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Climate Model Tests of the Early Anthropogenic Hypothesis Steve Vavrus Center for Climatic Research University of Wisconsin Bill Ruddiman (U. Virginia),

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Presentation on theme: "Climate Model Tests of the Early Anthropogenic Hypothesis Steve Vavrus Center for Climatic Research University of Wisconsin Bill Ruddiman (U. Virginia),"— Presentation transcript:

1 Climate Model Tests of the Early Anthropogenic Hypothesis Steve Vavrus Center for Climatic Research University of Wisconsin Bill Ruddiman (U. Virginia), John Kutzbach (U. Wisconsin)

2 Denton, G., and T. Hughes: “The Arctic Ice Sheet: An Outrageous Hypothesis” William Morris Davis The Value of Outrageous Geological Hypotheses, Science (1926) “ extolled the value of outrageous geological hypotheses as a means to force reexamination of the convictions of the day ” An Early Anthropocene?

3 Nature, February 12, 2004 Bill Ruddiman Deforestation ---------> Increased atmospheric CO 2 (8,000 years ago) Rice Cultivation ------> Increased atmospheric CH 4 (5,000 years ago) A New Outrageous Hypothesis

4 Insolation Trends

5 CH 4 CO 2 Insolation Trends

6 Spread of Agriculture

7 240 ppm CO 2 naturally 450 ppb CH 4 naturally 280 ppm pre-industrial CO 2 700 ppb pre-industrial CH 4

8

9 ~25%: direct agricultural carbon emissions (land clearance) ~25%: reduced ocean CO 2 solubility (warmer ocean) ~50%: enhanced S. Ocean CO 2 outgassing (sea ice, winds, stratification) Difference Between Natural and Pre-Industrial CO 2

10 Testing Hypothesis with 3 Climate Model Simulations 1. CAM3: Atmosphere/Mixed-Layer Ocean (CAM3+MLO): “NOANTHRO” 2. CAM3 + Interactive Vegetation (CAM3+MLO+VEG): “NOANTHRO_VEG” 3. CCSM3: Fully Coupled (includes deep ocean but fixed vegetation) T42 Resolution

11 Testing Hypothesis with 3 Climate Model Simulations 1. CAM3: Atmosphere/Mixed-Layer Ocean (CAM3+MLO): “NOANTHRO” 2. CAM3 + Interactive Vegetation (CAM3+MLO+VEG): “NOANTHRO_VEG” 3. CCSM3: Fully Coupled (includes deep ocean but fixed vegetation) Modern Control: Recent Greenhouse Gases (355 ppm CO 2, 1700 CH 4 ) LOWGHG: “Natural” Present-Day Gases (240 ppm CO 2, 450 ppb CH 4 ) Pre-Industrial: Pre-industrial Gases (280 ppm CO 2, 700 ppb CH 4 )

12 Testing Hypothesis with 3 Climate Model Simulations 1. CAM3: Atmosphere/Mixed-Layer Ocean (CAM3+MLO): “NOANTHRO” 2. CAM3 + Interactive Vegetation (CAM3+MLO+VEG): “NOANTHRO_VEG” 3. CCSM3: Fully Coupled (includes deep ocean but fixed vegetation) Modern Control: Recent Greenhouse Gases (355 ppm CO 2, 1700 CH 4 ) LOWGHG: “Natural” Present-Day Gases (240 ppm CO 2, 450 ppb CH 4 ) Pre-Industrial: Pre-industrial Gases (280 ppm CO 2, 700 ppb CH 4 ) Total anthropogenic influence (industrialization, early farming)

13 Testing Hypothesis with 3 Climate Model Simulations 1. CAM3: Atmosphere/Mixed-Layer Ocean (CAM3+MLO): “NOANTHRO” 2. CAM3 + Interactive Vegetation (CAM3+MLO+VEG): “NOANTHRO_VEG” 3. CCSM3: Fully Coupled (includes deep ocean but fixed vegetation) Modern Control: Recent Greenhouse Gases (355 ppm CO 2, 1700 CH 4 ) LOWGHG: “Natural” Present-Day Gases (240 ppm CO 2, 450 ppb CH 4 ) Pre-Industrial: Pre-industrial Gases (280 ppm CO 2, 700 ppb CH 4 ) Total anthropogenic influence (industrialization, early farming)Anthropogenic influence of early farming only

14 2.75-3.0 o C Global Cooling Temperature Change under “Natural” GHG Forcing

15 Permanent Snow Cover

16 Change in Tundra (%) Change in Boreal Forest (%) Vegetation Feedbacks

17 2.8 o C Global Cooling Annual Temperature Change Fully Coupled CCSM3

18 2.8 o C Global Cooling 2.75-3.0 o C Global Cooling Annual Temperature Change Fully Coupled CCSM3CAM3-SOM

19 Permanent Snow Cover Fully Coupled CCSM3CAM3-SOM CONTROL LOWGHG

20 -1.01 W/m 2 -2.05 W/m 2 -1.04 o -1.71 o 2.5E6 km 2 Pre-Industrial vs. CONTROL NOANTHRO vs. Pre-Industrial NOANTHRO Minus CONTROL

21 Conclusions Ruddiman’s early anthropogenic hypothesis is outrageous Holocene GHG trends are very unusual compared with recent interglacials Model evidence suggests a climate nearly 3 o C cooler than present in the absence of ancient agriculture and modern industrialization About 40% of the total artificial anthropogenic warming is early agricultural Improved physics (interactive veg and dynamical ocean) enhances response Future work: 1. Higher resolution runs (T85), 2. Plant CO 2 physiology, 3. Coupled carbon cycle in CCSM4 Special session at AGU Fall Meeting

22

23 X -0.87 W m -2

24 Schurgers, 2006 EMIC Simulation of Eemian and Holocene

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