According to the newspaper, the year is 2050. We see an old man with a beard sitting in a battered armchair on his porch. He is wearing a white vest. The landscape resembles a desert, and the wreck of a car is half-buried in the sand. The gaunt-looking girl sitting on the old man's knee asks, "Grandpa, what did you do in the war against climate change?' Her equally gaunt brother is playing with some toys which include a polar bear and an oil tanker (nice touch!) The message is clear: if we don't act now to curb global warming, this is the future which awaits us (and our children).
The cartoon is based on this First World War recruitment poster. The phrase "What did you do in the war, Daddy?" (along with its variants) has become a cliché and was the title of a 1966 Blake Edwards movie.
Actually, someone beat Chris Riddel to it. I found this parody of the original poster on Google Images, but I couldn't discover where or when it first appeared.