Measuring and monitoring ocean CO 2 sources and sinks Andrew Watson

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 CO 2 sources and sinks

How well is the global ocean sink known? Estimates of the global ocean sink ReferenceSink (GtC yr -1 ) IPCC (2001) 1.7+/- 0.5 Estimate (O2/N2 ratio) OCMIP-2 Model 2.5+/- 0.4 Intercomparison (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 CO 2 fluxes in the ocean is much larger Than the equivalent on the land surface. Land ~ 100m, Ocean > 10km

Direct measurement by air-side techniques –Very challenging, under development Surface pCO 2 + gas transfer equation –Need lots of data, better parameterization of gas transfer Remote sensing extension of pCO 2, gas transfer –(CASIX; eventually, direct sensing of CO 2 ) 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 CO 2 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 pCO 2 tracks atmosphere? Poleward of 50º, no change in sea surface pCO 2 ? 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 CASIX Currently in its infancy Interesting 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 CO 2 Removal of large metabolic DIC and pre-industrial solubility signals -steady state assumption-O 2 surface saturation -uniform Redfield ratios-seasonal sampling bias -analogy of CFC and C anthro Sarmiento and Gruber (2002)

80W 60W 40W 20W 0 E 60S 40S 20S 0 N 40N 60N AR7W (CAN) A16 (US, 2009) A13S (US, 2009) OVIDE (ES/FR, 2004) A22 (US, 2004) AMT Drake Passage repeat 24N A02 (GE, 2004) 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 NO 3 fertilisation pH change mediates against calcite- precipitating organisms Reduction in MOC offset by increased efficiency of nutrient utilisation Other unforeseen ecosystem changes ProcessEffect on CO 2 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 C ant –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.