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

2 21 st 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 Climate Model Anthropogenic forcing Climate change Input Output

6 Emission Scenarios Emission scenario: Possible future emissions Reference: htm htm htm

7 CO 2 Emissions “Scenarios” B1

8 Calculation of Future CO 2 Concentrations -- Method Carbon Cycle Model Anthropogenic Emissions CO 2 Concentration increase Model Input Model output

9 Projected CO 2 Concentrations for Various Emission Scenarios B1

10 Some Scenario Results “Middle-of-the-road” scenario B Moderate economic growth; much less reliance on fossil fuels B Rapid economic growth; strong reliance on fossil fuels A1FI CO 2 Concentration in 2100 (ppm) Emission Rate in 2100 (Pg/yr) CharacterizationScenario

11 Past and Projected Future CO 2 Concentrations on same graph See the IPCC Synthesis Report, Summary for Policymakers, Figure SPM-10a, p. 33. See the IPCC Synthesis Report, Summary for Policymakers, Figure SPM-10a, p. 33. (URL: sh/fig9-1a.htm)

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

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

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

15 Model Projections of Climate Change Fig in IPCC Scientific Assessment. Link: From 2001 Report

16 Summary: Two Causes for Large Range in Projections 1.Wide range in emission scenarios 2.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 compared to average for 1980 – 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 Achievable IF 1. Emissions are cut significantly 2. Lowest-sensitivity model is correct High end: Warming of 6.4  C May occur if May occur if 1. Emissions continue to rapidly increase 2. Highest-sensitivity model is correct

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

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

22 Realized Warming This is the warming that has been observed at time t stop

23 time Temperature t stop Temperature Response Warming continues until equilibrium is restored Additional Warming “Realized” warming

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 Doesn’t include effect of any future increases in greenhouse gases

25 Published by AAAS T. M. L. Wigley Science 307, (2005) 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) Low Sensitivity Medium Sensitivity High Sensitivity

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 Mid-range estimate: Commitment ~ 0.5  C High-end estimate: Commitment ~ 1  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 CO 2 mixing ratio is 380 ppm i.e., only 380 out of every million molecules of air are CO 2 i.e., only 380 out of every million molecules of air are CO 2 CO 2 removal would require processing enormous amounts of air Lots of energy probably required. Lots of energy probably required stm stm

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

30 Another Problem Removing CO 2 would create waste Where should it be stored?

31 Carbon Capture and Storage Would reduce future emissions, but wouldn’t actually remove CO 2 from atmosphere stm#carbon stm#carbon

Carbon Sequestration 32