A tiny pinwheel spins in the desert breeze atop the roof of the Global Research Technologies headquarters in Tucson. For seven months, the pinwheel has endured the blazing Arizona sun, blistering heat, wind, dust, and - finally - torrential rains. At the end of it all, the synthetic resin that makes up this seeming child’s toy has pulled carbon dioxide from the air that flowed through it and, with the rains, released it again.
The pinwheel is one of the first demonstrations of a technology that may one day be in great demand this century: devices that can extract from the air some of the billions of tons of heat-trapping CO2 being generated by industrial society. Known loosely as “artificial trees” for their ability to mimic a plant’s own uptake of carbon, such “air capture” technology has been touted as one of the most promising of the many proposed geoengineering schemes that could be used to cool an overheated planet.
“If we really do get into a situation where we realise that we’ve changed the atmosphere too much for our own well-being, there are at least ways to back off of that,” argues climate scientist Ken Caldeira of the Carnegie Institution of Washington at Stanford University, an expert on geoengineering. “There’s no fundamental limit on how much you could scale those activities up. It’s mostly a matter of how many resources you throw at it.”
Recent reports from the UK’s Royal Society and the Institution of Mechanical Engineers singled out air capture as the safest and potentially most effective of proposed geoengineering technologies. Although air capture is certainly not without its environmental impacts, the two groups noted that other geoengineering schemes - such as seeding the oceans with iron to stimulate the growth of CO2-absorbing algae, mimicking a volcanic eruption to shade the planet, or launching mirrors into space to deflect the sun’s energy away from Earth - could have far more unpredictable and potentially destabilising effects.
Proponents of air-capture technology acknowledge it is far from a perfect solution and will not enable humankind to continue spewing CO2 into the atmosphere with impunity. First, although it has been successfully tested on a small scale, air capture is at least five years away from being tested on a larger scale and, after that, could take at least two decades before it could be widely deployed. Second, to set up enough artificial trees to make a dent in reducing the vast amounts of CO2 being produced by humanity would require massive production at enormous expense.
“The cost estimates for capturing CO2 from ambient air are gross underestimates,” says principal research engineer Howard Herzog at the Massachusetts Institute of Technology. “It’s actually still a question whether it will take more energy to capture CO2 than the CO2 associated with [fossil fuel] energy in the first place.”
Even if artificial trees do prove capable of pulling large amounts of CO2 from the air, scientists then face the problem of what to do with that carbon dioxide. Underground sequestration - one possible solution - is still in the experimental stages. And deploying such artificial trees on a mass scale will have significant environmental costs, including producing the electricity needed to run them, the large land area the air capture devices would occupy, and the manufacture and installation of devices using resins, plastics, and other substances that could release air pollutants.
As the Royal Society report notes, air capture could “require the creation of an industry that moves material on a scale as large as (if not larger than) that of current fossil fuel extraction, with the risk of substantial local environmental degradation and significant energy requirements”.
In short, to extract enough CO2 from the atmosphere to begin to lower temperatures would require decades of building millions of air-capture devices that have been refined to minimise their environmental impact. Political scientist Roger Pielke, Jr. of the University of Colorado-Boulder estimates that 650 billion tons of carbon will need to be disposed of by 2100 to keep atmospheric concentrations of CO2 around 450 parts per million, a level that could easily lead to temperature rises of 2C or higher.
“You need 30 years of development time and 100 years of deployment before you start to see the effect you’re looking for,” says oceanographer John Shepherd, who led the Royal Society study of air capture and other geoengineering technologies.
That said, if humanity fails to rein in its greenhouse gas emissions, the need for air capture technology could be urgent. After all, concentrations of atmospheric carbon dioxide have reached 387 parts-per-million (ppm), more than 100 ppm higher than pre-industrial levels and quickly moving beyond what some consider to be a safe level of 350 ppm. Since the establishment of the United Nations Framework Convention on Climate Change in 1992, fossil fuel CO2 emissions have grown by more than 30 per cent and overall human-caused emissions have now reached roughly 30 billion tons per year.
“Unless future efforts to reduce greenhouse gas emissions are much more successful than they have been so far,” the Royal Society wrote in its September report, “additional action may be required should it become necessary to cool the Earth this century”.