Terrestrial ecosystems in the global carbon cycle

Chris FieldDepartment of Global Ecology

Carnegie Institutionwww.global-ecology.org

Carnegie Institution of Washington:Institutional commitment to research at the

frontier since 1902• 6 Departments

– Global Ecology, Plant Biology, Embryology, Observatories, Terrestrial Magnetism, Geophysical Laboratory

• One Department on the Stanford campus since 1928

Thanks to:• Nona Chiariello• Hal Mooney• Terry Chapin• Peter Vitousek• Brendan Bohannan• Shauna Somerville• Jim Tiedje

• Becky Shaw• Jeff Dukes• Hugh Henry• Bill Gomez• Bruce Hungate• Erika Zavaleta• Sue Thayer• Art Fredeen• Pep Canadell• Yiqi Luo• Rob Jackson• Osvalso Sala• Zoe Cardon• Teri Balser• Matthias Rillig• Beth Holland• Hans-Peter Horz• Sharon Avrahami• Shuijin Hu• Margaret Torn• Camille Parmesan• John Lussenhop• Ken Caldeira• Marty Hoffert• Jan Brown• Niki Gruber• Martin Heimann

And:• Elsa Cleland• Halton Peters• Lisa Moore• Brian Thomas• Geeske Joel• Chris Lund• Julia Verville• Carolyn Malmström• Julie Whitbeck• Heather Reynolds• Paul Higgins• Amber Kerr• Duncan Menge• Grace Hsu• Meg Andrew• Anna Hare• Valerie Franck• Wing Man Yeung• Jessica Guo• Joey Blankinship• Jessica Mentzer

• Todd Tobeck• David Kroodsma• Kathleen Brisgys• Yuka Estrada• Will Whitted• Barbara Mortimer• Eric Nelson• Julia Silvis• Ben Poulter• Julie Osborn• Julie Des Rosiers• Vivian Schoung• Christy Hipsley

• National Science Foundation• David and Lucile Packard Foundation• Carnegie Institution• Jasper Ridge Biological Preserve• Morgan Family Foundation• SCOPE• Global Carbon Project

Carbon storage & climate feedbacks• In coming decades, will the terrestrial

biosphere continue to store a substantial fraction of the carbon emitted from fossil fuel combustion and deforestation?

Global carbon cycle:

Before major human influences

Global carbon cycle:

Direct effects of humans

Global C cycle: past subsidies from nature

• Pre-industrial CO2 = 280 ppm• Current atmospheric CO2 = 375 ppm• Without subsidies, CO2 = 500 ppm

• Over the next century, models in IPCC project uncertain but large subsidies:– Oceans: 160 ppm– Land: 125 ppm

Implications for policy

• Have future free subsidies from nature been overestimated in past analyses?

• Is there a greater risk of positive feedbacks as warming becomes larger?

• Are we making realistic estimates of the need for emissions-free energy?

Stabilization wedges: Solving the climate problem for the next 50 years with current technologies –

S. Pacala & R. Socolow (Science, 2004)

Potential Wedges from P&S

• Economy wide carbon-intensity reduction

• Efficient vehicles• Reduced use of vehicles• Efficient buildings• Gas baseload power for coal

baseload power• Capture CO2 at baseload

power plant• Capture CO2 at H2 plant• Capture CO2 at coal to

synfuels plant

• Geological storage• Nuclear power for coal power• Wind power for coal power• PV for coal power• Wind H2 in fuel-cell car for

gasoline in hybrid car• Biomass fuel for fossil fuel• Reduced deforestation, plus

reforestation, afforestation, and new plantations

• Conservation tillage

Quantitative issues?

• Economic challenge of scaling wedges?• How many wedges do we need?

– Accounting for all forms of emissions reduction

– Accounting for land and ocean feedbacks

Kaya Identity

xPP

GWPxGWP

ExE

COCO ⎟

⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛= 2

2

Per Capita Carbon Emissions Versus Per Capita GDP of 100 Nations Marty Hoffert

10-31-04

Stabilization wedges

• Many of the potential wedges will fall under endogenous efficiency improvements

• Maintaining historical pace of endogenous efficiency improvements represents a real challenge

• Necessary wedges for constant emissions could be:– Up to 40 at 1 Pg C y-1 in 2054– A few much larger than 1 Pg C y-1 in 2054– Both of the above

Jasper Ridge C budget

+Plant

Production

CO2

179

1453939

440

279

323

Root Respiration,Turnover and Exudation

Belowground Respiration

201Litter+Soil

Accumulation

Hungate et al. 1997 Nature

Jasper Ridge Global Change Experiment:1997-

• How do responses to realistic multi-factor global change differ from responses to single factors?

• How are ecosystem-level responses to global change partitioned between plastic responses within species and changes in species composition?

• How do the interactions between plants and microbes change when plant composition changes?

• What is the role of herbivory in regulating global change responses?

N and Precip applied to quarter plots

Complete factorial, split-plot, randomized block design with 4 factors, at two levels, and 8 replicates

Ambient +CO2+Temp +CO2,Temp

N PNP

CO2 and heat applied at whole plot level

+CO2 = 700 ppm

+N = 7g N/m2/yr+P = 150% ambient + 20 day extension

+Temp = 1.5ºC above ambient

3 topicsPlant community diversity and composition

Reflectance-based measures of canopy greenness, NDVI

Net primary production, NPP Soil moisture Trace gases

“Suppressive” effect of elevated CO2 on NPP in 2001

• On its own, CO2 slightly increased NPP• Other factors also increased NPP• When added to other factors, CO2 suppressed that

NPP stimulation2001 NPP response

Shaw et al 2002 Science

“Suppressive” effect of elevated CO2 on NPP in 2001

• On its own, CO2 slightly increased NPP• Other factors also increased NPP• When added to other factors, CO2 suppressed that

NPP stimulation2001 NPP response

Shaw et al. 2002 Science

“Suppressive” effect of elevated CO2 on NPP in 2001

• On its own, CO2 slightly increased NPP• Other factors also increased NPP• When added to other factors, CO2 suppressed that

NPP stimulation2001 NPP response

Shaw et al. 2002 Science

Mechanism(s) of the CO2 suppression

• Some evidence for:– Phenology (nutrient competition)– Root:Shoot ratio (nutrient foraging)– Nutrient limitation, especially N&P

• General result:– Other resources limit CO2 response– Use these limits to estimate global

sensitivity to CO2

Carbon cycle consequences

• Modest CO2 fertilization under all circumstances• Constraint from N gives optimistic upper bound

for fertilization• Estimate global increase in N availability

– (2000-2100)• Compare with standard models for CO2

fertilization

• Hungate et al. 2003 Science

Yikes!

• Existing models may over-estimate century-scale CO2 uptake on land– Perhaps by up to 360 Pg or 180 ppm– Probably ½ compensated by oceans

Why have we overestimated CO2 fertilization?

• Unrealistic early experiments– Fast-growing plants– Abundant resources– Focus on aboveground NPP

• IPCC rules– Other sinks masqueraded as CO2 fertilization

• Regrowth• Overestimated deforestation• Forest thickening• Landfills and sediments

Gruber et al. SCOPE 62, 2004

Gruber et al. SCOPE 62, 2004

Gruber et al. SCOPE 62, 2004

Gruber et al. SCOPE 62, 2004

Land subsidies: New evidence

• Limited effects of CO2 fertilization– (decreased subsidies)

• Vulnerable pools– (decreased subsidies or carbon losses)

• Land: New uncertainty– Uptake of 125 ppm– Release of 200 ppm

• 325 ppm is most of the range among IPCC scenarios in 2100

From the natural side of the C cycle

• Challenge:– Preserve economic growth– Preserve integrity of global climate

• No single solution

• Dramatically increased commitments to:– Conservation– Efficiency– New technologies– Sequestration