For a decade, Canadian physicist David Keith has led the effort to explore the controversial idea of geoengineering , the deliberate tinkering of the planet to curb the effects of climate change. Ideas have ranged from sucking carbon dioxide out of the atmosphere  to seeding Earth’s stratosphere with sunlight-blocking particles . How practical are such approaches? What are the geopolitical implications? And what are the risks of these approaches compared to staying on our present course?
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Applied physicist David Keith holds joint appointments in Harvard's School of Engineering and Applied Sciences, as well as the Kennedy School for public policy. He is CEO of Climate Engineering, a company in Calgary building machines to remove CO2 from the air.
Contributing correspondent Eli Kintisch has covered science policy for seven years for Science magazine. In 2010 he published Hack the Planet: Science's Best Hope -- or Worst Nightmare -- for Averting Climate Catastrophe. He is the curator of the To Extremes  climate-themed art exhibition.