Global warming has snowballed into a political, social, and economic crisis, and the publicity it’s received has led to many misconceptions. A common one is that reducing carbon dioxide emissions is enough to stop global warming. However, even if environmental efforts succeed in reducing our rate of carbon dioxide emissions, the atmosphere has already been filled with so much of the greenhouse gas that we may have to actively remove it to fend off the potentially catastrophic effects of global warming.
The idea is catching on. Early this month, entrepreneur Sir Richard Branson offered a $25 million prize for a cost-effective technology to remove carbon dioxide from the atmosphere. A number of promising ideas are currently being tested for effectiveness, safety and cost.
A carbon grave
One popular option is to sequester carbon in soil, the largest reservoir for carbon on land. This strategy takes advantage of the natural terrestrial carbon cycle, but shifts the carbon balance towards storage in soil. Plants remove carbon dioxide from the atmosphere during photosynthesis, and the carbon enters the soil when the plant dies.
Carbon is present in the soil in two forms, called labile and recalcitrant. Labile carbon is free to re-enter the atmosphere quickly, while more stable recalcitrant carbon stays in the soil much longer. Soil carbon sequestration strategies mainly involve planting “green” crops that store carbon in its recalcitrant form, which will keep it out of the atmosphere for up to 1,000 years, depending on the soil type. This strategy would also use techniques to retain carbon already stored in the soil. This method of sequestration works best in dry lands, which cover 43 per cent of the Earth’s surface and have large potential for carbon storage.
However, many questions remain about soil carbon storage. Until proper and accurate models are developed, the true costs and effectiveness of soil carbon sequestration remain unknown. Also, scientists do not know the upper limit for carbon storage in the soil, leaving unanswered the question of how long this strategy would last.
Down in the deep blue
Another way to tackle the effects of climate change is by capturing carbon from fossil fuels at the site of production and storing it in our oceans. CO2-rich exhaust gases from fossil fuel power plants are first mixed with seawater in a reactor vessel to produce a carbonic acid solution. This solution is immediately combined with a mineral such as limestone, producing calcium ions and a bicarbonate solution. The bicarbonate is then released and diluted in the ocean. Power plant carbon dioxide could be effectively stored in the oceans, largely as bicarbonate, a form that does not readily escape into the atmosphere.
Another approach is to physically bury carbon dioxide beneath our oceans. According to U of T professor of geography Dr. Jing Ming Chen, this approach may be the most feasible, but will be challenging nonetheless, since the technology does not yet exist to bury carbon dioxide at one kilometre, the minimal depth required to maintain its liquid state.
Dr. Ulrich Wortmann, a professor of geography at U of T, warns that though burial of carbon beneath our oceans may eventually be physically possible, it poses huge risks. In the case of an accident, enormous amounts of the oceanic carbon storage could be released. In 1986, a catastrophic eruption of a natural carbon dioxide reservoir in a western African volcanic lake pushed breathable oxygen out of the local atmosphere, asphyxiating 6,000 people. As well, the effect of a large amount of carbon on oceanic pH, or acidity, is unknown. Many oceanic organisms can only survive within a narrow pH range, so altering oceanic pH could have a more detrimental effect than the global warming itself.
Storing carbon in the deep ocean-diluted in water or buried in reservoirs-is a temporary solution. A study by Dr. Ken Caldeira of Lawrence Livermore National Laboratory at the University of California suggests that in only 300 years, 20 per cent of the submerged carbon dioxide would leak back into the atmosphere.
Just add iron
A different way of storing carbon in the ocean is facilitating the growth of organisms that store carbon. This can be done by “fertilizing” the surface of the ocean with iron to promote the growth of plankton, which absorbs carbon dioxide. When plankton dies, it sinks to the bottom of the ocean, carrying its carbon with it. In an experiment known as IronEx II in May 1995, a total of 450 kg of iron was added over a 72 km-wide area of the equatorial Pacific over one week. Plankton bloomed in the area, becoming over 20 times more abundant over the course of the experiment. It worked-the plankton significantly reduced atmospheric carbon dioxide concentrations. However, as with other carbon sequestration strategies, the long-term consequences of large introductions of iron into iron-poor regions of the ocean remain uncertain. For example, some species of plankton produce dimethyl sulfide, a compound with the potential to affect cloud formation if surface-water concentrations increase.
Researchers have shown that iron seeding would be the least effective of the proposed sequestration strategies. According to Wortmann, experiments performed years after IronEx II have shown that the majority of the carbon absorbed by the plankton is immediately returned to the atmosphere within a short time, with almost none of it sinking into deep ocean.
Mega-engineering: this is where it gets crazy
The main goal of climate mega-engineering is to increase the Earth’s albedo, the amount of light reflected by the planet’s surface. Increasing the planet’s albedo would theoretically cool the earth, which could balance out global warming. Some estimate that a 1.5 to 2 per cent increase of the earth’s albedo could counterbalance the harmful warming effects that result from a doubling of carbon dioxide emissions.
One way to alter the earth’s albedo, one scientist claims, is to surround the earth with tiny aluminum balloons. The proposed hydrogen-filled balloons would be around 4 mm in diameter with a skin thickness of 20 nanometres. They would be designed to float at altitudes of 25 km above the earth.
Another approach is to make clouds whiter. Dr. Stephen Salter, professor of engineering and design at Edinburgh University, has invented a technique that involves spraying seawater into the air to whiten clouds. As the water evaporates, small salt particles move up into low-hanging stratocumulus clouds. The salt would not only whiten the clouds, but would cause more water droplets to form, further reducing the amount of the sun’s rays penetrating the atmosphere. He aims to deploy 500 radio-controlled boats (costing about $1.8 million US each) off the west coast of Africa and west of Peru, where stratocumulus clouds are most prevalent.
As fascinating as these mega-engineering approaches may sound, scientists are concerned about the dangers of tinkering with a weather system we know very little about. Most papers on mega-engineering also emphasize unexpected environmental effects due to the lack of a complete understanding of the systems involved.
Where, oh where, should our carbon go?
Soil carbon sequestering, mega-engineering, and other innovative solutions to the threat of climate change may one day prove feasible. However, they address the symptoms of climate change rather than the cause. These solutions may buy time until our carbon emissions are reduced to sustainable levels, but are probably not the silver bullet we’re looking for.
“I look at these solutions as what we would potentially do if we were desperate. At the same time, we should definitely encourage people to think about such mega-engineering ideas, even though they are not immediate solutions,” said Chen.
According to Dr. Chen, a more pragmatic and effective solution would be to promote use of alternative energy and reduce our reliance on carbon-generating fossil fuels. Other scientists worry that these strategies would take the moral pressure off policy-makers and citizens to reduce emissions.
“To meet the Kyoto target, Canadians have to reduce atmospheric [carbon dioxide] levels by 30 per cent in five years. If the government wants to be serious about this, they should use…renewable sources of energy, such as wind, solar and biofuel,” said Chen. According to Dr. Chen, individuals working together can have a great influence on climate change.
“If every Canadian participated, this would solve about 20 per cent of the problem,” he said.
