Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Does warm or cold water take up more CO2?

E Maier-Reimer, J. Segschneider, and K. Six

Max-Planck-Institute for Meteorology, Hamburg, Germany

EU FP6 IP 511176 (GOCE)

supported by CARBOOCEAN

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Motivation

Recently, the high solubility of CO2 in cold water has been used to argue that the polar regions are key regions for CO2 uptake

Is this a valid argument?

It neglects the buffer system, and, possibly, ocean dynamics

Let‘s have a more complete look at the carbonate buffer system

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

this is meant to be a reconsideration and clarification of well known facts about carbonate chemistry rather than a piece of ongoing research!

Motivation:

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Motivation

to answer if warm or cold water takes up more CO2 we need to investigate what determines

∂2DIC / (∂ pCO2 ∂ T) = ?

based on and further reading:DOE 1994. Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water

Zeebe & Wolf-Gladrow, 2001. CO2 in Seawater: Equilibrium, Kinetics, Isotopes

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Inorganic carbon chemistry in sea water

transfer of CO2 from atmosphere to ocean

CO2

K0 K1 K2

CO2 + H2O HCO3- + H+ CO3

2- + 2 H+

K0 = solubility (T,S,p)K1,K2 = equilibrium constants (T,S,p)

Atmosphere

Ocean

solubility buffering

simplified view leaving aside carbonic acid and boron compounds

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Solubility and equilibrium constants

based on Weiss 1974, Millero 1995

solubility constant K0:

equilibrium constant CO2 + H2O HCO3- + H+

equilibrium constant HCO3- + H+ CO3

2- + 2 H+

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

K0 (T=30oC,S=35) =0.025mol/(kg bar)

Solubility and equilibrium constants

S=35TA=2350

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

the carbonate system at 360 ppm

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

The Revelle factor traditionally relates the changes of the carbon pools DIC and [CO2] to their actual size:

(d[CO2] / [CO2])

(dDIC / DIC)

some words on buffering: Revelle factor

R=

1 (d[CO2] / [CO2])

R DICdDIC=

is no mystery!

TA=const!

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

The Revelle factor traditionally relates the change of the carbon pools DIC and [CO2] to their actual size:

(d[CO2] / [CO2])

(dDIC / DIC)

Revelle factor

R=TA=const!

1 (d[CO2] / [CO2])

R DICdDIC=

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

How is the Revelle factor determined?

for simplicity: s:=[CO2], h:=[H+] +[H3O+]+ ..

(1) DIC = s(1+K1/h + K1K2/h2)

(2) TA = s(K1/h+2K1K2/h2) + BT/(1+h/KB)+KW/h-h

change of pCO2 yields change in s and h:

(3) dDIC = ∂DIC/∂s ds + ∂DIC/∂h dh(4) dTA = ∂TA/∂s ds + ∂TA/∂h dh (=0)

(5) dh/ds = - (∂TA/ ∂s ) / (∂TA/ ∂h ) | using dTA=0, and Henry‘s law inserting (5) into (3) and derivation with respect to s yields:

(6) dDIC/ds=(∂DIC/∂s)ds - (∂DIC/∂h) ((∂TA/∂s)/(∂TA/∂h))

explicit differentiation and multiplication of (6) yields (quite) lengthy expression (Zeebe&Wolf-Gladrow, p 71/72)

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Revelle factor - continued-

dDIC = K0 dpCO2 (∂DIC/∂s - (∂DIC/∂h)(∂TA/∂s)/(∂TA/∂h))

in words: The Revelle factor is the total derivative of DIC with respect to [CO2] (s)

or: Revelle factor is given by the ratio of the relative change of CO2 to the relative change of DIC

we only discuss the resulting terms:

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Factor for buffering

dDIC = K0 dpCO2 (∂DIC/∂s - (∂DIC/∂h)(∂TA/∂s)/(∂TA/∂h))re

lati

ve u

nit

s

x20

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Factors for buffering: RF(pCO2, T)

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Revelle factor

the Revelle factor decreases with rising temperature (which may intuitively –and erroneous- imply that the uptake of anthropogenic CO2 increases along with temperature during global warming), while the increase of the Revelle factor with rising pCO2 implies reduced uptake

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Change of pools with increasing pCO2(T)

(CO32- concentration,

not charge)

dpCO2=+1x10-5 ppm/K

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Surface and deep circulation (Wüst, 1950)

NADW

AABW

Gulf Stream

subtropical gyre

AAIW

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Anthropogenic CO2 in the North Atlantic

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

warm or cold? inventory divided into temperature bands

Third annual CarboOcean meeting, 4.-7.December 2007, Bremen, Segschneider et al.

Recent discussions indicate that some confusion may be prevalent in the climate research community about the role of cold water in the uptake of anthropogenic CO2. This may be due to

the facts that: the solubility of CO2 in sea water is higher in cold

water - however, the buffer effect is not Warm water takes up relatively more of an

imposed atmospheric pCO2 increase Ocean physics play a role: deep water formation

regions are major sinks for CO2 because of deep mixing that transports CO2 to the ocean interior

Summary: