CLM-CN update: Sensitivity to CO 2, temperature, and precipitation in C-only vs. C-N mode Peter Thornton, Jean-Francois Lamarque, Mariana Vertenstein, Nan Rosenbloom, Jeff Lee

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

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

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

offline CLM-CN (CAM drivers) coupled (CAM – CLM-CN) transient, control(transient-control) GPP (CO 2 +Ndep)

offline CLM-CN (CAM drivers) coupled (CAM – CLM-CN) transient, control(transient-control) NEE (CO 2 +Ndep)

offline CLM-CN (CAM drivers) coupled (CAM – CLM-CN) transient, control(transient-control) TotC (CO 2 +Ndep)

Total C uptake (PgC) (land fraction) Mean NEE (PgC/y) Expt N dep16 (6%)50 (3%) CO 2 fert61 (22%)220 (13%) CO 2 +Ndep79 (29%)301 (17%) CLM-C223 (81%)843 (49%) Cumulative land carbon uptake and net ecosystem exchange, with constant climate (25-yr cycle), prescribed [CO 2 ] atm Land fractions referenced against cumulative fossil fuel emissions of 276 PgC for and 1732 PgC for (SRES A2)

CLM-C CLM-CN (CO2,Nfix,dep) CLM-CN (CO2,Nfix) CLM-CN (CO2)  C4MIP models  C4MIP mean Land biosphere sensitivity to increasing atmospheric CO 2 (  L ) 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 CO 2 concentrations.

CLM-CN (CO2,Nfix,dep) CLM-CN (CO2,Nfix) CLM-CN (CO2) Land biosphere sensitivity to increasing atmospheric CO 2 (  L ) Evidence that increasing N- limitation under rising CO 2 has an important effect on the transient behavior of  L, and that consideration of anthropogenic N deposition reverses this trend by around 2060.

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).

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

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

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.

NEE sensitivity to Tair and Prcp: effects of rising CO 2 and anthropogenic N deposition Carbon-only model has increased sensitivity to Tair and Prcp under rising CO 2. CLM-CN has decreased sensitivity to both Tair and Prcp, due to increasing N-limitation.

CLM-CN Summary: C-cycle response to nitrogen coupling, CO 2, temperature, and precipitation 1.Nitrogen coupling reduces sensitivity to CO 2. This effect increases with increasing CO 2. 2.Anthropogenic nitrogen deposition alleviates this effect. 3.Nitrogen coupling reduces global mean sensitivity to temperature and precipitation. 4.Complex spatial patterns of NEE response to T and P. These responses would be in tension under warmer-wetter climate. 5.CO 2 increases T and P sensitivities in carbon- only model, decreases sensitivities in carbon- nitrogen model.

CLM-CN summary contd: Nitrogen cycle buffers land carbon- climate feedbacks Nitrogen coupling… reduces CO 2 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.

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.

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.