Lecture 15: Projections of Future Climate Change

1. Lecture 15: Projections of Future Climate Change Global Mean Temperature

2. 21st Century Climate Change • Dominant influence likely to be increase in greenhouse gases (anthropogenic) • Projections of temperature change are made using climate models

3. Climate Models – 1 (Not Necessary) • A climate model is a mathematical representation of the physical processes that control climate • Equations are complicated: Computers are used to solve them

4. Climate Models –2 (Not Necessary) • The most sophisticated climate models are called General Circulation Models (GCMs) • These models attempt to simulate all processes in the atmosphere and ocean relevant to climate

5. Climate Models – 3 Anthropogenic forcing Input Climate Model Output Climate change

6. Emission Scenarios • Emission scenario: Possible future emissions • Reference: http://www.grida.no/climate/ipcc_tar/wg1/029.htm

7. CO2 Emissions “Scenarios” 25 20 15 10 B1

8. Calculation of Future CO2 Concentrations -- Method Model Input Anthropogenic Emissions Carbon Cycle Model Model output CO2Concentration increase

9. Projected CO2 Concentrations for Various Emission Scenarios B1 B1

10. Some Scenario Results Scenario Characterization Emission Rate in 2100 (Pg/yr) CO2 Concentration in 2100 (ppm) Rapid economic growth; strong reliance on fossil fuels A1FI 28 980 B1 Moderate economic growth; much less reliance on fossil fuels 5 550 B2 “Middle-of-the-road” scenario 13 700

11. Past and Projected Future CO2 Concentrations on same graph • See the IPCC Synthesis Report, Summary for Policymakers, Figure SPM-10a, p. 33. • (URL: http://www.grida.no/climate/ipcc_tar/vol4/english/fig9-1a.htm)

12. Climate Models -- 5 • Complication: Models have differing sensitivities  models produce different results for same emission scenarios

13. Differing Response of Models for Same Scenario High Sensitivity Global Mean Temperature Low Sensitivity Time

14. Three Scenarios; three models Model 1 (Medium sensitivity) Model 2 (High sensitivity) Model 3 (Low sensitivity) Medium emissions Scenario A Scenario B Low emissions Scenario C High emissions Temp Lowest sensitivity, lowest emissions Highest sensitivity, highest emissions time

15. Model Projections of Climate Change From 2001 Report Fig. 9.14 in IPCC Scientific Assessment. Link: http://www.grida.no/climate/ipcc_tar/wg1/353.htm

16. Summary: Two Causes for Large Range in Projections • Wide range in emission scenarios • Wide range in model sensitivities #1 due to uncertainty in future human actions (i.e., it is not a fault of the models) #2 is due to our imperfect understanding of the climate system (i.e., it is a fault of the models)

17. From 2007 Report

18. Model Projections • Average temp. for 2090-2100 compared to average for 1980 – 1990. • All models project warming for all scenarios • Range: 1.1C – 6.4C

19. Meaning • Low end: warming of 1.1C Achievable IF • Emissions are cut significantly • Lowest-sensitivity model is correct • High end: Warming of 6.4 C May occur if • Emissions continue to rapidly increase • Highest-sensitivity model is correct

20. Constant Concentrations Suppose concentrations of greenhouse gases have been rising, but suddenly stopped rising at time = tstop Greenhouse-gas Concentrations tstop time

21. Temperature Response System is not yet in equilibrium Temp. rises as g.h. gases increase Temperature “Realized” warming tstop time

22. Realized Warming • This is the warming that has been observed at time tstop

23. Temperature Response Temperature Additional Warming “Realized” warming tstop time Warming continuesuntil equilibrium is restored

24. Warming Commitment • Def: Warming that will occur in the future due to greenhouse gases added in the past • Doesn’t include effect of any future increases in greenhouse gases

25. Fig. 1. CC warming commitment (constant concentrations after 2000) for different climate sensitivities and aerosol forcing levels (L, M, and H on the right of the figure indicate low, mid-, and high magnitudes for aerosol forcing, respectively) High Sensitivity Medium Sensitivity Low Sensitivity T. M. L. Wigley Science 307, 1766 -1769 (2005) Published by AAAS

26. Meaning of Graphs • With greenhouse-gas composition constant at 2000 levels, Earth would warm for centuries (warming commitment) • Commitment depends on sensitivity and amount of aerosol forcing • Mid-range estimate: Commitment ~ 0.5C • High-end estimate: Commitment ~ 1C • This is warming that is “in the pipeline”

27. Reducing Commitment • Only way: Reduce amount of greenhouse gas in atmosphere • The sooner the reductions occur, the greater the reduction in the commitment • Problem: Removing greenhouse gases is very difficult! • More later

28. Principal Difficulty • CO2 mixing ratio is 380 ppm • i.e., only 380 out of every million molecules of air are CO2 • CO2 removal would require processing enormous amounts of air • Lots of energy probably required. • http://news.bbc.co.uk/2/hi/science/nature/6345557.stm

29. What the link reveals Branson launches $25m climate bid

30. Another Problem • Removing CO2 would create waste • Where should it be stored?

31. Carbon Capture and Storage • Would reduce future emissions, but wouldn’t actually remove CO2 from atmosphere • http://news.bbc.co.uk/2/hi/science/nature/6345557.stm#carbon

32. Carbon Sequestration