Is the ocean carbon sink sinking?
1 November 2007 - david @ 3:09 PM

The past few weeks and years have seen a bushel of papers finding that
the natural world, in particular perhaps the ocean, is getting fed up
with absorbing our CO2. There are uncertainties and caveats associated
with each study, but taken as a whole, they provide convincing
evidence that the hypothesized carbon cycle positive feedback has
begun.

Of the new carbon released to the atmosphere from fossil fuel
combustion and deforestation, some remains in the atmosphere, while
some is taken up into the land biosphere (in places other than those
which are being cut) and into the ocean. The natural uptake has been
taking up more than half of the carbon emission. If changing climate
were to cause the natural world to slow down its carbon uptake, or
even begin to release carbon, that would exacerbate the climate
forcing from fossil fuels: a positive feedback.

The ocean has a tendency to take up more carbon as the CO2
concentration in the air rises, because of Henry's Law, which states
that in equilibrium, more in the air means more dissolved in the
water. Stratification of the waters in the ocean, due to warming at
the surface for example, tends to oppose CO2 invasion, by slowing the
rate of replenishing surface waters by deep waters which haven't taken
up fossil fuel CO2 yet.

The Southern Ocean is an important avenue of carbon invasion into the
ocean, because the deep ocean outcrops here. Le Quere et al. [2007]
diagnosed the uptake of CO2 into the Southern Ocean using atmospheric
CO2 concentration data from a dozen or so sites in the Southern
hemisphere. They find that the Southern Ocean has begun to release
carbon since about 1990, in contrast to the model predictions that
Southern Ocean carbon uptake should be increasing because of the
Henry's Law thing. We have to keep in mind that it is a tricky
business to invert the atmospheric CO2 concentration to get sources
and sinks. The history of this type of study tells us to wait for
independent replication before taking this result to the bank.

Le Quere et al propose that the sluggish Southern Ocean CO2 uptake
could be due to a windier Southern Ocean. Here the literature gets
complicated. The deep ocean contains high concentrations of CO2, the
product of organic carbon degradation (think exhaling fish). The
effect of the winds is to open a ventilation channel between the
atmosphere and the deep ocean. Stratification, especially some decades
from now, would tend to shut down this ventilation channel. The
ventilation channel could let the deep ocean carbon out, or it could
let atmospheric carbon in, especially in a few decades as the CO2
concentration gets ever higher (Henry's Law again). I guess it's fair
to say that models are not decisive in their assessment about which of
these two factors should be dominating at present. The atmospheric
inversion method, once it passes the test of independent replication,
would trump model predictions of what ought to be happening, in my
book.

A decrease in ocean uptake is more clearly documented in the North
Atlantic by Schuster and Watson [2007]. They show surface ocean CO2
measurements from ships of opportunity from the period 1994-1995, and
from 2002-2005. Their surface ocean chemistry data is expressed in
terms of partial pressure of CO2 that would be in equilibrium with the
water. If the pCO2 of the air is higher than the calculated pCO2 of
the water for example, then CO2 will be dissolving into the water.

The pCO2 of the air rose by about 15 microatmospheres in that decade.
The strongest Henry's Law scenario would be for the ocean pCO2 to
remain constant through that time, so that the air/sea difference
would increase by the 15 microatmospheres of the atmospheric rise.
Instead what happened is that the pCO2 of the water rose twice as fast
as the atmosphere did, by about 30 microatmospheres. The air-sea
difference in pCO2 collapsed to zero in the high latitudes, meaning no
CO2 uptake at all in a place where the CO2 uptake might be expected to
be strongest.

One factor that might be changing the pressure of CO2 coming from the
sea surface might be the warming surface waters, because CO2 becomes
less soluble as the temperature rises. But that ain't it, as it turns
out. The surface ocean is warming in their data, except for the two
most tropical regions, but the amount of warming can only explain a
small fraction of the CO2 pressure change. The culprit is not in hand
exactly, but is described as some change in ocean circulation, caused
maybe by stratification or by the North Atlantic Oscillation, bringing
a different crop of water to the surface. At any event, the decrease
in ocean uptake in the North Atlantic is convincing. It's real, all
right.

Canadell et al [2007] claim to see the recent sluggishness of natural
CO2 uptake in the rate of atmospheric CO2 rise relative to the total
rate of CO2 release (from fossil fuels plus land use changes). They
construct records of the atmospheric fraction of the total carbon
release, and find that it has increased from 0.4 back in about 1960,
to 0.45 today. Carbon cycle models (13 of them, from the SRES A2
scenario) also predict that the atmospheric fraction should increase,
but not yet. For the time period from 1960 to 2000, the models predict
that we would find the opposite of what is observed: a slight decrease
in the atmospheric fraction, driven by increasing carbon uptake into
the natural world. Positive feedbacks in the real-world carbon cycle
seem to be kicking in faster than anticipated, Canadell et al
conclude.

There is no real new information in the Canadell et al [2007] analysis
on whether the sinking sink is in the ocean or on land. They use an
ocean model to do this bookkeeping, but we have just seen how hard it
is to model or even understand some of the observed changes in ocean
uptake. In addition to the changing ocean sink, drought and heat wave
conditions may change the uptake of carbon on land. The infamously hot
summer of 2003 in Europe for example cut the rate of photosynthesis by
50%, dumping as much carbon into the air as had been taken up by that
same area for the four previous years [Ciais et al., 2005].

The warming at the end of the last ice age was prompted by changes in
Earth's orbit around the sun, but it was greatly amplified by the
rising CO2 concentration in the atmosphere. The orbits pushed on ice
sheets, which pushed on climate. The climate changes triggered a
strong positive carbon cycle feedback which is, yes, still poorly
understood.

Now industrial activity is pushing on atmospheric CO2 directly. The
question is when and how strongly the carbon cycle will push back.

http://www.realclimate.org/index.php...-sink-sinking/
http://www.realclimate.org/index.php...-carbon-cycle/