1. CLM-CN update: Sensitivity to CO2, temperature, and precipitation in C-only vs. C-N mode
Peter Thornton, Jean-Francois Lamarque, Mariana Vertenstein, Nan Rosenbloom, Jeff Lee

2. CLM-CN: Summary Model Structure and Fluxes
Current
Storage
Leaf
Live
Stem
Live
Coarse Root
Plant
Pools
Previous
Storage
Fine
Root
Dead
Stem
Dead
Coarse Root
Wood Litter
(CWD)
Litter
Pools
Litter 1
(Labile)
Litter 2
(Cellulose)
Litter 3
(Lignin)
Soil Organic
Matter Pools
SOM 1
(fast)
SOM 2
(medium)
SOM 3
(slow)

4. Coupled Carbon-Nitrogen dynamics
This illustrates the potential feedback introduced by the coupled nitrogen cycle.
Coupled Carbon-Nitrogen dynamics
Strong feedback between decomposition and plant growth: soil mineral N is the primary source of N for plant growth.
Can result in a shift from C source to C sink under warming.
P.E. Thornton, NCAR

5. CLM-CN spinup summary: global total C pools (CAM drivers)
using accelerated decomposition method of Thornton and Rosenbloom, Ecol Mod (2005)

6. GPP (CO2+Ndep) transient, control (transient-control) offline CLM-CN
(CAM drivers)
coupled
(CAM – CLM-CN)

7. NEE (CO2+Ndep) transient, control (transient-control) offline CLM-CN
(CAM drivers)
coupled
(CAM – CLM-CN)

8. TotC (CO2+Ndep) transient, control (transient-control) offline CLM-CN
(CAM drivers)
coupled
(CAM – CLM-CN)

9. Expt Total C uptake (PgC) Mean NEE (PgC/y) (land fraction)
Cumulative land carbon uptake and net ecosystem exchange,
with constant climate (25-yr cycle), prescribed [CO2]atm
Total C uptake (PgC)
(land fraction)
Mean NEE (PgC/y)
Expt
N dep
16 (6%)
50 (3%)
-0.24
-0.73
CO2 fert
61 (22%)
220 (13%)
-0.98
-2.56
CO2+Ndep
79 (29%)
301 (17%)
-1.31
-4.13
CLM-C
223 (81%)
843 (49%)
-3.80
-10.75
Land fractions referenced against cumulative fossil fuel emissions of
276 PgC for and 1732 PgC for (SRES A2)

10. Land biosphere sensitivity to increasing atmospheric CO2 (L)
CLM-C
CLM-CN (CO2,Nfix,dep)
CLM-CN (CO2,Nfix)
CLM-CN (CO2) 
C4MIP models 
C4MIP mean
Results from offline CLM-CN, driven with CAM climate, in carbon-only (CLM-C) and carbon-nitrogen (CLM-CN) mode, from present to Using SRES A2 scenario assumed CO2 concentrations.

11. Land biosphere sensitivity to increasing atmospheric CO2 (L)
CLM-CN (CO2,Nfix,dep)
CLM-CN (CO2,Nfix)
CLM-CN (CO2)
Evidence that increasing N-limitation under rising CO2 has an important effect on the transient behavior of L, and that consideration of anthropogenic N deposition reverses this trend by around 2060.

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

13. NEE sensitivity to Tair and Prcp (CLM-CN vs CLM-C)

14. Components of NEE temperature response
NPP
NEE HR
NPP dominates NEE response to temperature in most regions. Exceptions include Pacific Northwest, Scandanavia.
FIRE

15. Dissection of NPP temperature response
GPP
NPP
Btran
Warmer temperatures lead to drying in warm soils (increased evaporative demand), and wetting in cold soils (less soil water held as ice).
Soil ice

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

17. Dissection of HR precipitation response
NEE
Snow depth
Higher Precip in arctic/cold climate produces deeper snowpack, warmer soils, increased HR.
Tsoil

18. Potential for complex climate feedbacks depending on the spatial patterns of changing temperature and precipitation.
NPP variability dominates the Tair and Prcp response in most locations, but HR dominates the Prcp response in cold climates, due to feedback between snowpack, soil warming, and enhanced HR.

19. NEE sensitivity to Tair and Prcp: effects of rising CO2 and
anthropogenic 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.

20. CLM-CN Summary: C-cycle response to nitrogen coupling,
CO2, temperature, and precipitation
Nitrogen coupling reduces sensitivity to CO2. This effect increases with increasing CO2.
Anthropogenic nitrogen deposition alleviates this effect.
Nitrogen coupling reduces global mean sensitivity to temperature and precipitation.
Complex spatial patterns of NEE response to T and P. These responses would be in tension under warmer-wetter climate.
CO2 increases T and P sensitivities in carbon-only model, decreases sensitivities in carbon-nitrogen model.

21. CLM-CN summary contd: Nitrogen cycle buffers land carbon-climate feedbacks
Nitrogen coupling…
reduces CO2 fertilization (reduces a negative feedback on climate system)
reduces T sensitivity (reduces a positive feedback)
reduces P sensitivity (uncertain feedback sign)
So, not a simple result with respect to total carbon-climate system gain.

22. CLM-CN development path
3-pool vs 4-pool switch implemented and tested, with expected results.
Landcover change effects (product pools) underway.
13C now incorporated and tested, transient runs underway, collaboration with CSU.
Collaboration with LLNL to add 14C on the same framework.
Implement age-class distributions.
Introduce N speciation and direct effects of ozone on physiology, in collaboration with CCSM Chem-Climate WG.

23. Speciation of land N emissions
Nitrification vs. denitrification depends on aerobic state of soil, probably at the microscopic scale.
Sophisticated models already exist, and it should be possible to adapt them for use in CLM-CN.
Agricultural emissions could be tied to new efforts with crop modeling.