Sinking wood under the Black Sea: a carbon sequestration strategy
Ning Zeng and Jim Carton, University of Maryland, December 3, 2008
Bob Ballard is well known for his discovery of the Titanic, a luxurius cruise ship that sank in the middle of the North Atlantic Ocean in 1934 on its maiden voyge from London to New York. Less known was his recovery of a 1500 year old Roman battleship at the bottom of the Black Sea. The Roman ship was found to be preserved in near perfect condition with all its wooden structure, in contrast to the rusty condition of the Titanic. [photo of the Roman ship]
The secret of the good preservation of the Roman ship is the lack of oxygen. From 500 meters down, the Black Sea is nearly devoid of oxygen. The bottom water of the Black Sea is stagnant because it is heavy due to the high salt content. The surface fresh water from the Danube and other rivers is lighter than salty water, so it does not mix with deep water and the salty water stays at the bottom. In contrast, the North Atlantic Ocean is mixed because the cold water at the surface in winter becomes heavy enough to sink to the bottom, the main driving force of the Atlantic meridional over-turning circulation. As a result, any initial oxygen at the bottom of the Black Sea would be quickly depleted. Wood is decomposed biologically by fungi and bacteria, both need oxygen to grow. Thus the anoxic condition under the Black Sea prevented the degradation of the Roman ship.
Because the decomposition of dead wood in the forest releases CO2 into the atmosphere, Zeng (2008, Carbon Balance and Management) proposed to bury dead wood under soil and other anerobic environment. Because forests continue to absorb CO2 from the atmosphere, cutting off its return pathway forms an effective carbon sink. While this method is technically possible with large potential, the feasibility will depends on local circumstances that make it economically competitive. For instance, steep mountains where forests are typically better preserved tend to have too shallow soil for burying wood which then has to be transported, adding to the cost both in terms of money and energy needed and a CO2 emission 'penalty'.
So why not preserve some dead wood in the Black Sea? The Black Sea drainage basin consists major rivers such as the Danube, which collects water from some of Europe and Central Asia's better preserved and managed forests such as in the Alps , the Carpathians, and the Caucasus. Wood logs collected in these areas can be rafted down the rivers, and tied with weight such as stone or metal and sink to the bottom of the Black Sea, when they can be preserved and thus the carbon be locked away for thousands of years or longer. Colleting logs and send them down the rivers, especially in the mountainous regions have been practiced widely in the past and present, and the feasibility is obvious. [log raftting photo here]
So how big is the potential for CO2 sequestration? A quick estimate shows that about 1 million km2 of area may be available. Note that we are not proposing to cut down the forest, but rather to collect dead wood or selectively cut less productive trees. Zeng (2008) estimated a 0.1 kg/m2/y sustainable rate of coarse wood harvest. It turns out that European forests have already been harvested at a comparable rate, without apparent negative impact on the forests' health, and even more, the wood production rate is a factor of 2-3 higher than the harvest rate (Ciais et al., 2008; Nature Geoscience). Using 0.1 kg/m2/y wood harvest rate multiplied by the area, we would have 0.1 GtC/y sequestration rate. [Black Sea Map here]
This sequestration rate would offset Europe's current carbon emission by 10%, or 10 billion dollars per year, assuming $100 per tonne of carbon (this is fairly low according to International Atomic Energy Agency's 2008 estimate of $500/tC or higher). The cost of doing this may be around 5 billion dollars.
Ideally, one would like to leave the forest as intact as possible. However, given the urgency of the climate problem, some carbon sequestration will likely be needed in the future. The proposed method is an attempt at calling upon the forests to do yet another service, but presumably in a sustainable way if done right.
Zeng, N., 2008: Carbon sequestration via wood burial. Carbon Balance and Management, 3:1; https://doi.org/10.1186/1750-0680-3-1.
Draft by N. Zeng and J. Carton. Please do not distribute. Comments/feedbacks welcome.