Fundamental influence of carbon-nitrogen cycle coupling on climate-

carbon cycle feedbacks

Peter Thornton NCAR, CGD/TSS

Collaborators:

Keith Lindsay, Scott Doney, Keith Moore, Natalie Mahowald, Mariana Vertenstein, Forrest Hoffman, Jean-Francois Lamarque, Nan Rosenbloom, Beth

Holland, Johannes Feddema

Overview

• Review carbon-nitrogen coupling hypotheses

• Summarize results from offline simulations

• New results from fully-coupled simulations

• Review disturbance history hypotheses

• Preliminary results from fully-coupled simulations with landcover change

Recent findings from field experiments

• Norby et al., Finzi et al., Hungate et al., Gill et al., 2006 (Ecology)– CO2 fertilization shifts N from soil organic matter to

vegetation and litter– Some sites show progressive N limitation

• Reich et al., 2006 (Nature)– Low availability of N progressively suppresses the

positive response of plant biomass to elevated CO2

• Perennial grassland: CO2 x N x (sp. diversity)

• van Groenigen et al., 2006 (PNAS)– Soil C accumulation under elevated CO2 depends on

increased N inputs (fixation or deposition).

Brief history of coupled C-N modeling• Pre-1983: Decomposition models• Parton et al., 1983, 1987, 1988, 1989, etc.

– CENTURY: soil organic matter and vegetation model, coupled C, N, P, and S dynamics

• McGuire et al., 1992– TEM: C, N interactions control net primary production (N.Am.)

• Rastetter et al., 1992, 1997– MEL: Effects of CO2, N deposition, and warming on carbon

uptake• Schimel et al., 2000

– VEMAP: Terrestrial biogeochemistry model intercomparison• Thornton et al., 2002

– Biome-BGC: C, N, and disturbance history control net ecosystem carbon exchange (evaluation against eddy flux obs.)

• Liu et al., 2005– IBIS: NPP estimates improved when N dynamics added to a C-

only model (evaluation against remote sensing NPP est.)

Brief history of coupled climate – carbon cycle modeling

• Cox et al., 2000 – HADCM3 / MOSES / TRIFFID: Strong positive carbon-climate

feedback.• Joos et al., 2001

– BERN-CC: • Dufresne et al., 2002

– LMD5 / SLAVE: Moderate positive carbon-climate feedback• Thompson et al., 2004

– CCM3 / IBIS: Strong negative CO2 feedback• Fung et al., Doney et al., 2005

– CCM3 / LSM / CASA’: moderate CO2 feedback, weak climate feedback

• Friedlingstein et al., 2006– C4MIP Phase 2: Eleven models, wide range of CO2 and climate

(temperature) sensitivities.

‘86 ‘90‘88 ‘94‘92 ‘96 ‘00‘98 ‘04‘02 ‘06

Carbon-Nitrogen

CENTURY MELTEM VEMAP

Biome-BGCIBIS+N

Carbon-Climate

HadleyIPSL

C4MIP Phase 2

Carbon-Nitrogen-Climate

CCSM/CLM-CN

Convergence toward a fully

coupled carbon-nitrogen-climate

model

Atm CO2

Plant

Litter / CWD

Soil Organic Matter

Carbon cycle

Soil Mineral N

N deposition

N fixation

denitrification

N leaching

Nitrogen cycle

respiration

Internal(fast)

External(slow)

mineralization

assimilation

photosynthesis

litterfall & mortality

decomposition

Schematic of terrestrial nitrogen cycle

Image source: US EPA

C-N coupling: hypotheses

• Smaller CO2 fertilization effect, compared to C-only model– Reduced ecosystem base state– Stoichiometric constraints on new growth– Progressive N-limitation: carbon accumulation in litter

and soil leads to increased nitrogen immobilization

• Reduced carbon cycle sensitivity to temperature and precipitation variability– Reduced ecosystem base state– Internal N cycling links plant and microbial

communities

C-N coupling: hypotheses (cont.)

• Climate x CO2 response– Expect changes over time in land carbon cycle

sensitivity to variability in temperature and precipitation, forced by land carbon cycle response to increasing CO2.

• Different fertilization responses for increased CO2 and increased mineral N

– CO2: increased storage in vegetation carbon

– N: increased storage in vegetation and soil carbon

Offline simulation protocol

1. Drive CLM-CN with 25 years of hourly surface weather from coupled CAM / CLM-CN.

2. Spinup at pre-industrial CO2, N deposition, landcover

3. Transient experiments (1850-2100)

• Increasing CO2

• Increasing N deposition

• Increasing CO2 and N deposition

4. Repeat experiments in C-only mode

• Supplemental N addition eliminates N-limitation

• Test dependence on base state

Evaluation of fluxes and states

Thornton and Zimmermann, J. Clim. (in press)

Land biosphere sensitivity to increasing atmospheric CO2 (L)

CLM-C

CLM-CN (CO2)

C4MIP models

C4MIP mean

CLM-C2

Veg C (Pg C)

GPP (PgC/y)

L (2100)

CLM-C 1014 177 1.44

CLM-C2 771 146 1.25

CLM-CN 653 102 0.35

% change Veg C GPP L

CLM-C2 -24% -18% -13%

CLM-CN -35% -42% -76%

CN : C2 1.5 2.3 5.8

Thornton et al., GBC (in review)

Spatial distribution of L

C-N C-N

C-only C-only

2000 2100

Thornton et al., GBC (in review)

NEE sensitivity to Tair and Prcp (at steady-state)

% change VegC GPP Tair Prcp

CLM-C2 -24% -18% -17% -32%

CLM-CN -35% -42% -77% -58%

CN : C2 1.5 2.3 4.5 1.8

Thornton et al., GBC (in review)

NEE sensitivity to Tair and Prcp (at steady-state)

C-N C-N

C-only C-only

Tair Prcp

Thornton et al., GBC (in review)

NEE sensitivity to Tair and Prcp: effects of rising CO2 andanthropogenic N deposition

Carbon-only model has increased sensitivity to Tair and Prcp under rising CO2. CLM-CN has decreased sensitivity to both Tair and Prcp, due to increasing N-limitation.

Tair Prcp

% c

han

ge

fro

m c

on

tro

l

-40

-20

0

20

40

60

80

CLM-C: +CO2

CLM-C(2): +CO2

CLM-CN: +CO2

CLM-CN: +CO2 +Nmin

Thornton et al., GBC (in review)

Fertilization responses to CO2 and mineral N deposition

(period 2000 – 2100)

Tot C (PgC)

% Veg C % Lit C % SOM C

C-N: CO2 204 79 13 8C-N: Ndep 50 56 13 31C-only: CO2 843 66 14 20C-only(2): CO2 740 66 13 21

• C-N gives qualitative match to observations from field experiments

• Changing the base state has negligible effect on partitioning of fertilization response

• Important because of order-of-magnitude differences in turnover times for vegetation and SOM pools, regionalization of sink permanence.

Thornton et al., GBC (in review)

Summary of offline results

• C-N coupling results in large decrease in CO2 fertilization of land ecosystems.

• Sensitivities of net carbon flux to temperature and precipitation variation are damped by C-N coupling.

• C-N coupling reverses sign of trend in carbon-climate sensitivity under increasing CO2.

• Model agrees with experimental studies on contrasting effects of CO2 and N fertilization

Fully-coupled climate-carbon-nitrogen simulations

• CCSM3 framework: CAM + CICE + POP Doney/Moore/Lindsay ocean ecosystem + CLM-CN

• 1000-year stable pre-industrial control• Historical + A2 (1890-2100), forced with

fossil fuel emissions and nitrogen deposition.

• Coupled vs. fixed radiative effects of atmospheric CO2.

Results from fully-coupled runs:

Land sensitivity to increasing CO2 is not significantly different between offline and

fully-coupled simulations

Offline results

Negative climate feedback of land

carbon cycle

Land biosphere sensitivity to increasing atmospheric CO2 (L)

Changes in land carbon stocks: 1870-2100

PoolChange

PgC % of total

Plant 300 89%

CWD 20 6%

Litter 1 <1%

Soil 17 5%

Total 338 (25%)

Tair Prcp

NE

E s

ens

itiv

ity

to T

air

(P

gC

/ K

)

0

1

2

3

4

5

NE

E s

ens

itiv

ity

to P

rcp

(P

gC

/ m

m d

-1)

-25

-20

-15

-10

-5

0

CLM-CCLM-CN

NEE sensitivity to Tair and Prcp (at steady-state)

CCSM3

Land carbon cycle sensitivity to variation in temperature and precipitation is not significantly different between

offline and fully-coupled simulations

Climate-carbon cycle feedbacks

CO2-induced climate change (warmer and wetter) leads to increased land carbon storage

Climate-carbon cycle feedbacks

Vegetation C CWD C

Soil CLitter C

Climate change feeds back to carbon cycle in part through

change in nitrogen availability

Climate-carbon cycle feedbacks

Climate change impact on land carbon uptake closely related to changes in

precipitation

Climate-carbon cycle feedbacks

CCSM3.1-BGC C4MIP mean

uncoupled coupled uncoupled coupled

Land-borne fraction 0.15 0.18 0.29 0.21

Ocean-borne fraction 0.22 0.21 0.25 0.25

Airborne fraction 0.63 0.61 0.46 0.54

Fractions of 1870-2100 anthropogenic emissions in land, ocean, and atmosphere pools

What about disturbance history and landcover change?

Results: NEE response to disturbance, changing [CO2]atm, and Ndep shows strong interaction effects

Fully-coupled simulations with prescribed landcover changes (prognostic fluxes)

• Using subset of data from Johannes Feddema– Annual time slices from 1870-2100, originally on half-degree

grid.– Historical data from Ramankutty and Foley (1999), Goldewijk

(2001), integration with present-day MODIS landcover.– Landcover change for 2000-2100 based on SRES A2 scenario.

• New prognostic component in CLM-CN to handle carbon and nitrogen fluxes and mass balance associated with landcover change.– Conversions to/from all plant functional types.– Tracking two wood product pools in each land gridcell (10-yr and

100-yr turnover times).

• Fossil-fuel emissions, N deposition, radiatively coupled

Influence of landcover change on land carbon flux and global pools

Agric

Shrub

Tree

Grass

+200 ppmv net (by 2100)

+250 ppmv from land

-50 ppmv from ocean

A

S

T

G

• GPP falls and then recovers during landcover transition• Plant respiration increases, due to replacement of woody with herbaceous biomass• NPP falls and remains lower (GPP is offset by plant respiration)• Soil respiration rises in response to conversion fluxes, then falls in response to reduced NPP

Carbon cycle impacts of landcover change

• Transient fluctuations in GPP as new ecosystem is established

• Reduced NPP due to increased plant respiration– Depends on tree-to-ag vs. grass-to-ag transition and

regional climate (biggest effect for tropical forest conversion)

• Increased soil respiration due to more labile litter inputs.

• Reduced potential for CO2 fertilization of land ecosystems, due to loss of area of woody vegetation.

Changes in vegetation carbon pools due to landcover change

Comparison to previous results (preliminary)

• IPCC TAR includes estimates of fluxes due to landcover change, for the A2 scenario, which are more than an order of magnitude smaller than those predicted here for the 21st century.– Appear to ignore the mechanism of reduced

sink potential.

• Gitz and Ciais (2003): EMIC. Landcover flux much larger than IPCC, smaller than CCSM3-BGC.

Conclusions

• Carbon-nitrogen cycle coupling has a fundamental influence on climate-carbon cycle dynamics

• Mechanistic treatment of landcover change suggests this signal will be a major factor influencing future atmospheric CO2 concentrations, and regional patterns of climate change.

• Next generation models should explicitly include the factors driving fossil fuel emissions and landcover change.