Carbon – Nitrogen – Climate Coupling

Peter Thornton

NCAR, CGD/TSS

June 2006

The global carbon cycle: fluxes and storage

The global carbon cycle: fluxes and storage

The global C cycle: changes over time

The global C cycle: changes over time

The global nitrogen cycle: fluxes and storage

The global N cycle: changes over time

Atm CO2

Plant

Litter / CWD

Soil Organic Matter

Carbon cycle

Soil Mineral N

N dep

N fix

nit/denit

N leaching

Nitrogen cycle

Respiration

Internal(fast)

External(slow)

Elements of design

• Establish design goals

• Persistence in the pursuit of quality

• Probe, test, explore…

• …build prototypes, and be prepared to abandon them

• Build to last, and archive your efforts

C-N coupling: hypotheses

• N-limitation reduces land ecosystem response to increasing CO2 concentration– Reduced base state– Stoichiometric constraints, internal N cycling– Progressive N-limitation due to biomass accumulation

• N-limitation damps carbon cycle sensitivity to temperature and precipitation variability– Reduced base state– Persistence due to internal N cycling

C-N coupling: hypotheses (cont.)

• Climate x CO2 response

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

Simulation protocol

1. Spinup at pre-industrial CO2 and N deposition

• ~700 yrs, following Thornton and Rosenbloom (2005)

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

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, following Thornton and Zimmermann (in review)

offline CLM-CN(CAM drivers)

coupled(CAM – CLM-CN)

transient, control (transient-control)GPP(CO2+Ndep)

CLM-C

CLM-CN (CO2,Nfix,dep)

CLM-CN (CO2,Nfix)

CLM-CN (CO2)

C4MIP models

C4MIP mean

Land biosphere sensitivity to increasing atmospheric CO2 (L)

Results from offline CLM-CN, driven with CAM climate, in carbon-only (CLM-C) and carbon-nitrogen (CLM-CN) mode, from present to 2100. Using SRES A2 scenario assumed CO2 concentrations.

Spatial distribution of L

C-N C-N

C-only C-only

2000 2100

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)

Coupling C-N cycles buffers the interannual variability of NEE due to variation in temperature and precipitation (global means, control simulations).

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

C-N C-N

C-only C-only

Tair Prcp

FIRE

HR

NPP

NEE

Components of NEE temperature response

NPP dominates NEE response to temperature in most regions. Exceptions include Pacific Northwest, Scandanavia.

Dissection of NPP temperature response

GPP

Soil ice

BtranNPP

Warmer temperatures lead to drying in warm soils (increased evaporative demand), and wetting in cold soils (less soil water held as ice).

FIRE

HR

NPP

NEE

Components of NEE precipitation response

NPP dominates NEE response to precipitation in tropics, midlatitudes, HR dominates in arctic and coldest climates.

Dissection of HR precipitation response

Snow depthNEE

HR

Tsoil

Higher Precip in arctic/cold climate produces deeper snowpack, warmer soils, increased HR.

Tair Prcp

% c

han

ge

fro

m c

on

tro

l

-40

-20

0

20

40

60

CLM-C: +CO2

CLM-CN: +CO2

CLM-CN: +CO2 +Nmin

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.

Conclusions (1)

• C-N coupling significantly reduces L

– not primarily the result of altered base state– strongest in the tropics and above 40N

Conclusions (2)

• C-N coupling significantly reduces NEE sensitivity to interannual variation in Tair and Prcp at steady-state– Tair effect is not primarily due to altered base state– Prcp effect consistent with alteration to base state– Tair: NPP dominates, with warming leading to drying in

warm soils, wetting in cold soils.– Prcp: NPP dominates in tropics/temperate, but HR

dominates in cold climates, with wetting leading to deeper snow, warmer soil, increased HR.

– Tair and Prcp responses likely in tension for warmer-wetter future climate.

Conclusions (3)

• Increasing CO2 amplifies the sensitivity of land carbon cycle to Tair and Prcp in C-only model, but damps these sensitivities in C-N model– This difference is not primarily due to a difference in

base state.– Tair response is consistent with increasing N

limitation under increasing CO2

The role of disturbance in C-N-climate coupling

Wildfire

The role of disturbance in C-N-climate coupling

Wind damage

The role of disturbance in C-N-climate coupling

Insects

The role of disturbance in C-N-climate coupling

Forest harvest

Simulated disturbance effects: Duke Forest, NC

Harvest loss: 11278 gC m-2

Thornton, in prep.