Effectiveness of CO2 sequestration in the pre- and post-industrial oceans

Ocean carbon cycle modeling is expected to play a key role in decisions concerning the purposeful sequestration of CO2 in the oceans. Modeling is probably the only practical way to know how much good sequestration is doing, or would do if implemented, since even with sequestration, atmospheric CO2 would be expected to continue to increase because of the release of non-sequestered CO2 and the return to the atmosphere of sequestered CO2. We have employed a carbon cycle model based on an ocean general circulation model to estimate the return of sequestered CO2 to the atmosphere for sequestration in the pre-industrial ocean at sites near Tokyo, San Francisco, New York and Miami. Significant differences in the effectiveness of sequestration are found. Off the East Coast of the USA, the atmospheric concentration due to the return of sequestered CO2 quickly rises to near the final equilibrium value, for all depths below about 800 m. Off the West Coast of the USA, the CO2 is effectively hidden from the atmosphere for several hundred years, for depths below 800 m. Then the atmospheric concentration rises to larger values than for sequestration off the East Coast. Sequestration off the coast of Japan at 800 m is similar to sequestration off the West Coast of the USA, but without the time delay. These predicted differences are understandable in terms of ocean circulation. Sequestration in the post-industrial ocean beginning in the year 2000 has been modeled also and sites near Tokyo and New York. This calculation includes additional CO2 from a representation of anthropogenic CO2 for the entire fossil fuel era. No-sequestered CO2 tends to flush the sequestered CO2 out of the ocean by reducing the concentration of the carbonate ion, with which CO2 reacts. Results are less favorable in the post-industrial ocean. The pre-industrial ocean model can help in the selection of location and depth, but the post-industrial ocean model is more realistic. “Thus human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years” Roger Revelle and Hans Suess.1