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 high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.