填空题sustainable[听力原文]Klaus Lackner, director of the Lenfest Center for Sustainable Energy at Columbia University, has come up with a technique that he thinks could solve the problem of carbon dioxide emissions. He has designed an artificial tree that passively soaks up carbon dioxide from the air using leaves that are 1,000 times more efficient than true leaves that use photosynthesis. We don t need to expose the leaves to sunlight for photosynthesis like a real tree does, he explains. So our leaves can be much more closely spaced and overlapped, even configured in a honeycomb formation to make them more efficient. The leaves look like sheets of papery plastic and are coated in a resin that contains sodium carbonate, which pulls carbon dioxide out of the air and stores it as a bicarbonate (baking soda) on the leaf. To remove the carbon dioxide, the leaves are rinsed in water vapor and can dry naturally in the wind, soaking up more carbon dioxide.He calculates that his tree can remove one ton of carbon dioxide a day. Ten million of these trees could remove 3.6 billion tons of carbon dioxide a year, equivalent to about 10% of our global annual carbon dioxide emissions. Our total emissions could be removed with 100 million trees, whereas we would need 1,000 times that in real trees to have the same effect.If the trees were mass-produced they would each initially cost around $20,000, just below the price of the average family car in the United States. And each would fit on a truck to be positioned at sites around the world. The great thing about the atmosphere is it s a good mixer, so carbon dioxide produced in an American city can be removed in Oman.The carbon dioxide from the process can be cooled and stored; however, many scientists are concerned that even if we did remove all our carbon dioxide, there isn t enough space to store it securely in saline aquifers or oil wells. But geologists are coming up with alternatives. For example, peridotite, which is a mixture of serpentine and olivine rock, is a great sucker of carbon dioxide, sealing the absorbed gas as stable magnesium carbonate mineral. In Oman alone, there is a mountain that contains some 30,000 cubic km of peridotite.Another option could be the basalt rock cliffs, which contain holes, solidified gas bubbles from the basalt s formation from volcanic lava flows millions of years ago. Pumping carbon dioxide into these ancient bubbles causes it to react to form stable limestone—calcium carbonate.These carbon dioxide absorption processes occur naturally, but on geological timescales. To speed up the reaction, scientists are experimenting with dissolving the gas in water first and then injecting it into the rocks under high pressures.However, Lackner thinks the gas is too useful to petrify. His idea is to use the carbon dioxide to make liquid fuels for transport vehicles. Carbon dioxide can react with water to produce carbon monoxide and hydrogen—a combination known as syngas because it can be readily turned into hydrocarbon fuels such as methanol or diesel. The process requires an energy input, but this could be provided by renewable sources, such as wind energy.We have the technology to suck carbon dioxide out of the air, and keep it out, but whether it is economically viable is a different question. These trees would do the job for around $200 per ton of removed carbon dioxide, dropping to $30 a ton as the project is scaled up. At that price, which has been criticized as wildly optimistic, it starts to make economic sense for oil companies who would pay in the region of $100 per ton to use the gas in enhanced oil recovery.Ultimately, we have to decide whether the cost of the technology is socially worth the price, and that social price is likely to fall as climate change brings its own mounting costs. Economically too, if the price of carbon rises, then this could lead to two effects. Investing in air capture will likely be seen as an equivalent to avoided emissions . And then it will become a worthy investment.
