Scientists, firms and consultants are coming up with some innovative solutions to greenhouse gas emissions and global warming. But the profile some of the ideas have gained in the media takes no account of their practicality.
Two climate geoengineering schemes - generating clouds to cool the Earth’s surface and installing pipes to boost ocean uptake of carbon dioxide - have been investigated in recent papers.
Meanwhile back-of-the-envelope calculations by ENDS show that plans to mitigate power station CO2 emissions with algae, or sequester CO2by putting lime in the ocean face near impossible obstacles.
• Ocean fertilisation through recirculation: In 2007, James Lovelock, originator of the Gaia theory, and Chris Rapley, head of the Science Museum, proposed a scheme where nutrient-rich water from the deep ocean would be piped to the surface where enhanced biological productivity would lead to more CO2 sequestration (ENDS Report 393, p 24).
The idea was blasted as the "equivalent of a perpetual motion machine," but a study has shown it really could work.1
Unfortunately, it would not work very well. Andrew Yool and colleagues at the National Oceanography Centre in Southampton examined the outcome of the scheme in a global ocean model including an idealised representation of the floating ocean pipes which would pump the deep water to the ocean surface.
They looked at pipes of 200, 500 or 1,000 metres in length. The pipes actually boosted atmospheric CO2 levels to start with, as deep, CO2-rich water released some gas at the surface. Biological productivity was stimulated however, eventually producing a small net increase in CO2sequestration.
To sequester one gigatonne of CO2 per year - the amount emitted by two weeks of global human activity - 212 million pipes of 1,000m length would be required. The sheer number of pipes needed seems likely to prohibit the scheme on economic grounds alone, the authors conclude.
•Salter’s clouds: Professor Stephen Salter at the University of Edinburgh suggests low-level clouds over the ocean could be thickened by ships spraying a fine mist of sea water into the air. The thickened clouds would reflect more sunlight back into space cooling the ocean and ultimately the whole planet.
Unlike some geoengineering schemes, cloud modification is "not completely stupid" and warrants further investigation, according to a scientific expert who asked not be named. He said the biggest question mark was feasibility, as nothing had been demonstrated.
According to Professor Salter, 50 self-powered robotic ships could provide cooling equivalent to one year’s worth of global greenhouse gas emissions.
If atmospheric CO2doubled, a fleet of 1,500 ships, each costing £1-2m, could offset the warming, says Professor Salter. He estimates a full-scale prototype ship could be ready in five years.
A team led by Andy Jones at the Met Office Hadley Centre has modelled the effects of cloud modification focused on three small areas of the south Pacific, north Pacific and south Atlantic.2Their results showed that the technique offset about 0.6°C, delaying global warming by about 25 years. But the results are not uniform, with strong cooling of up to 3°C over the Arctic and little cooling over Europe.
The paper also highlights potentially damaging side-effects, including a 50% decline in rainfall in some areas over the Amazon rainforest. The effects were probably due to cloud modification in the south Atlantic, suggesting the area should be avoided.
Lower rainfall could have long-lasting effects on ecosystems, the authors say, even after the cloud modification is ‘turned off’.
Professor Salter said he felt the study was an attempt to sabotage his idea, and criticised the areas chosen for cloud modification. Olivier Boucher, a study co-author, said they had tried to be objective, highlighting both positive and negative outcomes.
Dr Boucher agreed that the model was a bit extreme with strong modification in small regions, but added that a more even distribution of spray ships would still result in uneven effects.
But cloud modification only addresses global temperature, leaving ocean acidification to continue unabated.
• Pouring lime into the oceans: Boosting the uptake of CO2 by adding lime to the sea tackles increasing CO2levels and ocean acidification. The idea is to mine limestone or chalk (calcium carbonate) and turn it into lime (calcium oxide), a process already widely used in the cement industry.
This calcination reaction requires a lot of heat, which would have to come from a low- or no-carbon energy source. It also emits CO2which would have to be captured and sequestered.
The idea is being promoted by Cquestrate, a company with a small amount of funding from Shell. Each tonne of CO2sequestered would need four tonnes of limestone, assuming no carbon emissions during mining, calcination or transport to the ocean.
For example to sequester one gigatonne of CO2, about 1.5 cubic kilometres of limestone weighing four billion tonnes would be needed. Mining this would be a massive enterprise, 600 times larger than current world coal production which is just 7 million tonnes per year.
• Green CO2 sequestration: Rather than dealing with the consequences of excess CO2 emissions, the Sustainable Development Commission has promoted an idea for preventing emissions in the first place. The plan is to capture CO2from power station chimneys using the photosynthetic power of algae.
It was one of 19 "breakthroughs for the 21st century" unveiled by the commission at the start of July. Engineering firm Arup and the UK Centre for Process Innovation are developing the idea.
With current plausible productivity of 60 tonnes biomass per hectare per year and assuming algae are 50% carbon, one hectare of farmed algae could capture 110tCO2per year.
Drax, the UK’s largest and most efficient coal-fired power station generates 4,000 megawatts of power and emits about 22 MtCO2 per year. It would require 200,000ha of algae to capture all its carbon emissions - an area larger than Greater London. Land is the "Achilles’ heel" of the proposal admitted Peter Head, global head of planning at Arup. "We need an order of magnitude [ten fold] increase in productivity." Under natural sunlight that would be impossible (ENDS Report 413, pp 32-35).
Arup is investigating the use of highly efficient LED lighting to allow round-the-clock algal growth. "We can’t prove that we can do this, but we think it might be possible," said Mr Head. He added that "it could take 10-15 years".