This one's simple. There is no energy source that creates less carbon than nuclear power. And switching to nuclear can make the biggest change; electric power generates 26 percent of the world's greenhouse gasses and is the biggest contributor to global warming.
Unfortunately when the term "nuclear power" is mentioned, the first association usually isn't "most energy efficient." More likely it is something along the lines of "radioactive waste," "proliferating weapons," "high costs for construction and decommissioning," or "Fukushuma."
Wind power comes close to being as energy efficent as nuclear, but that's only if the wind is blowing. Here's the good news: the gap between solar power and nuclear power in terms of carbon emissions is closing. As production of photovoltic panels becomes greener, some day nuclear may no longer be the answer. But right now, it still is.