Far-fetched fixes for climate change, from solar shields to air-scrubbing

The world's first direct air capture plant opened in Switzerland in May.

(CNN)In the battle against climate change, efforts to reduce emissions of harmful gases like carbon dioxide have consumed most of the headlines.

But many scientists say that simply minimizing man-made pollution is not enough to limit global warming to two degrees Celsius, as agreed in the 2015 Paris climate agreement -- humans must also devise other means to help cool the planet.
There's no shortage of imaginative proposals to achieve this goal. Some involve sucking carbon dioxide out of the atmosphere; others, including a giant space-based solar shield, would reduce the amount of sunlight that reaches Earth.
    While many of these solutions are considered radical and impractical, others have been operational for years. But given their often high costs, significant side effects, vast scale and international impact, there's plenty of skepticism about the viability of the existing options.
    "There's no way around reducing emissions," said Stefan Schaefer, climate engineering program leader at the Institute for Advanced Sustainability Studies in Potsdam, Germany. "Without that, none of the techniques can do anything useful."
    "None of them can be a silver bullet," he added. "But investing some money to research these approaches makes sense."
    Here are some of the existing technologies and a look at how realistic they are.

    Scrubbing the air

    The world's first operating DAC plant. Experts say many more would need to be built to make a serious difference.
    While some projects remove carbon dioxide at its source -- such as power stations or gas fields -- before it enters the atmosphere, direct air capture (DAC) involves taking CO2 out of the air and storing it, often in underground reservoirs.
    Chemical techniques for capturing CO2 -- such as "scrubbing" (using an alkaline to absorb CO2) -- are already being used in power stations and could be transferable to DAC projects.
    Other suggested methods include using artificial trees to suck up carbon or building towers that suck in air and filter out the CO2.
    Pros:
    • Far-reaching: DAC can combat emissions from disparate sources including homes and vehicles, not just single points (such as power stations).
    • Mobile: Since the technology targets CO2 in the atmosphere, DAC facilities can be set up anywhere on Earth.
    Cons:
    • High costs: Factoring in the building of plants and assembling new infrastructure, DAC is very expensive, both financially and in terms of energy input.
    • Storage risks: Some scientists are worried about storing CO2 underground because of the possibility of leaks and the question of who would be responsible for monitoring the gas over time.
    How realistic is it?
    Unlike some other technologies, DAC has made it beyond the drawing board.
    The world's first commercially operational DAC plant opened in Switzerland in May. Fans suck air into filters that absorb carbon dioxide. The gas is then piped into greenhouses and used as fertilizer to grow crops.
    But Schaefer is skeptical about the scope of this technology: "The question with direct air capture is: how much can it actually achieve? How much CO2 can it actually remove from the atmosphere? And to what extent would you have to scale it up to impact the global climate?"
    Building hundreds or thousands of DAC plants would take vast amounts of energy and materials and produce CO2 in the process, contributing to the problem the plants are meant to help solve.
    Richard Darton, emeritus professor and co-director of the Oxford Geoengineering Programme at the University of Oxford, is more hopeful. He said that if the plants themselves use renewable energy to run, their carbon footprint could be very small.

    Good old-fashioned trees

    Since the 1970s, China has planted billions of trees in an effort to hold back the encroaching Gobi Desert. The swathe of forest is known as the Great Green Wall.