Long term climate projection: tension between continuity and novelty, between describing and understanding

Transcription

1 Long term climate projection: tension between continuity and novelty, between describing and understanding Jean-Louis Dufresne Laboratoire de Météorologie Dynamique (CNRS, UPMC, ENS, X) Institut Pierre Simon Laplace Confidence, Credibility, and Authority in Climate Sciences and Politics, November 2014, Paris

2 Contributing to the IPCC reports: an uncommon and rich experience A permanent diffuse pressure AR5 started in 2010 after climategate, COP 15 in Copenhagen, InterAcademy Council review of IPCC Identity fraud via ; TSU received false from "Co-Chair Thomas Stocker" with a malicious attachment Lawyers offer their services Permanent anticipation of possible criticisms Writing a text with about fifteen co-authors you haven't choose Review of a rapid evolving literature, beyond your own expertise

3 The IPCC heartbeat Number of s on IPCC sent and received daily for one coordinating lead author Courtesy of Reto Knutti, CLA, chapter 12

4 Chapter 12: Long term climate Change: Projections, Commitments and Irreversibility Some specificities of this chapter: Highly visible chapter: Provides 3 out of 10 figures of the SPM, 2 out of 3 Tables (contributes to the third) Interface with other IPCC working groups (WG 2 and 3) Chapter with a long history: it exists since the beginning of the Assessment Reports (AR), with some variations Structure of the chapter: in continuity from that of AR4. => Few general discussions on the overall goal and content

5 Idealized experiments: Understanding climate response Quantified climate sensitivity Idealized tests CO2 concentration increase 1%/year Instantaneous quadrupling of CO2 concentration Idealized geoengineering: decrease solar constant Ocean surface temperature increase by +4 C radiative forcing Future projections time Scenarios. Their roles: radiative forcing Represent realistic possible futures? Explore contrasted hypothesis of possible futures? Train simplified models (climate emulators)? Understanding climate responses? Currently, it is mainly the two first points that are addressed time

6 Scenarios for future projections RCP : «Representative concentration pathways» Scenarios are defined with a target value of the radiative forcing (W.m-2) Integrated Assessment Models (climate and socioeconomic, very simplified) (IAM) Earth System Models (ESM)

7 Total anthropogenic radiative forcing (W.m-2) Scenarios for future projections Absolutely no link between the four RCP scenarios For climate sciences: Four disconnected scenarios, with two extreme cases Only possible metric to characterize them: RF or Temp Complexity due to realism has little value... but creates complications and difficulties The price for credibility?

8 Presenting individual scenarios or gathering them? Annual mean, multi model mean, surface temperature change Individual scenario end 21th century ( ) All scenarios Temp. scaled by global mean temp ( C/ C) RCP2.6 RCP4.5 Global mean temp change ( C) RCP6.0 Temperature scaling condense results and separate questions Departure from temperature scaling is a useful information But not adopted (continuity, habit, limited validity) RCP8.5

9 Variability and response to forcings. Climate variations have different origins: variation internal variability response to natural forcings response to anthropogenic forcings natural variability Difference between simulations includes all these terms Their relative difference depends on time average and on the amplitude of the forcings All these terms may differ among models

10 default in AR5 How to quantify model agreements in Maps? Relative mean precipitation change (%) around 2100, in DJF, for RCP8.5 Same mean, different methods to estimate agreement among models Stippling: ~ large, significant or robust changes Hatching: ~ small changes compared to natural variability None: ~ no model agreement White: ~ model disagreement The estimate adopted in the AR5 has the smallest areas with «agreement» and the larger areas with «no significant changes»

11 Projected global mean surface temperature change in the IPCC SPMs TAR (2001) AR4 (2007) All scenarios envelope model mean model spread AR5 (20013) model mean and spread model results + other estimates + other processes (climate-carbon feedback) model mean and spread

12 Strong separation between the paste and the future Underlying framework of AR5 approach Observations of climate change Estimate of forcings Detection and attribution of climate changes Historical climate change simulations Models evaluation and ranking Future climate change simulations Best estimate of future changes has not be achieved Very few discussions of links between recent and future changes Some end to end chapters for some thematics Clouds and aerosols Sea level rise Carbon cycle (Radiative forcing)

13 Strong separation between the paste and the future Detailed analysis of paste radiative forcings (also included in the SPM) No equivalent detailed analysis of future radiative forcings although their relative contribution will change in the future (and probably their uncertainty)

14 IPCC: prudent and conservative rather than alarmist It is easy to justify an increase of the uncertainty of results, it is much more difficult to justify a decrease Multi-model mean always smooth the responses Future projections is not well defined as it may also include natural variability: future response to anthropogenic forcings?

15 IPCC: prudent and conservative rather than alarmist Recent climate changes: Observations, models, possible test of hypothesis: the scientific methodology is efficient. The IPCC conclude that human influence has been detected in recent changes of some climate variables. Future climate change: The same methodology has difficulties to lead (can not?) to strong statements: No observations, only models with limited constrains Difficulty to definitively refute many hypothesis What is the questions: we want to be sure of future climate changes or we want to avoid some future risks? Future climate change projections and uncertainties changed little in the paste, and this will probably continue in the near future as the questions is to broad, to general Answers to some very specific questions can make progress in the future, eventually with partial answers The global assessment by the IPCC should be complemented, supplied by other assessments driven by very specific questions

16 Thank you for your attention

17 Estimates of climate sensitivity Transient Climate Response ( C) Equilibrium Climate Sensitivity ( C)