You would have to go back at least 15 million years to find carbon dioxide levels on Earth as high as they are today, a UCLA scientist and colleagues report in the online edition of the journal Science.

Global warming, some have argued, can be reversed with a large-scale “geoengineering” fix, such as having a giant blimp spray liquefied sulfur dioxide in the stratosphere or building tens of millions of chemical filter systems in the atmosphere to filter out carbon dioxide.