However, it is not clear how effective different types of sequestration and associated leakage are in the long term, and what their consequences might be. Here I present projections over 100,000 years for five scenarios of carbon sequestration and leakage with an Earth system model.
Most of the investigated scenarios result in a large, delayed warming in the atmosphere as well as oxygen depletion, acidification and elevated CO2 concentrations in the ocean.
Specifically, deep-ocean carbon storage leads to extreme acidification and CO2 concentrations in the deep ocean, together with a return to the adverse conditions of a business-as-usual projection with no sequestration over several thousand years.
Geological storage may be more effective in delaying the return to the conditions of a business-as-usual projection, especially for storage in offshore sediments. However, leakage of 1% or less per thousand years from an underground stored reservoir, or continuous resequestration far into the future, would be required to maintain conditions close to those of a low-emission projection with no sequestration.
Gary Shaffer, professor at the Niels Bohr Institute, and leader of the Danish Center for Earth System Science, made long model projections for a number of sequestration/leakage scenarios. His results show that leakage of the stored CO2 may bring about large atmosphere warming, large sea level rise and oxygen depletion, acidification and elevated CO2 concentrations in the ocean.
Storage of CO2 in the deep ocean is a poor choice since this creates grave problems for deep sea life and since CO2 stored this way returns to the atmosphere relatively quickly, bringing back the global warming.
Geological storage may be more effective in delaying the return of the warming and associated consequences but only if a CO2 leakage of 1 % or less per thousand years can be obtained.
Dreams of braking global warming by storing carbon emissions from power plants could be undermined by the risk of leakage, according to a study published on Sunday.
Rich countries have earmarked tens of billions of dollars of investment in carbon capture and storage (CCS), a technology that is still only at an experimental stage. Under CCS, carbon dioxide (CO2) would be snared at source from plants that are big burners of oil, gas and coal.
Instead of being released into the atmosphere, where it would contribute to global warming, the gas would be buried in the deep ocean or piped into underground chambers such as disused gas fields.
CCS supporters say the sequestered carbon would slow the pace of man-made warming. It would buy time for politicians to forge an effective treaty on greenhouse gases and wean the global economy off cheap but dirty fossil fuels.
Critics say CCS could be dangerous if the stored gas returns to the atmosphere. They also argue that its financial cost, still unknown, could be far greater than tackling the source of the problem itself.
The new research, published by the journal Nature Geoscience, wades into the debate with an estimate of capturing enough carbon to help limit warming to two degrees Celsius (3.6 degrees Fahrenheit), the figure set in last December’s Copenhagen Accord.
Storing CO2 in the ocean will contribute to acidification of the sea, with dangers that reverberate up the food chain, says its author, Gary Shaffer, a professor at the Danish Centre for Earth System Science in Humlebaek, Denmark. It also carries a higher risk of being returned to the atmosphere by ocean currents and storms.
Underground storage is a better option, but only if the geological chamber does not have a significant leak or is breached by an earthquake or some other movement, says the paper.
The gas will have to be stored for tens of thousands of years to avoid becoming a threat to future generations, a scenario similar to that for nuclear waste, it says.
This means less than one percent of the stored volume can be allowed to leak from the chamber per 1,000 years. To offset any bigger leak, re-sequestration would be needed — in other words, grabbing an equivalent amount of CO2 from the air and storing it. But this would be a cost burden that could last for millennia.
"The dangers of carbon sequestration are real and the development of CCS should not be used as a way of justifying continued high fossil fuel emissions," said Shaffer.
"On the contrary, we should greatly limit CO2 emissions in our time to reduce the need for massive carbon sequestration and thus reduce unwanted consequences and burdens over many future generations from the leakage of sequestered CO2."
Until only recently, CCS was widely dismissed as fantasy or a last-ditch option. In the last five years, though, it has become progressively enshrined as a favourite policy among rich economies.
In 2008, the Group of Eight (G8) summit recommended launching 20 large-scale CCS demonstration projects by 2010.
This target "remains a challenge," according to the International Energy Agency (IEA) in Paris.
It estimates that over the past two years, countries have committed 26 billion dollars in CCS projects. Thanks to this funding, "between 19 and 43" large-scale demonstration projects would be launched by 2020.
"CO2 sequestration has many potential advantages over other forms of climate geoengineering. It makes good sense to modify the Earth’s radiation balance by putting carbon back in where it came from. Atmospheric CO2 is long-lived and evenly-distributed globally making it possible to manage it in a long-term, controlled way with less chance for unpleasant climate surprises.
However, one should not underestimate potential short and long-term problems with leakage from underground reservoirs. Carbon in light form will seek its way out of the ground or seabed. The present situation in the Gulf of Mexico is a poignant reminder of that", says Gary Shaffer.
Professor Shaffer concludes that "the dangers of carbon sequestration are real and the development of this technique should not be used as an argument for continued high fossil fuel emissions. On the contrary, we should greatly limit CO2 emissions in our time to reduce the need for massive carbon sequestration and thus reduce unwanted consequences and burdens over many future generations from the leakage of sequestered CO2."
www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo896.html
