Ali Garrigan, currently one of six Greenpeace activists climbing exterior of The Shard, has compared the rise in temperatures in London to that of the Arctic.
Near the top thinking about the 1.4 degree rise here and the 10 degree rise in the Arctic. Scary. #IceClimb
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The meeting was arranged in the wake of Wednesday's terrorist murder of four people in Westminster.
Dozens of women gathered on Westminster Bridge to form a human chain in a show of solidarity with the victims of Wednesday's terror attack.
In a wide-ranging Peston on Sunday interview, he said Parliament security needs reviewing and the PM's counter-terror strategy is flawed.