When U.S. Sens. Jeff Flake and Tom Udall fly back from Cuba to the United States on Wednesday, they will carry the regret of not being able to take with them Alan Gross, a U.S. government subcontractor serving a 15-year prison sentence on the island.

"Alan wants to come home," Flake, an Arizona Republican, said at a news conference after a two-hour meeting with the imprisoned American on Tuesday.

The two senators, who are critics of the U.S. trade embargo on Cuba, said they had come to Cuba to again push for Gross' release. They said his imprisonment is an impediment to improved U.S.-Cuba relations.

"Other people have tried to come and meet with him and not been able to," said Udall, a New Mexico Democrat. "That's an optimistic thing."

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In a historic climate change deal, U.S. President Barack Obama and Chinese President Xi Jinping announced both countries will curb their greenhouse gas emissions over the next two decades.

Under the agreement, the United States would cut its 2005 level of carbon emissions by 26-28% before the year 2025. China would peak its carbon emissions by 2030 and will also aim to get 20% of its energy from zero-carbon emission sources by the same year.

"As the world's two largest economies, energy consumers and emitters of greenhouse gases, we have a special responsibility to lead the global effort against climate change," Obama said Wednesday in a joint news conference with Xi.

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There are two questions we all want answers to: where did we come from? And are we alone in the universe? Bill Nye explains how the comet landing might help us solve these msyteries.