Carbon-Water-Climate
Interactions
Dennis P. Lettenmaier
Ted Bohn
Department of Civil and Environmental
Engineering
University of Washington
for
NEESPI Science Team Meeting
Vienna
February 22, 2006
Outline
• Distribution of carbon in the climate
system
– Sources/sinks
– Transformations
• Carbon-water-climate interactions
• Specific freshwater-influenced interactions
– Wetlands
– Rivers
• Modeling perspective
Hydrologic Cycle
Atmosphere
Transpiration
Evap
Precip
Evap
Evap
Evap
Evap
Precip
Terrestrial Biosphere
Snow
Throughfall
Surface
Runoff
Soil
Wetlands
Upper
Ocean
Subsurface
Runoff
Rivers
Permafrost
Ocean
Bottom
Carbon Cycle
(to stratosphere)
CH4
-
OH + CH4
oxidation
Atmosphere
-
H2O + CH3
CH4
CO2
CH4
CO2
Respiration &
outgassing
ruminants,
termites,
and plants
Terrestrial Biosphere*
Litter
Soil
Burial
microbial respiration
Org C
CO2
Rh
NPP
phytoplankton
Wetlands
microbial
methanogenesis
* Excluding soil microbes
CO2
Upper
Ocean
CH4
Org C
DOC
Rivers
Ocean
Bottom
CO2
Combustion of fossil fuels
NPP
Burial
CH4 Rh
Fires
CO2
Distribution of Carbon in the
Climate System
CO2: 380 ppm
CH4: 1.75 ppm
Influenced by
Hydrology
Storages: Gt C
Fluxes: Gt C yr-1
(IPCC 2001)
Terrestrial Carbon Stocks
(IPCC 2001)
•Wetland soils store the most carbon per unit area
•Wetland extent depends on hydrology
•Wetland behavior depends on hydrology
High-Latitude Carbon Stocks
• High latitudes comprise much of NEESPI
domain
• 20-60% of global soil carbon pool stored in highlatitude soils
Biome
Soil C storage (Gt or Pg)
Upland forest
Northern peatland
Arctic tundra
90-290
120-460
60-190
(Schlesinger 1977, Post et al 1982, Post et al 1985, Oechel 1989, Gorham
1991, Chapin & Matthews 1992)
Sources of Atmospheric C
• Respiration (autotrophic and heterotrophic)
• Burning of fossil fuels
– over past 2 centuries, source of 480-500 Gt (Pg) C (IPCC 2001)
• comparable in size to terrestrial plant pool
– current rate: 5.3 GtCyr-1 (IPCC 2001)
• Natural fires
• Outgassing of CO2:
– from fresh water: 0.9 GtCyr-1 (Richey et al 2002)
– from ocean
– from soils
• Methane (CH4) production:
– CH4 has 23 times the greenhouse potential of CO2
Source
Flux (MtCyr-1)
Flux (GtCequivalentyr-1)
plants
63-243 (Keppler et al 2006)
1.45-5.59
microbes in wetlands
92-237 (IPCC 2001)
2.12-5.45
ruminants and termites
100-135 (IPCC 2001)
2.30-3.11
anthropogenic
170-340 (IPCC 2001)
3.91-7.82
Sinks of Atmospheric C
• Gross Primary Production (GPP)
(terrestrial and oceanic)
• Post-fire recovery
• Dissolution of CO2 into:
– precipitation
– freshwater
– oceans
– soils
• Methane oxidation in stratosphere
Either sources or sinks
• Changes in land use
– Over past 2 centuries, deforestation = source
of 180-200 Gt (Pg) C (IPCC 2001)
– Reforestation of abandoned farmland = sink
of up to 100 Gt (Pg) C (IPCC 2001)
• Fire and other disturbances
– Initial source followed by sink until equilibrium
again
– Change in disturbance frequency can cause
biological & soil pools to grow/shrink
Transformations
• Oxidation of CH4
– Stratosphere: 0.5 GtCyr-1
– Microbes in unsaturated soil in wetlands
• Riverine transport of C from terrestrial to
oceanic pools
– 0.4-1.2 GtCyr-1 (Richey et al 2002)
• Burial in deep sediments (land/ocean)
Global Methane Sources/Sinks
Influenced by
Hydrology
(Mt CH4 yr-1)
These numbers may
be revised in light of
recent estimates of
plant emissions of
63-243 MtCyr-1
(Keppler et al 2006)
(Lowe 2006)
Interactions (a partial list)
below saturation
CO2 uptake
CO2 fert.
NPP
waterlogging
evaporation
permafrost
outgassing,
respiration
plants,
wetlands,
ruminants,
etc.
negative
positive
either
clouds
Atm
[CH4]
Surface
T
evaporation
greenhouse
precipitation
greenhouse
Atm
[CO2]
Soil
evaporation Moisture
Atm
[H2O]
albedo,
sensible/
latent
heat flux
Species
Distrib.
transpiration
precipitation
Feedbacks – non-hydrological
Negative:
• CO2 fertilization
– Higher rate of NPP
– CO2 fertilization effect may decrease in
amplitude at higher levels of CO2 (Fung et al
2005)
• Dissolution of more CO2, CH4 into
oceans, freshwater
– This may have very limited effect
Feedbacks – non-hydrological
Positive:
• Increase in CH4 production in plants
(Keppler et al 2006)
• Emissions of CO2 from soils (Bellamy et al
2005)
Feedbacks - hydrological
Positive:
• Water vapor feedback
• Snow/Ice albedo
Uncertain direction:
• Clouds
• Change in species distribution
– replacement of moss with shrubs = less evaporation =
warmer T
– expansion of forests = more evaporation = cooler T;
but possibly lower albedo (if coniferous) = warmer T
High-latitude climate feedbacks
Hydrology
influences all
of these
(Chapin et al 2000)
Response time (y)
High-latitude climate feedbacks
Hydrology
influences all
of these
(Chapin et al 2000)
Wetlands
Competing processes:
•Carbon sink (peat
accumulation)
•Methane source
Hydrology plays key
role:
•Wetland extent
•Balance between CO2,
CH4
Global Distribution of Wetlands
(GLWD-3, Lehner and Doll 2004)
Global Wetland Distribution
(Lehner and Doll 2004)
Areas in 103 km2
Eurasia contains 30-50% of world’s wetlands
Wetland Peat Accumulation
Rates:
• All northern peatlands: 0.07 GtCyr-1 (Clymo et al 1998)
• Historical average rate during Holocene:
– Pan-Arctic: 97 TgCyr-1 (0.097 GtCYr-1) or 28 gCm-2yr-1 (Corradi et al 2005)
– West Siberia: 6.1 TgCyr-1 (0.0061 GtCyr-1) or 12 gCm-2yr-1 (Smith et al 2004)
Peat accumulation depends on:
• NPP
• Species assemblage
• pH
• Soil T
• Water table depth
• Thaw depth (where present)
• Leaching of DOC
These influence each other, but hydrology influences all of them
Wetland CH4 Emissions
Rates:
• Global: 0.09-0.24 TgCyr-1 (IPCC 2001)
• Can be up to 25% of CO2 uptake (Corradi et al 2005), or 5 X
greenhouse potential of CO2 uptake
• Historical average rate during Holocene:
– West Siberia: 0.3-84 TgCyr-1 (Smith et al 2004)
CH4 emissions depend on:
• Water table depth
• Soil temperature
• Substrate availability (approximated by NPP)
• Thaw depth (when permafrost is present)
Hydrology influences all of these
Wetland CH4 Emissions
(Huissteden 2004)
Example: Finnish bog
Where water table is near surface:
•aerobic respiration and methane
oxidation rates are small
•methane production is large
Where water table is deeper:
•aerobic respiration and methane
oxidation rates are large
•methane production is small
At all locations, methane
production rises and falls with soil
temperature
Walter and Heimann
(2000)
Example: bog
underlain by
permafrost
More substrate is made
available for consumption
as the thaw depth
increases.
This prolongs the period of
CH4 production through
September.
Walter and Heimann
(2000)
Example: West Siberian Peatlands
West Siberian
peatlands:
•Cover 600,000 km2
•Store 70 Gt (Pg) C
(Sheng et al, 2004, Smith
et al, 2004)
Permafrost is a factor here
Permafrost extent is shrinking
Future extent uncertain
Estimates depend on:
•CO2 scenario
•Climate model
(Frey and Smith 2005)
Boreal wetland extent is changing
Depends on hydrological
conditions:
•Precipitation excess (P-E)
•Permafrost extent
Northern Siberia:
•continuous permafrost thaws
•wetlands form
(Smith et al 2005)
Southern Siberia:
•discontinuous permafrost
disappears
•wetlands disappear
Potential Consequences
One scenario:
• West Siberian peatlands thaw completely and
water table drops
• Complete shutdown of CH4 emission
• Complete oxidation of C over 500 years = flux to
atm. of 140 TgCyr-1 (Smith et al 2004)
• Without enhanced uptake by biosphere and
oceans, growth rate of atm. [CO2] will increase
by 0.07 ppm per year, 4% faster than current
rate
• Net decrease in greenhouse potential
Rivers
Transport carbon (DOC, DIC) to
oceans:
•0.4-1.2 GtCyr-1 globally (Degens
et al 1991)
Emit CO2 to atmosphere (evasion):
•At least 1 GtCyr-1 (Richey et al 2002)
•Majority comes from from respiration
of particulate organic carbon (POC)
within the river
(Richey et al 2002)
Hydrology plays key role:
•River surface area
•POC donated by uplands during
floods
Rivers: CO2 Evasion
Depends on:
•Surface area of river network
•POC content
•Temperature
Currently more important in tropics than at
high latitudes
•Tropics: 0.9 GtCyr-1 (Richey et al 2002)
•Northern Peatlands: 0.02-0.05 GtCyr-1 (Hope
et al 2001) but this may be an underestimate
Ob, Yenisei, Lena:
0.3-0.4 ton km-1 yr-1
Amazon:
2.9 ton km-1 yr-1
POC Yields (Beusen et al 2005)
Rivers: CO2 Evasion at high latitudes
• Frey et al (2005): DOC transport from west
Siberian peatlands to Arctic Ocean is likely
to increase dramatically (29-46%) during
next century, due to thawing of permafrost
• This may increase evasion of CO2 at high
latitudes
Terrestrial Carbon Uncertainties
General Issues
• Balance between increased NPP and increased Rh
– Particularly boreal forests vs tropical forests
• Shape of NPP response curve at high [CO2] – when
does CO2 fertilization saturate?
• Upper limits on C storage in ecosystems (mechanical
and resource constraints)
– Thawing of permafrost may decrease the available storage
capacity at high latitudes
• Future land-use changes
• Changes in fire regimes
• Changes in N cycle?
Terrestrial Carbon Uncertainties
(cont.)
Future wetland extent
• Non-uniform increases in precipitation and evaporation
over most of high latitudes
• Where will precipitation excess (P-E) be higher? Lower?
By how much?
• Influence of topography, soil characteristics
• How quickly will permafrost thaw, and where?
High-latitude river evasion
• How much organic C will be released as permafrost
thaws?
• How fast will it decompose in transit?
Terrestrial Carbon Uncertainties
Competing influences of warmer T and drier soils on NPP, peat
accumulation, CH4 emissions
•
Relative importance of cold T, permafrost, waterlogging, and substrate quality in
stabilizing soil organic matter are poorly known (Hobbie et al 2000)
–
•
Contribution of wintertime soil respiration to C fluxes (Hobbie et al 2000)
–
•
Observations have been primarily at site level
Corradi et al (2005): global warming will increase the carbon sink of boreal wetlands
–
•
appears substantial but magnitude is unknown
Influence of fire, permafrost, and drainage on large-scale C fluxes poorly known
(Hobbie et al 2000)
–
•
behaviors under climate change?
whether this will entail a reduction in methane emissions depends on whether higher T
results in drier or wetter conditions
Angert et al (2005): drier summers produced by warming climate 1985-2002 canceled
out the CO2 uptake enhancement induced by warmer springs
How will changes in species assemblage affect rates of peat
accumulation and decomposition? (Strack et al 2004, Bauer 2004)
Effect of emission of methane by plants? (Keppler et al 2006)
Terrestrial Carbon Modeling Issues
Dynamic global vegetation models (DGVMs)
• Simulate biological processes based on soil and climate
– Contain some soil hydrology formulation
– Focus is on reproducing soil moisture trends
– But best available observations are for stream flow
• Vegetation distribution depends on:
–
–
–
–
Soil and climate
Growth rates
Competition for resources
Disturbance frequency (fire, mortality)
• Take account of lagged response of veg to climate
• Typically do not simulate methane emissions
• Examples: HYBRID, IBIS, LPJ, SDGVM, TRIFFID, VECODE,
BIOME3, DOLY
• Reproduction of observed land surface water cycle dynamics is
problematic
Terrestrial Carbon Modeling Issues
(cont.)
To adequately model water-mediated processes,
carbon models need sophisticated soil
hydrological components:
Most terrestrial carbon
models handle these, but
• Hydraulics
formulations are not
sophisticated
• Thermodynamics
Ex: HYBRID, IBIS, LPJ,
CASA, CENTURY
• Snow formulation
Large-scale hydrology
• Realistic stream flow
models handle these with
varying success
Ex: VIC, CLM, ECMWF
• Lakes/wetlands
Few large-scale models
• Permafrost
handle these
Ex: VIC
Conclusions
• Current separation of water and carbon
communities is primarily an administrative
artifact, which can only retard the scientific
advances in both areas
• Many examples of strong linkage between water
and carbon cycle at high latitudes, and
specifically within the NEESPI region
• Makeup of NEESPI science team provides an
opportunity to break down barriers between the
two communities in a region where carbon/CH4
dynamics have strong implications for the global
climate system
Diagnosis and Prognosis of Changes in Lake and
Wetland Extent on the Regional Carbon Balance of
Northern Eurasia
PI:
Co-PIs:
D.P. Lettenmaier (University of Washington
L.C. Bowling (Purdue University)
K. McDonald (Jet Propulsion Laboratory)
Collaborators:
N. Speranskaya and K. Tysentko (State Hydrological Institute,
Russia)
Daniil Kozlov and Yury N. Bochkarev (Moscow State
University)
Reiner Schnur and Martin Heimann (Max Planck Institute for
Biogeochemistry)
Gianfranco De Grandi (Joint Research Centre, Italy)
Overarching Science Question:
How have changes in lake and wetland extent in northern
Eurasia over the last half-century affected the region’s carbon
balance, and how are changes in lakes and wetlands over the
regional likely to affect its carbon balance over the next century?
Specific Questions



What areas within the region have been, and are most likely to
be in the future, affected by changes in lake and wetland extent?
How are ongoing changes in the tundra region (especially
changes in permafrost active layer depth) affecting the dynamics
of wetlands, and how are and will these changes affect the
carbon balance of the region?
How well can current sensors (MODIS, SAR) detect changes in
wetland extent, and can high resolution SAR products be used to
extend the rapid repeat cycle of lower resolution products like
MODIS to provide information about seasonal and interannual
variations in lake and wetland extent?
General approach:
Use high resolution remote sensing and in situ data to test and
evaluate new lake and wetland, and permafrost dynamics models
within the Variable Infiltration Capacity (VIC) macroscale
hydrology model.
 The VIC model will then be linked (through collaborations with ongoing
work at MPI-Hamburg and Jena with a dynamic terrestrial carbon model,
and with a lake and wetland methane model.
 Evaluation will be performed with respect to large area estimates of
carbon production and sequestration based on a combination of
extrapolation of direct measurements, inverse modeling methods, and
other modeling studies.
 Finally, we will attempt to reconstruct, using the extended VIC construct,
the time history of terrestrial carbon and methane balances over the
arctic Eurasia drainage, and, using a range of climate scenarios, to
interpret how these balances might change over the next century.
Tasks
Task 1: Model improvements
– Task 1a: VIC Lake and Wetlands model
extensions
– Task 1b: Methane model extensions
– Task 1c: Integration of VIC in MPI
VIC/BETHY/LPJ framework
Task 2: Data preparation and analysis
– Task 2a: In situ data
– Task 2b: Satellite data
Task 3: Model testing and evaluation
Task 4: Retrospective reconstruction of
regional carbon balance
VIC daily average soil temperatures for the 100-km
EASE grid cell centered at 60.6 N, 65.5 E, 1979
Simulated methane fluxes for the 100-km EASE
grid cell centered at 60.6 N, 65.5 E, 1979.
BETHY/LPJ net primary productivity for the 1degree grid cell centered at 60.5 N, 65.5 E, 1979
Modeling: Recent Progress
Better parameterizations of peat accumulation
•
Bauer (2004): changes in T, pH, water table over time
Incorporate CH4 emissions in wetlands into large-scale framework
•
Walter et al (2001)
– coupled Walter-Heimann methane model to simple large-scale hydrological
model
– hydrological model simplistic – didn’t handle snow, frozen soil, wetland evolution,
etc.
•
Shindell et al (2004) estimated future global methane emissions by:
– correlating simulated methane emissions (Walter-Heimann model) to simulated
anomalies of soil moisture and precipitation (ECMWF) for period 1982-1993
– applying correlations to anomalies from GISS GCM double CO2 scenario
– good first pass at predicting wetland extent, but ignores surface processes
(especially permafrost dynamics)
Modeling: Recent Progress
Combining hydrology and river chemistry
• Seitzinger et al (2005), Beusen et al (2005)
– Global Nutrient Export from Watersheds (GNEWS)
– coupled carbon, chemistry and hydrologic models to estimate river
transport of POC
– Evasion?
Permafrost, wetland dynamics, and large-scale carbon cycling
• Zhuang et al (2001)
– coupled 1D permafrost model to Terrestrial Ecosystem Model (TEM)
• Joint Simulation of Biosphere Atmosphere Coupling in Hamburg
(JSBACH)
– coupled LPJ (carbon), ECHAM (climate), VIC (hydrology), BETHY (plant
phenology)
– Soon will add Walter-Heimann methane model
Pathways for CH4 to reach
atmosphere
• Diffusion
– Slow
– Always available
– Many opportunities for oxidation when diffusing through aerobic soil
• Ebullition
– Rising of bubbles through water column
– Fast
– Occurs in saturated soil and in open water
• Plant-aided transport
– CH4 travels through aerenchyma (snorkel-like air tubes in wetland plant
tissue)
– Fast
– Some opportunity for oxidation by microbes in vicinity of plant roots
before CH4 reaches aerenchyma
– Requires presence of vascular wetland plants
– Requires that plant roots extend into saturated soil
Wetland Peat Accumulation
Peat
Accumulation
Soil T
NPP
Permafrost
pH
Leaching
of DOC
Water
Table
Species
Assemblage
Process-Based Terrestrial Carbon
Models
•
Terrestrial Biogeochemical Models (TBMs)
– Satellite based
• Estimate fluxes directly from satellite data
• KGBM, GLO-PEM, SDBM, TURC, SIB2
– Static vegetation models
• Simulate biological processes based on soil and climate
• Veg distribution may be prescribed or can be instantaneous function of soil and climate
• CASA, CENTURY, HRBM, TEM, CARAIB, FBM, PLAI, SILVAN, BIOME-BGC
•
Dynamic global vegetation models (DGVMs)
– Simulate biological processes based on soil and climate
– Veg distribution depends on:
•
•
•
•
Soil and climate
Growth rates
Competition for resources
Disturbance frequency (fire, mortality)
– Veg distribution may be out of equilibrium with climate due to lagged response
– HYBRID, IBIS, LPJ, SDGVM, TRIFFID, VECODE, BIOME3, DOLY
Hydrological Modeling Issues
•
Wetland dynamics
–
–
Energetics
Existence/growth/destruction as function of:
•
•
•
•
•
–
Climate
Topography
Water table depth
Soil characteristics
Stream inflows/outflows
Influence on downstream drainage network
•
Chemical concentrations/transport/exchange
•
Permafrost dynamics
–
Time evolution of soil characteristics usually treated as “constant” calibration parameters:
• Soil depth
• Porosity
• Permeability
(due to formation/melting of ice wedges, etc)
NOTE: I may be misinterpreting the conventional usage of permafrost – I’m making a
distinction between it and seasonally frozen soils)