Geoscience Reference
In-Depth Information
Advantages and Disadvantages
Although the surface of the ocean is becoming more concentrated with
CO
2
, the surface waters and the deep ocean waters generally mix very slowly,
on the order of decades to centuries. Injecting CO
2
directly into the deep ocean
would take advantage of the slow rate of mixing, allowing the injected CO
2
to
remain sequestered until the surface and deep waters mix and CO
2
concentrations equilibrate with the atmosphere. What happens to the CO
2
would depend on how it is released into the ocean, the depth of injection, and
the temperature of the seawater.
Carbon dioxide injected at depths shallower than 500 meters typically
would be released as a gas, and would rise towards the surface. Most of it
would dissolve into seawater if the injected CO
2
gas bubbles were small
enough.
41
At depths below 500 meters, CO
2
can exist as a liquid in the ocean,
although it is less dense than seawater. After injection below 500 meters, CO
2
would also rise, but an estimated 90% would dissolve in the first 200 meters.
Below 3,000 meters in depth, CO
2
is a liquid and is denser than seawater; the
injected CO
2
would sink and dissolve in the water column or possibly form a
CO
2
pool or lake on the sea bottom. Some researchers have proposed injecting
CO
2
into the ocean bottom sediments below depths of 3,000 meters, and
immobilizing the CO
2
as a dense liquid or solid CO
2
hydrate.
42
Deep storage in
ocean bottom sediments, below 3,000 meters in depth, might potentially
sequester CO
2
for thousands of years.
43
The potential for ocean storage of captured CO
2
is huge, but
environmental impacts on marine ecosystems and other issues may determine
whether large quantities of captured CO
2
will ultimately be stored in the
oceans. Also, deep ocean storage is in a research stage, and the effects of
scaling up from small research experiments, using less than 100 liters of
CO
2
,
44
to injecting several GtCO
2
into the deep ocean are unknown.
Injecting CO
2
into the deep ocean would change ocean chemistry, locally
at first, and assuming that hundreds of GtCO
2
were injected, would eventually
produce measurable changes over the entire ocean.
45
The most significant and
immediate effect would be the lowering of pH, increasing the acidity of the
water. A lower pH may harm some ocean organisms, depending on the
magnitude of the pH change and the type of organism. Actual impacts of deep
sea CO
2
sequestration are largely unknown, however, because scientists know
very little about deep ocean ecosystems.
46
Environmental concerns led to the cancellation of the largest planned
experiment to test the feasibility of ocean sequestration in 2002. A scientific
consortium had planned to inject 60 tCO
2
into water over 800 meters deep near
