In his Feb. 12 State of the Union address, President Obama singled out climate change as a top priority for his second administration. "We can choose to believe that Superstorm Sandy, and the most severe drought in decades, and the worst wildfires some states have ever seen were all just a freak（奇异的） coincidence," he said. "Or we can choose to believe in the overwhelming judgment of science -- and act before it's too late." Four years ago, the president addressed rising global temperatures by pledging a 17 percent cut in carbon dioxide (CO2) and other greenhouse gas emissions in the United States by 2020, and an 80 percent cut by 2050. The administration has taken a number of steps to meet those goals, such as investing billions of dollars in wind, solar and other carbon-neutral energy technologies.

But reducing CO2 emissions may not be enough to curb global warming, according to scientists at Stanford University. The solution, they say, could also require developing carbon-negative technologies that remove large amounts of CO2 from the atmosphere. Their findings are summarized in a report by Stanford's Global Climate and Energy Project (GCEP).

"To achieve the targeted cuts, we would need a scenario where, by the middle of the century, the global economy is transitioning from net positive to net negative CO2 emissions," said report co-author Chris Field, a professor of biology and of environmental Earth system science at Stanford. "We need to start thinking about how to implement a negative-emissions energy strategy on a global scale."

In the GCEP report, Field and lead author Jennifer Milne describe a suite of emerging carbon-negative solutions to global warming -- from bioenergy technologies to ocean sequestration（隔离，扣押）. Many of the examples cited were initially presented at a negative carbon emissions workshop hosted by GCEP in 2012.