Atmospheric inversion of
CO2 sources and sinks
Northern Hemisphere sink
Jay S. Gregg
Goal
Inverse modeling identifies carbon sources and
sinks, and coupled with a planetary transport
model, generates predicted CO2 concentrations.
Ideally, the model is adjusted so that the predicted
flux measurements best match those measured
at various locations around the globe.
Paraphrased from Gurney, 2006, http://www.purdue.edu/eas/carbon/inverse_modeling.html
Components

Observed Atmospheric Concentrations of CO2


Observed Sea Surface Concentrations of CO2


Spatiotemporal concentrations (ppm) of CO2
Partial Pressure of CO2
General Circulation Model
Sources and Sinks Involved
Fossil-Fuel-Based Emissions (Confidence: High)
Land Use Change (Confidence: Low)
Terrestrial Ecosystem Response to Elevated CO2
(Confidence: Low)
Terrestrial Sink (Confidence: Low)
Ocean Sink (Confidence: Low)
*Confidence refers to amount, temporal pattern, and
spatial location
Atmospheric CO2 Observations
Geophysical Monitoring for Climate Change
(GMCC) Network
Based on flask measurements
20 cites since 1980
Atmospheric CO2 Sampling Sites
ppm +300
Mountainous Sites (e.g., Mauna Loa) were not used due to difficulty in elevation for the
transport models
Tans et al., 1990
Atmospheric CO2 Concentration
observed
concentrations
Evidence for
missing northern
hemisphere sink
predicted concentrations
from known sources and
sinks (b, c, d)
Tans et al., 1990
Oceanic Observations
Observed pCO2 difference between surface ocean and
atmosphere
Transect Sampling, some data gaps in Indian and
Southern Ocean- extrapolation based on Sea Surface
Temperatures
Oceans divided into 2o x 2o grids, and mean DpCO2 is
calculated for the periods (January through April) and
(July through October)
Oceanic CO2 Calculations
Working Formula for F (CO2 flux across air-sea interface):
E: gas transfer coefficient, depends on wind speed
Vp: gas transfer piston velocity, depends on turbulence,
atmospheric and oceanic
S: solubility of CO2 in seawater
DpCO2: Sea surface – Atmosphere
(>0 is a ocean sink, <0 is an ocean source)
Tans et al., 1990
Oceanic CO2 Calculations
Transect Samples as of 1972
Tans et al., 1990
Oceanic CO2 Fluxes
Largest positive fluxes (sinks) are in the equatorial oceans
Largest negative fluxes (sources) are in the Southern gyres
Tans et al., 1990
Jan-Apr
Oceanic
CO2 Fluxes
Jul-Oct
Tans et al., 1990
Transport Model

3-D General Circulation Model (GCM) from
Goddard Space Flight Center, NASA

Seasonal, diurnal
Transport Model (vs. Observed)
Scandinavia
Bass Strait
observed
modeled
Tans et al., 1990
Relative to Global Mean Concentration
Modeled Atmospheric CO2 Concentrations
observed
modeled
Tans et al., 1990
Modeled
Fluxes
(C. Roedenbeck et al., 2002)
Modeled Fluxes
(C. Roedenbeck et al., 2002)
(C. Roedenbeck et al., 2002)
(C. Roedenbeck et al., 2002)
Modeled NPP
arbitrary units
(linear)
(C. Roedenbeck et al., 2002)
Modeled CO2 Sources and Sinks


Atmospheric CO2 increases about 3 Gt C/yr
Sinks are larger in northern hemisphere than
southern
ocean sink is largest at equator
 must be a larger northern terrestrial sink



El Nino and La Nina cycles changes fluxes
Still a lot of uncertainty in global carbon cycle
Which Transport Model to Use?

Many different transport models can give
different results

Underscores uncertainty in inverse model results

Transcom 3 Project (Gurney, 2002) seeks to
compare the outcome from various models
Which
Transport
Model to
Use?
Comparison of two transport
models, confidence range for all
models are in boxes
(Gurney et al., 2002)
Which Transport Model to Use?
Confidence range for all models based on latitude
(Gurney et al., 2002)
Factors in CO2 Flux Variability



El Nino and La Nina (increased biomass
burning), changes in NPP
Volcanic Eruptions (e.g., Pinatubo- changes in
NPP from sunlight limitations)
Temperature and humidity affect microbial
respiration (soil respiration increases at higher
temperatures)
(C. Roedenbeck et al., 2002)
References
I.G. Enting, C.M. Trudinger, R..J.A. Francey (1995) A synthesis inversion of the
concentration of d13C of atmospheric CO2. Tellus B 47, 35-52.
S. Fan, et al., (1998) A large terrestrial carbon sink in North America implied by
atmospheric and oceanic carbon dioxide data and models. Science 282, 442446.
K. R. Gurney et al., Towards robust regional estimates of CO2 sources and sinks
using atmospheric transport models, Nature 415, 626 (2002).
C. Roedenbeck, S. Houweling, M. Gloor, and M. Heimann (2003) CO2 flux
history 1982–2001 inferred from atmospheric data using a global inversion of
atmospheric transport, Atmos. Chem. Phys., 3, 1919–1964.
P. P. Tans, I. Y. Fung, T. Takahashi, (1990) Observational Constraints on the
Global Atmospheric CO2 Budget, Science 247, 1431-1438.