYCLE UNCERTAINTIES MEDIAN CO ONLY PROBABILITY DENSITY (degc**-) ALL UNCERTAINTIES TEMPERATURE CHANGE (degc).8 EFFECT OF REMOVING EMISSIONS UNCERTAINTIES.7 B PROBABILITY DENSITY (degc**-) AB AFI ALL UNCERTAI NTIES TEMPERATURE CHANGE (degc) 7

8 .7 DTx =.5 o C EFFECT OF REMOVING SENSITIVITY UNCERTAINTIES.6 PROBABILITY DENSITY (degc**-) o C ALL UNCERTAI NTIES 4.5 o C TEMPERATURE CHANGE (degc) EXPERIMENT 9% CONF. INT. (degc) RANGE (degc) RANGE REL. TO ALL UNCERTS ALL UNCERTS MEDIAN Kz MEDIAN AEROSOL MEDIAN CO EMIS. = B EMIS. = AB EMIS. = AFI Tx =.5 o C Tx =.6 o C Tx = 4.5 o C % CONFIDENCE INTERVALS FOR 99- WARMING 8

9 GLOBAL-MEAN TEMPERATURE UNCERTAINTIES : SUMMARY () Uncertainties are dominated by uncertainties in future emissions and in the climate sensitivity () Carbon cycle uncertainties may affect the probability of extreme future warming (3) Even with emissions or Tx specified, uncertainties may be very large, especially for high emissions or sensitivity SOURCES OF UNCERTAINTY REGIONAL SCALE () Uncertainties in global-mean temperature (due to uncertainties in emissions, climate sensitivity, etc.) () Uncertainties in normalized patterns of change: i.e., patterns of change per unit global-mean warming (quantifiable by comparing results from different models) 9

10 RESULTS FOR PATTERNS OF CLIMATE CHANGE (per o C global-mean warming) [Average results for 6 coupled ocean-atmosphere climate models] Normalized annual-mean temperature and precipitation changes in CMIP Greenhouse Warming Experiments (%/year CO increase) Normalized temperature change L a t i t u d e _ t a s Longitude_tas Normalized precipitation change L a t i t u d e _ p r Longitude_pr

11 Inter-Model Signal-to-Noise Ratios in CMIP Greenhouse Warming Experiments (%/year CO increase) Normalized S/N L a t i t u d e _ t a s Longitude_tas Surface temperature changes Raw S/N L a t i t u d e _ t a s Longitude_tas Inter-Model Signal-to-Noise Ratios in CMIP Greenhouse Warming Experiments (%/year CO increase) Normalized S/N L a t i t u d e _ p r Longitude_pr Changes in total precipitation rate 3 Raw S/N L a t i t u d e _ p r Longitude_pr

12 REGIONAL UNCERTAINTIES : SUMMARY For temperature, models agree best in tropics and sub-tropics, and worst in mid-latitude storm regions For precipitation, models agree best in high latitudes and worst in subtropics Regions of good/poor agreement for temperature tend to be regions of poor/good agreement for precipitation WHAT CAN WE DO TO REDUCE THE MAGNITUDE OF FUTURE CLIMATE CHANGE?

13 ARTICLE OF THE U.N. FRAMEWORK CONVENTION ON CLIMATE CHANGE The ultimate objective is stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous interference with the climate system achieved within a time frame to enable economic development to proceed in a sustainable manner EMISSIONS REQUIREMENTS FOR CO CONCENTRATION STABILIZATION (Note: stabilization of emissions does not stabilize concentrations, but leads to steadily increasing concentrations at around 4 ppm/century.) 3

14 8 75 CO CONCENTRATION STABILIZATION PATHWAYS WRE75 7 P5 BASELINE CO CONCENTRATION (ppm) CONST EFOSS() WRE65 WRE55 45 WRE YEAR 8 CO CONCENTRATION STABILIZATION PATHWAYS 75 WRE75 7 P5 BASELINE KEY POINTS CO CONCENTRATION (ppm) WRE65 CONST EFOSS() WRE55 45 WRE YEAR () Stabilizing emissions does not stabilize concentrations (magenta line in plot) () These concentration stabilization pathways depart from the no-policy baseline (black P5 line in plot) in 5 (45ppm stabilization), (55ppm), 5 (65ppm) and (75ppm) (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 4

15 TOTAL CO EMISSIONS (GtC/yr) CO EMISSIONS TO ACHIEVE STABILIZATION P5 BASELINE WRE75 WRE65 WRE55 WRE YEAR KEY POINTS CO EMISSIONS TO ACHIEVE STABILIZATION P5 BASELINE WRE75 Except for the 45ppm case, emissions can rise substantially above present levels and still allow concentration stabilization to be achieved TOTAL CO EMISSIONS (GtC/yr) WRE65 WRE55 WRE YEAR 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 (CO emissions per unit of energy) Eventually, emissions must fall substantially below current levels Note that these results are for CO alone in practice the effects of other greenhouse gases must also be accounted for 5

16 ALTERNATIVE PATHWAYS TO STABILIZATION : 55ppm OVERSHOOT CASE 8 CO CONC. STABILIZATION PATHWAYS : 55 OVERSHOOT 75 WRE75. CO CONCENTRATION (ppm) P5 BASELINE. WRE65. CONST EFOSS(). WRE55. WRE YEAR 6

17 8 7 CO EMISSIONS TO ACHIEVE STABILIZATION : 55 OVERSHOOT P5 BASELINE. 6 TOTAL CO EMISSIONS (GtC/yr) WRE75. WRE65. WRE55. WRE YEAR 3 COMPARISON OF STANDARD AND OVERSHOOT PATHWAYS TO 55ppm OVERSHOOT GLOBAL-MEAN TEMPERATURE CHANGE (degc) STANDARD YEAR 7

18 WHAT SHOULD THE STABILIZATION TARGET BE? STABILIZATION TARGET: KEY QUESTIONS () What determines the target? [Avoid dangerous interference with the climate system] () What does dangerous interference mean? [Limiting the magnitude of future global warming] (3) How do we restrict global warming? [Restrict equivalent CO* buildup] (4) What does this mean for actual CO? * equivalent CO is the amount of CO that gives the same radiative forcing as all gases combined (CO + CH4 + NO + etc.) 8

19 HOW IS FUTURE GLOBAL WARMING RELATED TO FUTURE CO? T = f( Tx,total forcing) = f( Tx, QCO, Qother) = f( Tx,C, Qother) where : T = warming from 99 Tx = climate sensitivity C = actual CO concentration QCO = CO forcing from 765 Qother = non-co forcing from 765 HOW IS FUTURE GLOBAL WARMING RELATED TO FUTURE CO? C =78 [**{( T+.7)/ Tx Qother/3.7}] where : T = warming from 99 Tx = climate sensitivity C = actual CO concentration Qother = non-co forcing from 765 9

20 3 NON-CO FORCING FOR THE SRES SCENARIOS MAXIMUM.5 RADIATIVE FORCING FROM 765 (W/m**).5.5 LOW SO4 MEAN MINIMUM -.5 HIGH SO YEAR CO STABILIZATION TARGETS FOR DIFFERENT WARMING LIMITS DTx =.5 9 Qother =. Qother=. CO STABILIZATION TARGET (ppm) Q=. DTx = 3. Qother =. DTx = 4.5 Qother = WARMING LIMIT, CHANGE FROM (degc)

21 INPUT PDFs : CO STABILIZATION CONCENTRATION IS CONTROLLED BY WARMING LIMIT, CLIMATE SENSITIVITY AND NON-CO FORCING PROBABILITY DENSITY (degc**-) INPUT PDF FOR GLOBAL WARMING LIMIT (from ) GLOBAL WARMING LIMIT (degc).5 INPUT PDF FOR CLIMATE SENSITIVITY.7 INPUT PDF FOR NON-CO FORCING PROBABILITY DENSITY (degc**-) PROBABILITY DENSITY (m**/w) CLIMATE SENSITIVITY, DTx (degc) NON-CO FORCING (W/m**).8 CO CONCENTRATION STABILIZATION TARGET Median (65ppm) PROBABILITY DENSITY (ppm**-) HIGH SENSITIVITY HIGH NON-CO FORCING LOW WARMING LIMIT LOW SENSITIVITY LOW NON-CO FORCING HIGH WARMING LIMIT. % CO CONCENTRATION (ppm)

22 .8 CO CONCENTRATION STABILIZATION TARGET Median (65ppm) CONCLUSIONS RE FUTURE CO PROBABILITY DENSITY (ppm**-).6 HIGH SENSITIVITY HIGH NON-CO.4 FORCING LOW WARMING LIMIT...8 LOW SENSITIVITY LOW NON-CO FORCING.6 HIGH WARMING LIMIT.4 % CO CONCENTRATION (ppm) BASED ON: Current Tx uncertainties, SRES-based uncertainties in Qother, future warming with median of 3.5 o C and 9% C.I. Of o C.. () % chance that CO stabilization level < today () 6% chance that CO stabilization level > 55ppm (3) 7% chance that CO stabilization level >ppm OVERALL CONCLUSIONS () 99- warming is in the range o C (9% C.I.) () The main sources of uncertainty are future emissions and the climate sensitivity (3) Eliminating either one of these may not change the uncertainty range much, but it will change the central estimate (4) CO stabilization requires, eventually, large reductions in emissions relative to today (5) The CO stabilization target depends on the warming target ( dangerous interference ), the climate sensitivity, and forcing from non-co sources (6) The stabilization target could be less than today or >ppm, with a median value of around 6ppm (7) The choice of target depends on the acceptable risk