Forecasts for the Future: Understanding Climate Models and Climate Projections for the 21 st Century

Transcription

1 Forecasts for the Future: Understanding Climate Models and Climate Projections for the 21 st Century Linda E. Sohl NASA Goddard Institute for Space Studies and the Center for Climate Systems Reseearch at Columbia University Climate Modeling and Simulation Workshop NOAA Science Center, Silver Spring, MD August 5, 2015

2 Earth s Climate System Source: GFDL/NOAA NOAA Workshop August

3 Historical Records of Climate Change Source: Wikimedia Commons NOAA Workshop August

4 Overview What is a climate model, and how does it work? How do scientists test the quality of model output? How does the climate science community develop forecasts of future climate? NOAA Workshop August

5 What is a GCM? Global Climate Model NOAA Workshop August

6 Tools of the Trade: Global Climate Models (Henderson-Sellers, 1985) NOAA Workshop August

7 Tools of the Trade: Global Climate Models Fundamental Physical Equations: NOAA Workshop August

8 Physical Processes Simulated by GCMs Source: NASA/GISS " Seasonal and Diurnal Cycles " Latent and Sensible Heat Fluxes " Clouds and Convection " Planetary Boundary Layer " Greenhouse Gases " Aerosols and Atmospheric Chemistry " Dynamic Sea Ice " Ground Hydrology " Chemical Tracers " Ocean Circulation " Dynamic Vegetation " Dynamic Ice Sheets " Carbon Cycle Chemistry NOAA Workshop August

9 Subgrid-scale Processes Source: The COMET Program/UCAR NOAA Workshop August

10 What Goes Into a Climate Simulation? Need to supply the GCM with information Boundary conditions: inputs that do not change over the course of the simulation Examples include topography, land/sea distribution, land ice extent, vegetation Initial conditions: inputs prescribed at the beginning of a run that change as the simulation proceeds Examples include greenhouse gas trends, solar trends, sea surface temperatures Some model inputs, such as greenhouse gas levels, can fall into either category of input NOAA Workshop August

11 What Comes Out of a Climate Simulation? The GCM produces raw data files of various diagnostic quantities that have been calculated by the physical equations in the model Data files must be post-processed so that usable information can be extracted more readily Diagnostic variables are visualized and analyzed to check the model accuracy as well as the outcome of the climate experiment NOAA Workshop August

12 Overview What is a climate model, and how does it work? How do scientists test the quality of model output? How does the climate science community develop forecasts of future climate? NOAA Workshop August

13 Two Approaches to Model Evaluation 1. Single model development Ongoing process Comparisons with past and current climate Focus on identifying areas of potential upgrade/ improvement NOAA Workshop August

14 Example: NASA/GISS ModelE2-R PRISM3 Pliocene - Preindustrial (Proxy Data and Observations) Mid-Pliocene - Preindustrial Control (NASA/GISS Coupled Ocean Atmosphere GCM) Sea Surface Temperature Anomalies NOAA Workshop August 2015 (Chandler et al., 2013) 13

15 Two Approaches to Model Evaluation 1. Single model development Ongoing process Comparisons with past and current climate Focus on identifying areas of potential upgrade/ improvement 2. Multi-model intercomparison projects (MIPs) - Periodic assessment (e.g., CMIP for IPCC assessments) - Focus on identifying ranges of potential future change at varying levels of confidence - Identifies which models perform better under certain circumstances NOAA Workshop August

16 Example: IPCC Fourth Assessment (AR4) NOAA Workshop August

17 Example: IPCC Fourth Assessment (AR4) NOAA Workshop August

18 What About Local Climate Change? Source: UCAR NOAA Workshop August

19 What About Local Climate Change? Downscaling of GCM output Statistical methods (e.g., delta method) Dynamical methods (regional modeling, e.g., NARCCAP) Source: NARCCAP/UCAR NOAA Workshop August

20 Model Evaluation Summary Models are under continual development/ improvement start with IPCC assessments but don t stop there Certain parameterizations reflect uncertainty in current knowledge (clouds, sea ice), so models are less likely to reproduce related processes for the right reasons Model output has to be viewed at the appropriate scale for the research question(s) posed Impacts research benefits from realistic expectations about what the climate models say NOAA Workshop August

21 Overview What is a climate model, and how does it work? How do scientists test the quality of model output? How does the climate science community develop forecasts of future climate? NOAA Workshop August

22 Framework for Understanding Climate Change Source: IPCC AR4, 2007 NOAA Workshop August

23 Coupled Model Intercomparison Project (CMIP) Organized by the World Climate Research Programme (WCRP) Includes contrbutions from 20+ climate and Earth system models Provides physical basis for IPCC assessment reports Scope of experiments established through community input NOAA Workshop August

24 CMIP Design Process NOAA Workshop August

25 CMIP Contribution to IPCC Assessment Process CMIP experiments are designed, completed and submitted to archives for open access Peer-reviewed papers evaluating model performance ONLY are permitted for consideration in IPCC assessment process IPCC assessment process takes 5-7 years to complete 1990: First Assessment Report (FAR) 1996: Second Assessment Report (SAR) 2001: Third Assessment Report (TAR) 2007: Fourth Assessment Report (AR4) 2013: Fifth Assessment Report (AR5) 2020: Sixth Assessment Report (AR6) NOAA Workshop August

26 IPCC Assessment Process Source: IPCC NOAA Workshop August

27 Summary Climate models are quantitative models based on physical laws that provide insight into climate processes Scientists regularly challenge model performance and evaluate outcomes through comparisons with past climate data, and model intercomparisons Evaluated model output is provided to broader community of stakeholders for inclusion in global climate change impact assessment reports NOAA Workshop August