Wednesday, May 23, 2007

Antarctic Ocean a Blocked Sink for CO2?

The Southern (Antarctic) Ocean is thought to account for 15% of the total global "carbon sink" into which CO2 is drained from the atmosphere, but recent evidence is that it is being "blocked" by climate change, meaning that CO2 will increasingly accumulate in the atmosphere and possibly exacerbate global warming. It is a subtle feedback mechanism, since normally the oceans store CO2 in their deep waters, but as the atmospheric circulation (winds) increases over the Antarctic, the ocean water mixes more, and shuffles the deep, cold reservoir of CO2 toward the surface, where it can out-gas to the atmosphere. If this in turn leads to more warming and stronger winds, more of the deep waters will be churned upward in a symbiotic acceleration of effects that increasingly weaken the efficiency of the sink, and push-up both CO2 levels and temperature if CO2 really is the key to the Earth system's thermostat.

The effect had been predicted by climate scientists, and is thought at least partly to be included in climate models, though to what extent is debatable. The alarm-bell sounding now is that this is happening a good 40 years before its predicted time. Now this might be simply a feature of the enormously complex and interconnected nature of a truly "complex" system, in the mathematical sense of "complexity" where operating components can no longer be perceived in isolation from one another and conspire actively to bring about certain outcomes that might not be predicted by the reductionist logic that is standard to science. It is nonetheless worrying since it either indicates that the Earth systems are breaking down or that our simulations of reality may not be able to forewarn us of imminent catastrophes.

Only about half (I worked it out at 40% since 1950, in the posting "Carbon in the Atmosphere") of the CO2 that is released into the atmosphere actually stays there - the rest is absorbed into carbon sinks. A good proportion of CO2 is absorbed by plants (including the phytoplankton of the oceans), thought to be about 50:50 into land and marine based flora. Some of it is simply dissolved into the oceans and over time it is taken-up into plant and animal life, ultimately becoming mineralised as calcium carbonate, in the form of sedimentary deposits. CO2 is also absorbed directly from the atmosphere in the weathering of silicate rocks, which are thus converted into silicon dioxide and calcium carbonate. The CO2 that is not absorbed by some particular sink therefore accumulates in the atmosphere.

It was believed that ocean sinks would maintain pace with increasing levels of CO2 emitted from human activities, absorbing a comparable and consistent proportion of this greenhouse gas. It was expected that the sink-system might eventually break down but not before around 2050. Data from 11 sampling stations were noted in the Southern Ocean and from another 40 stations across the globe, which allowed the determination of how much CO2 was being absorbed by each according sink, and to compare their various efficiencies in performing this task. According to researcher Corinne LeQuere, "Ever since observations started in 1981, we see that the sinks have not increased. They have remained the same as they were 24 years ago, even thought the emissions have risen by 40%." This does appear significant, but is there a delay (a lag) behind CO2 emission and absorption? Are we pouring CO2 into the sky at such a rate that it will take some time before Nature activats her sinks (maybe by increasing the available quantity of phytoplankton) to absorb this (in geological terms) "abrupt"aberration in skyward CO2 levels?

Two factors have been cited as being culpable in the cause of this effect. Firstly the increased churning of the ocean waters by winds, as already noted, and secondly the increased warming of the Antarctic region as more solar radiation can get through the widening hole in the thinning ozone layer, which is particularly marked in the polar regions. This is thought to cause localised warming. The ocean surface becomes saturated with CO2 by the up-draught of CO2-laden waters from the depths, and so cannot take-up more of it from the air.

One further consequence is that these near-surface waters become more acidic, with a detrimental effect on marine organisms such as coral. Phytoplankton also do not tolerate unusually acidic environments well, and this important sink too may decline, in a similar fashion to the "clearing" rainforests. If our activities are not only raising CO2 levels directly in the atmosphere but blocking-up the sinks to remove it as well, we are surely in a condition of CO2-forcing... with unknown but probably not benign consequences.

Related Reading.
"Polar Ocean 'soaking up less CO2'," by Paul Rincon.