June 11, 2005

How Much Effect Is There From the Sea?

Follow-up to Humanity's Greenhouse Footprint from The Militant Wing of Pacifism

CS responded to one of the previous posts with:

A little known fact is that much of the world's CO2 is stored in sea-water, and a rise in temperature reduces the water's capability to retain CO2, so the oceans give off CO2 as they warm. While I don't have exact figures to hand, I think the contribution of natural warming to CO2 levels in this manner is significant.

I pretty much ignored it, previously, because it wasn't really significant to my point – even if human global warming is triggering other carbon sources, this still counts as human caused climate change, in that it wouldn't be happening if we weren't releasing. It just adds a degree of superfluous indirectness to the preceeding. It didn't occur to me to investigate this particular 'fact'.

So, from RealClimate :

This question keeps coming back, although we know the answer very well: all of the recent CO2 increase in the atmosphere is due to human activities, in spite of the fact that both the oceans and the land biosphere respond to global warming.

Why? Because we've actually measured the Carbon content of the seas. The following is pasted pretty much directly from that entry. (Because I can't get blockquotes working right…) They've given references, so you can check up their assessments if you desire:

Pasted section begins

On time-scales of ~100 years, there are only two reservoirs that can naturally exchange large quantities of CO2 with the atmosphere: the oceans and the land biosphere (forests and soils). The mass of carbon (carbon is the "C" in CO2) must be conserved. If the atmospheric CO2 increase was caused, even in part, by carbon emitted from the oceans or the land, we would measure a carbon decrease in these two reservoirs.

Number of observations of carbon decreasing in the global oceans: zero.

Number of observations of carbon increasing in the global oceans: more than 20 published studies using 6 independent methods.
The methods are:
(1) direct observations of the partial pressure of CO2 at the ocean surface (Takahashi et al. 2002),
(2) observations of the spatial distribution of atmospheric CO2 which show how much carbon goes in and out of the different oceanic regions (Bousquet et al. 2000),
(3) observations of carbon, oxygen, nutrients and CFCs combined to remove the mean imprint of biological processes (Sabine et al. 2004),
(4) observations of carbon and alkalinity for two time-periods combined with an estimate of water age based on CFCs (McNeil et al. 2002), and the simultaneous observations of atmospheric CO2 increase and the decrease in (5) oxygen (Keeling et al. 1996), and (6) carbon 13 (Ciais et al. 1995) in the atmosphere.

The principle of the last two methods is that both fossil fuel burning and biospheric respiration consume oxygen and reduce carbon 13 as they produce CO2, but the exchange of CO2 with the oceans has only a small impact on atmospheric oxygen and carbon 13. The measure of atmospheric CO2 increase together with oxygen or carbon 13 decrease gives the distribution between the different reservoirs.

All the estimates show that the carbon content of the oceans is increasing by 21 PgC every year (current burning of fossil fuel is 7 PgC per year). One method is able to go back in time and shows that the carbon content of the oceans has increased by 11819 PgC in the last 200 years. There is some uncertainty about the exact amount that the oceans have taken up, but not about the direction of the change. The oceans cannot be a source of carbon to the atmosphere, because we observe them to be a sink of carbon from the atmosphere.

Why are the ocean and land taking up carbon, when we know that warming of the oceans reduces the solubility of CO2 and warming of the land accelerates bacterial degradation of the soils? The answer is that warming is not the only process that influences the oceans and land biosphere. The dominant process in the oceans is the response to increasing atmospheric CO2 itself.


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