Synthetic trees

    Living trees absorb carbon dioxide slowly. Scientists are proposing to build synthetic trees, which would not grow of produce leaves，but absorb carbon dioxide. One synthetic tree would remove about 90，000 tons  of carbon dioxide in a year-the output of more than 15,000 cars and a thousand-fold improvement on the natural behavior of a real living tree.

    Carbon sequestration(隔离)technology is likely to play a major role in the battle against climate change.  But critics say that engineers could end up using more energy in making the synthetic trees than they would save.

Sea grass

     Blooms of plankton are the grasslands of the oceans.  They absorb carbon dioxide, die and then sink to the seabed carrying the carbon dioxide they absorbed during their lifetimes. Increase such blooms and you could take out more carbon dioxide from the atmosphere, scientists argue.

    The favored method for stimulating plankton growth is to use iron fertilizers. Several groups of US entrepreneurs have begun experiments pumping iron compounds(化合物)into the sea.

    But some critics warn that the iron compounds added to the sea might cause serious pollution.

Cloud shield

    Before reaching the Earth, part of the incoming solar radiation is reflected by the clouds in the sky. As the Earth warms up, scientists plan to spray seawater into the sky to increase cloud cover. It's estimated that in this way cloud cover can increase by 4 percent-enough to shield the Earth from solar radiation.

    This plan is cheap and relatively low-tech. But scientists say this could cause other dangers, such as changing weather patterns.

Ocean pumps

    Cold ocean water contains more life forms than warmer water. Such creatures could help absorb CO2. SO scientists plan t pump cold ocean water from the deep sea up to the surface. The life forms in cold water would absorb CO2, die and then sink to the seabed, storing the carbon away for thousands of years.

    Biologists say the scheme could threaten sea life, in particular whales and porpoises（海豚）.

Mirrors

    Besides making clouds, scientists are trying other ways to block the sun's radiation. One of the most ambitious ideas involves firing giant mirrors into orbit. The special high-tech mirrors would filter infrared radiation.

    But such a process would be extremely expensive. To produce a 1 percent cut in solar radiation, scientists would have to install a total mirror surface area of 600,000 square miles.