FILE - In this March 2, 2015 file photo, David Letterman attends "An Evening of SeriousFun Celebrating the Legacy of Paul Newman," hosted by the SeriousFun Children's Network at Avery Fisher Hall in New York. Letterman will return to TV next year to help draw attention to effects of climate change when he appears on an episode of "Years of Living Dangerously." The National Geographic Channel series, which Thursday, Sept. 17, 2015, was renewed for a second season, has signed Letterman for his first announced appearance since leaving his job hosting CBS’ "Late Show." (Photo by Evan Agostini/Invision/AP, File)

David Letterman-is that really you?

Wow, David Letterman — is that you? It’s been only 10 months since the late-night host did his farewell show on CBS, but he already looks qu…