ICOS Science Conference, Session 6

1. Substantially larger estimates of global ocean-
atmosphere fluxes of atmospheric CO₂ from surface data
obtained when temperature corrections are applied
Andy Watson1, Ute Schuster1, Peter Landschützer2, Jamie
Shutler1, Tom Holding1, Ian Ashton1, David Woolf3,
Lonneke Goddijn-Murphy4
1. College of Life and Environmental Sciences, University of Exeter, UK
2. Max Planck Institute for Meteorology, Hamburg, Germany
3. Herriot-Watt University, UK
4. University of the Highlands and Islands, UK
We gratefully acknowledge all those who have contributed data to SOCAT!

2. • Atlas of quality-
controlled
surface pCO2
data.
• The Northern
Hemisphere
oceans are well
covered in
SOCAT, from the
early 90s on.
• The Southern
Hemisphere is
much less well
covered.
www.socat.info
200 300 400 500
pCO2 (µatm)
(a)
(b)
Surface ocean pCO2 data in SOCAT:
(a) All good data
(b) Southern hemisphere winter (JJA) data, 2000-2018
SOCAT: www.socat.info
SOCAT (V2) number of
observations

3. Estimates of ocean-atmosphere CO2 fluxes from
SOCAT data
1. In recent years there have been several published estimates
of global ocean-atmosphere fluxes from SOCAT surface data.
2. However, none have corrected for small deviations in
temperature close to the ocean surface, which turn out to
have a substantial (~50%) effect on calculated fluxes.

4. Adjustments/corrections to SOCAT data.
In this work we:
1. Correct data to take account of mixed layer
temperature gradients between sampling
depth and “sub-skin” surface, using satellite
data for SST (Goddijn-Murphy et al, 2015) .
2. Then adjust flux calculation for skin
temperature deviation (Robertson and
Watson 1992, Woolf et al 2016).
“The OceanFlux Greenhouse Gases methodology for deriving a sea surface climatology of
CO2 fugacity in support of air–sea gas flux studies” L. M. Goddijn-Murphy , D. K. Woolf , P. E.
Land , J. D. Shutler , and C. Donlon”

5. Difference,
subskin - SOCAT
inlet temperature
mode = -0.32K
mean = -0.127K
ΔT (K)
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
0
2000
4000
6000
8000
10000
12000
14000
(averages over 1 x 1 degree and 1 month)
• Sub-skin temperature can
be either higher or lower
than the sampling
temperature.
• But lower temperatures
dominate, especially at
night time and at higher
wind speeds where fluxes
are larger.
• Net effect is to decrease
effective ocean pCO2, so to
increase the flux from
atmosphere to ocean.
• The presence of the cool
ocean skin further increases
the flux

6. ocean–atmosphereCO2flux,PgCyr-1
Uncorrected
pCO2sw corrected to subskin
Flux corrected for cool, salty skin
1990 1995 2000 2005 2010 2015 2020
-4
-3
-2
-1
0
Calendar year
Global ocean-atmosphere flux calculated by method of
Landschützer et al, (2015)

7. Uncertainties in interpolation of SOCAT fCO2 data: wintertime Southern Ocean
Red – data in SOCAT 1992-2000
Blue – data in SOCAT 2000-2018

8. What is the ocean sink and its uncertainty as calculated by
SOCAT coverage?
Start from SOCAT gridded data,
• Gap-filling by three methods.
• Apply to oceans divided up in each of three
ways, so 9 estimates in total.

9. What is the ocean sink and its uncertainty as calculated by
SOCAT coverage?
• Method 1: Fit seasonal cycle and linear time
trend of pCO2 to all monthly mean of data,
apply fitted values over whole region.
• Method 2: Multiple linear regressions of pCO2
on SST, SSS, Mixed layer depth, XCO2atm.
• Method 3: Landschützer feed-forward neural
net.

10. Polar N. Atlantic
Subtropical N. Atlantic
N. Pacific
Method 1:
• deliberately simplistic
• generally poor fits!

11. 1990 1995 2000 2005 2010 2015 2020
Year
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
CO2flux(PgCyr
-1
)
Global flux

12. 1990 1995 2000 2005 2010 2015 2020
Year
-3
-2.5
-2
-1.5
-1
-0.5
0
-1
-0.5
0
CO2flux(PgCyr
-1
)
-1
-0.5
0
-1.5
-2
Northern hemisphere
Southern hemisphere
-3
-2.5
-2
-1.5
-1

13. How does our estimate of uptake compare with
anthropogenic carbon from interior observations?
Gruber et al, Science, 2019)

14. Estimates of ocean CO2 uptake compared to interior inventory of anthropogenic carbon
Cumulative CO2 uptake through surface (–ve is into ocean) July 1994- June 2007 (PgC, ± 2σ)
Atlantic Pacific Indian Other basins global
north -5.68±0.97 -6.60±0.90 +1.16±0.43 -1.56±0.8 -12.7±1.6
south -3.22±0.91 -3.43±4.6 -7.41±0.96 - -14.1±4.6
basin -8.91±1.50 -10.04±4.3 -6.25±1.20 - -26.8±3.4
Gruber et al15 estimates of inventory increase 1994-2007 (PgC)
north
6.0±0.4 5.2±0.6 0.8±0.4 1.5±0.6 13.5±1.0
south
5.9±1.2 8.0±1.2 6.3±3.4 - 20.1±3.8
basin
11.9±1.3 13.2±1.3 7.1±3.4 - 33.7±4.0
• The difference is the pre-industrial “riverine” flux (plus estimate for the Arctic)
• 33.7-26.8 = 6.9 PgC in 13 years  0.53 PgC yr-1
• Consistent with previous estimates of 0.57 PgC yr-1
Estimates of ocean uptake compared to interior inventory of
anthropogenic carbon

15. 1990 1995 2000 2005 2010 2015 2020
Year
-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
CO2flux(PgCyr
-1
)
Global anthropogenic CO2 ocean-atmosphere flux
Global flux +0.57 compared to Gruber et al estimate.
Earlier surface flux
estimates
Landschützer et al
Rödenbeck et al
Gruber et al interior estimate

16. Conclusions
• Correcting “surface” pCO2 observations to true interface
temperatures increases the calculated fluxes by ~0.9 Pg C yr-1
• Globally and at basin level, fluxes can be specified with 90%
confidence intervals of around ±0.3 Pg C yr-1 after 2000, (and
before that in N. Hemisphere).
• Southern ocean and South Pacific contribute much of the
uncertainty.
• Corrections make surface fluxes consistent with observed
increase in anthropogenic CO2 calculated from ocean interior
observations. Both these major ocean-observation-based
estimates of ocean uptake now agree.

17. 29%
11.6 GtCO2/yr
Fate of anthropogenic CO2 emissions (2008–2017)
Source: CDIAC; NOAA-ESRL; Houghton and Nassikas 2017; Hansis et al 2015; Le Quéré et al 2018; Global Carbon Budget
2018
22%
8.9 GtCO2/yr
34.4 GtCO2/yr
87%
13%
5.3 GtCO2/yr
17.3 GtCO2/yr
44%
Sources = Sinks
5%
1.9 GtCO2/yr
Budget
Imbalance:

18. 29%
11.6 GtCO2/yr
Fate of anthropogenic CO2 emissions (2008–2017)
Source: CDIAC; NOAA-ESRL; Houghton and Nassikas 2017; Hansis et al 2015; Le Quéré et al 2018; Global Carbon Budget
2018
22%
8.9 GtCO2/yr
34.4 GtCO2/yr
87%
13%
5.3 GtCO2/yr
17.3 GtCO2/yr
44%
Sources = Sinks
5%
1.9 GtCO2/yr
Budget
Imbalance:
X
Oceans =12.8 Gt/yr CO2, 32%

19. paper just published,
Nature Communications

21. • Spare slides

22. Adjusting to sub-skin temperature
• Sub-skin temperature can be either higher or lower than the sampling
temperature.
• But lower temperatures dominate, especially at night time and at higher wind
speeds where fluxes are larger.
• Net effect is to decrease effective ocean pCO2, so to increase the flux from
atmosphere to ocean.
Graphic from:
https://www.ghrsst.org/ghrsst-data-
services/products/

23. “Transcom” regions
Fay and Mckinley (2012) biomes
Use regions defined in three different ways
SOM-”Takahashi” biomes (December)