Measuring and monitoring ocean CO2

sources and sinks

Andrew Watson

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• Discrimination of sources/sinks between latitude bands is relatively easy

• Localising sinks in the same latitude bands is subject to wide error.

• Difficult to up-scale measurements of local terrestrial sinks to the continental scale. Ocean sinks are easier to constrain over large regions.

Atmospheric Inversion calculations of CO2 sources and sinks

How well is the global ocean sink known?

Estimates of the global ocean sink 1990-1999

Reference Sink (GtC yr-1)

IPCC (2001) 1.7+/- 0.5Estimate(O2/N2 ratio)

OCMIP-2 Model 2.5+/- 0.4Intercomparison(ten ocean carbon models).

Where does the land flux end and the ocean flux begin?

• Pre-industrially, a “riverine” flux of ~0.7Gt Ca-1 ran in a circuit from land to ocean to atmosphere and back to land. Today it may be larger.

• This flux shows up as a net land sink and ocean source, yet it is not a response to global change.

• Its distribution (both the land and ocean component) is very poorly known).

Specifying land and ocean sinks: which is easier?

The spatial scale of variation of CO2 fluxes in the ocean is much largerThan the equivalent on the land surface. Land ~ 100m, Ocean > 10km

• Direct measurement by air-side techniques – Very challenging, under development

• Surface pCO2 + gas transfer equation– Need lots of data, better parameterization of gas transfer

• Remote sensing extension of pCO2, gas transfer

– (CASIX; eventually, direct sensing of CO2)

• Basin-wide, full depth sections and “anthropogenic carbon” budgets.– Integrated with physical oceanography and modelling

programmes– Links to global carbon observing system, Carbo-ocean.

Methods for measuring ocean fluxes

Current state of the art: the Takahashi et al. climatology for CO2 fluxes

•Data collected over > two decades, “collapsed” onto a seasonal climatology•Critical assumptions used to combine the data are open to question.

•Equatorward of 50º, seawater pCO2 tracks atmosphere?

•Poleward of 50º, no change in sea surface pCO2 ?

•In fact, we expect fluxes to change with time, on seasonal, interannual and decadal time scales.

Remote sensing of SST, Ocean colour to enable Interpolation/ extrapolation of surface CO2 observations

Approach adopted by CASIXCurrently in its infancyInteresting results soon!

Budgeting a basin for regional, decadal uptake

Measure:•transport across a section or sections

•content of anthropogenic carbon at two different times.

Then:Net air-sea exchange + net convergence into basin =

Rate of change of content.

Air-sea exchange

Carbon transport Through section

Empirical Anthropogenic CO2

Removal of large metabolic DIC and pre-industrial solubility signals -steady state assumption -O2 surface saturation-uniform Redfield ratios -seasonal sampling bias -analogy of CFC and Canthro

Sarmiento and Gruber (2002)

80 W 60 W 40 W 20 W 0 20 E

60 S

40 S

20 S

0

20 N

40 N

60 N AR7W (CAN)

A16 (US, 2009)

A13S (US, 2009)

OVIDE (ES/FR, 2004)

A22 (US, 2004)

A22 (US, 2004)

AMT

Drake Passage repeat

24N

A02 (GE, 2004)

SOC, proposed

SOC, proposed

36N

Extended Ellett Line

20W

Programme is necessarily international, long-term.

Overall umbrella: IOC ocean carbon co-ordination project.

Links to:European programmes

(Carbo-ocean)US & Canadian

programmes

Carbo-Ocean – proposed European FP VI integrated project.

• Observing system in the Atlantic (ships of opportunity, buoys, moorings.

• Full-depth hydrographic programme to implement basin-budgeting.

• Project targeted at Southern Ocean• Extensive modelling, data assimilation to

provide up-to-date estimates of how sink is changing on a seasonal, inter-annual basis.

• ~50 EU project partners, ~10 US partners.

Existing and planned regular Atlantic observations

Existing and planned regular global observations

Possible effects on marine carbon uptake, next 100 years.

Increased stratification, decrease in MOC,

convection

Iron fertilisation -- deliberate or

inadvertent

NO3 fertilisation

pH change mediates against calcite-

precipitating organisms

Reduction in MOC offset by increased

efficiency of nutrient utilisation

Other unforeseen ecosystem changes

Process Effect on CO2 uptake

?

Suggested marine carbon themes in a UK carbon programme

• Integrated with international programmes; Carbo-Ocean, US carbon cycle plan, International Ocean Carbon Co-ordination project…

• Integrated with terrestrial and atmospheric studies

• Clear aims: – Reduce (perhaps by a factor of two) current uncertainty on

N. Atlantic sink

– Detect change in Cant

– Detect change (or lack of it) in biological pump– Detect change (or lack of it) in physical transports. – Use models and theory to understand the underlying

processes.